HBW 10 - Foreword on the ecology and impact of non-indigenous birds by Daniel Sol, Tim Blackburn, Phillip Cassey, Richard Duncan and Jordi Clavell.

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1. Introduction

In March 1890, around 60–80 Common Starlings (Sturnus vulgaris) were released into Central Park, New York, as a plan to establish in North America all avian species cited in the works of Shakespeare (Wood 1924). Early attempts to introduce the species in 1872/73 had failed, but this one proved to be successful. Supplemented by subsequent introductions, the starlings spread to other states, reaching Pennsylvania in 1908, Alabama in 1918, and Kentucky in 1919. This range expansion continued during the following decades, and today the Common Starling is one of the most abundant and widespread birds in North America.

The introduction of plants and animals facilitated by human activities has been so common and widespread a phenomenon that alien species are currently recognized as a central element of global environmental change (Sakai et al. 2001). For birds alone, there have been more than 1900 documented attempts to introduce about 400 species all over the world (Cassey 2002a). A European visiting New Zealand, Australia or the Hawaiian Islands will likely feel at home listening to the familiar vocalizations of a variety of introduced European songbirds . Gerald Durrell (1966) noted on his trip to New Zealand: “We might as well have stayed at home if all we were going to see was blackbirds and skylarks”.

This unprecedented exchange of species between regions is not only leading to a rapid homogenization of the earth’s biota (Lockwood et al. 2000), but also poses a major threat to biodiversity and ecosystem functioning (Williamson 1996, Parker et al. 1999, Simberloff 2001, Fritts and Rodda 1998). Nowadays, non-indigenous species represent the second-most-frequent threat of species extinction after habitat loss, and they have become the primary threat to oceanic island biotas. Recent estimates indicate that one-quarter of all bird species at risk of extinction are threatened because of the effects of non-indigenous species (Stattersfield &Capper 2000). Non-indigenous species may affect native species through competition, predation or disease transmission; altering their genetics through hybridization and introgression; and can even change the structure and functioning of the entire ecosystem. In addition to ecological problems, invasive species are responsible for serious human health problems and important economic losses, which in the USA alone can exceed $100 billion per year (Vitousek et al. 1996, Everett 2000).

Given the considerable evidence currently available on the problems associated with past introductions, one would think that we would have learnt our lesson. Yet, surprisingly, potentially damaging introductions continue in many regions of the planet. In fact, many countries do not seem to perceive invasive species as an important problem. In a recent survey by the United Nations, fewer than 10% of the countries polled declared that most non-indigenous species in their territory had been identified, and only 2% acknowledged the problem enough to invest sufficient economic resources to deal with it.

What can we do to minimize the crisis generated by non-indigenous species? Given that once a species is established it is costly and often impossible to eradicate or control, prevention of new introductions is fundamental in order to minimize the impact of invaders (Mack et al. 2000). This demands that we understand the factors that allow certain species to establish and spread into foreign environments, so that we can identify situations where invasion risk is high (Duncan et al. 2003). Ecologists have long wondered what determines the success of species that are introduced into foreign environments (Mayr 1965, Ehrlich 1989). How is it that a species that has not evolved in a community not only is capable of invading it, but may even displace species adapted to live there through generations of selection?

Although researchers have for some time been pessimistic about the possibility of ever answering this question, recent studies have begun to uncover some of the rules that govern the invasion process (Williamson 1996, Kolar & Lodge 2001, Duncan et al. 2003). Much of this progress has come from the study of birds (Duncan et al. 2003, Cassey, Blackburn, Sol et al. 2004), for which detailed information is available on a large number of successful and failed introduction attempts (e.g. Long 1981, Lever 1987). Past avian introductions are undoubtedly an unfortunate natural experiment, but at the same time they have provided critical information for a better understanding of invasions and how we may prevent them. This has allowed the factors associated with their success and failure to be explored. Here, we describe the progress that has been made on this issue thanks to the study of avian introductions. We then examine the ecological and economic impact caused by avian invaders, and explore ways in which we might help mitigate the problem in the future.

2. A framework for understanding invasions


The process by which species establish exotic populations can be usefully illustrated as a sequence of stages (Figure 1; cf. Williamson 1996, Richardson et al. 2000, Daehler 2001, Kolar & Lodge 2001). First, the species must be transported from its native range to a new location. Second, it must be released or escape into that environment. Third, it must succeed in establishing a self-sustaining wild population, following release. Fourth, species that establish successfully may increase in abundance and spread beyond the release point, the extent of which defines their geographical range in the new environment. We term these stages transport, introduction, establishment and spread (see also Lockwood 1999, Kolar & Lodge 2001, Sakai et al. 2001), and for a species to have reached a given stage in the invasion process, it must have passed through all the previous stages (see Cassey, Blackburn, Jones & Lockwood 2004, Cassey, Blackburn, Russell et al.2004, for analysis of transitions through these stages).
It is useful at this point to clarify some of the terms we will use. An introduced species is one that passes through the first two stages and is released (or escapes) into a new environment. An established (also termed naturalized) species is one that forms a self-sustaining wild population following introduction. An established species that succeeds in spreading beyond the site of introduction is termed an invasive, while the introduction of a species to a location is termed an introduction event. We use the term ‘introduction’ to refer to a species that has been transported to, and been released or escaped into, an alien environment, regardless of whether or not it establishes or spreads.
The stages shown in Figure 1 provide a useful framework for reviewing what we know about the factors affecting the outcome of bird introductions around the world, and we consider each of these stages in turn.


3. Transport and introduction


a. History of avian introductions


Since humans began purposefully leaving their homelands to colonize new territories, they have taken or brought back with them exotic plants and animals. Evidence for bird introductions dates back at least 3000 years in India, for introductions of the Greylag Goose (Anser anser), Rock Dove (Columba livia), and Red Junglefowl (Gallus gallus) (Cassey 2002a, Brooke & Birkhead 1991), and for introductions of junglefowl by Polynesian seafarers to locations as remote as Easter Island (Hurles et al. 2003).
Nevertheless, most bird introductions occurred during the great migrations of ­European settlers in the 18th and 19th centuries. The people moving to new colonies, notably in the Americas, Australia, New Zealand and South Africa, encountered new environments very different to the highly modified European landscapes they left behind. Introducing species that reminded them of their European homeland was one motivation for bird introductions, along with purposeful introductions for hunting and pest control; accidental escapes of cage birds were another cause. The process of transporting and introducing species was facilitated by the setting up of acclimatization societies in many countries during the 19th century. In New Zealand, for example, acclimatization societies were set up with a stated aim being “the introduction, acclimatisation and domestication of all animals, birds, fishes, and plants, whether useful or ornamental” (McDowall 1994). Private individuals also took it upon themselves to “improve” the biota of new territories through introductions (e.g. Guild 1938 on Tahiti) and in some cases even formed amateur naturalist groups (Long 1981). In South Africa, Cecil Rhodes introduced several species in order to diversify the bird fauna of Cape Town, described previously as “poverty-stricken” (Cassey 2002a).
Quite quickly, however, it was realized that bird introductions frequently had undesirable side-effects. By 1899 sparrows introduced to North America had been described as “one of the worst of feathered pests” (Palmer 1899), while in New Zealand Drummond (1906) commented that the “company [of the introduced birds] was found not to be half as desirable as had been anticipated”. Indeed, in New Zealand the Protection of Certain Animals Act of 1861, which provided protection from hunting during most of the year for “swan, partridge, English plover, rook, starling, thrush and blackbird”, was soon replaced by the Small Birds Nuisance Act of 1882, and eventually by The Injurious Birds Act of 1908, with provision for the wholesale destruction of certain introduced species (Thompson 1922). Growing awareness of the negative impacts of introduced birds has led to a decline in the frequency of purposeful introductions, particularly among Passeriformes, where early introductions (before 1900) were almost all intentional (Figure 2). Intentional introductions of Galliformes have continued, primarily for game purposes, while accidental introductions of Psittaciformes have increased, almost certainly in response to an increase in the importation and trade of cage birds.


b. Which species were selected for introduction?


Humans have transported and introduced to a new location fewer than 5% of the world’s bird species (Blackburn & Duncan 2001a). The species chosen for transport and introduction are by no means a random subset of the world’s birds. Instead they tend to be concentrated in certain taxonomic groups and geographical regions, and to possess particular characteristics. We examine the patterns and causes of selectivity for each of these features in turn.

Taxonomic selectivity

Three studies have examined the patterns of taxonomic selectivity associated with bird introductions worldwide. Blackburn & Duncan (2001a) considered selectivity in the birds chosen for introduction, while Lockwood (1999) and Lockwood et al. (2000) considered selectivity in the birds that had become established following introduction. The results are similar in that the families over-represented at the introduction stage are also the families over-represented among established species. Consequently, we can infer that a major cause of taxonomic selectivity among established species is taxonomic selectivity in the species chosen for introduction.

Of the species chosen for introduction, around two-thirds belong to just six bird families: Anatidae; Phasianidae; Columbidae; Psittacidae; Passeridae; and Fringillidae (Blackburn & Duncan 2001a). The over-representation of species in these families undoubtedly reflect two of the major motivations behind bird introductions: species in the families Anatidae and Phasianidae were primarily introduced for hunting; while species in the families Psittacidae, Passeridae and Fringillidae are frequently kept as cage birds and would have been transported and introduced by people for aesthetic reasons (see Long 1981, Lever 1987). Nevertheless, even introductions in these families involve only a small subset of species, and most (69–94%) of the species in these families have never been introduced by people outside their native range (Blackburn & Duncan 2001a).

Geographical selectivity

The number of bird species increases towards the equator. However, a disproportionate number of the bird species chosen for introduction come from temperate regions, with their native ranges centred between 30° and 50° in both hemispheres (Blackburn & Duncan 2001a). While the Neotropics are a centre of bird diversity, being home to around 30% of all bird species (Rahbek 1997), only about 8% of introductions involve species from this region. Conversly, about 16% of introductions involve species from the Palearctic, despite only about 10% of bird species having geographical ranges that include this region (Sibley & Monroe 1990). Overall, species from tropical regions are greatly under-represented in lists of introduced species.

This pattern of geographical selectivity can largely be attributed to introductions carried out by European settlers, who mostly colonized other temperate parts of the world. Thus, opportunities for the transport and trade of birds during the 18th and 19th centuries would have been greatest between western Europe and the predominantly temperate locations where Europeans settled and with which they traded, including North America, South Africa, Australia and New Zealand. Around 60% of the bird species introduced to New Zealand originate from the Palearctic and Australasian regions (Duncan et al. 2005), the two areas with which trade and transport were most frequent during early European settlement of New Zealand.

Character selectivity

Birds in different taxa and from different regions of the world often possess characteristic life-history or ecological traits (e.g. Gaston & Blackburn 2000, Cardillo 2002). Given that the birds chosen for introduction are not a random subset of the world’s birds, we might expect them to possess traits characteristic of the taxa or regions that are over-represented. Thus, we would expect introduced birds to possess predominantly the characteristics of temperate gamebirds (Anatidae and Phasianidae) or cage birds (Psittacidae, Fringillidae and Passeridae). There have been few tests of character selectivity for introduced birds, although for those introduced to Australia a high proportion of species are ground-nesters that utilize grassland, cultivation or suburban habitats, and have largely vegetarian diets, such as seeds, fruit and the like (Newsome & Noble 1986). Introduced species are also larger-bodied, on average, than would be expected if they were a randomly chosen subset of bird species (Blackburn & Gaston 1994, Cassey 2001a).

Selection for certain characteristics also occurs independently of taxonomic and geographical selectivity. Within both a geographical subset (e.g. species in the British avifauna) and a taxonomic subset (e.g. order Anseriformes: wildfowl) of the world’s birds, the species chosen for introduction are those with larger population sizes (Blackburn & Duncan 2001a). For wildfowl, people tended also to select larger-­bodied species, while for British birds they tended to select widely distributed and resident species. Species with a large population size, wide distribution and that are resident throughout the year are the commonest species, and hence the most readily available for capture and transport to new locations. They are also likely to be familiar species, and people may have targeted them specially for introduction.
Taken together, these findings describe a general hierarchy of causes that have contributed to the selective introduction of certain birds. First, the species chosen for introduction were concentrated in geographical regions that in large part reflect the origin of European settlers and their subsequent patterns of settlement and trade. Second, from within geographical regions, people chose certain kinds of bird for introduction for a variety of reasons. An emphasis on birds for hunting and aesthetic purposes has resulted in birds from five families being significantly over-represented among those chosen. Finally, given that birds were chosen for particular purposes and from certain regions, the species that were finally caught, transported and introduced tended to be those that were common in the source locations. People preferentially selected abundant, widely distributed species, either because they were most readily available for capture, or because they were species with which they were most familiar, and therefore most desired to introduce.


c. To which locations were birds introduced?


While birds have been introduced to all major regions of the world (Fig. 3), relatively few introductions have been to equatorial regions. Most have been to latitudes between 10° and 40° on both sides of the equator (Blackburn & Duncan 2001a). An additional, and very striking pattern, is that people mostly introduced birds to islands (Cassey 2003). Around 70% of introductions have been to islands (Blackburn & Duncan 2001a) even though islands only make up about 3% of ice-free land area (Mielke 1989). Over half of all introductions were to islands in the Pacific (especially the Hawaiian Islands) and Australasia.
It is interesting to speculate why islands have been such a major focus for bird introductions. The answer may lie in the characteristically low numbers of bird species found on islands, relative to continents, and the extent to which island birds have suffered extinctions since human arrival. Only 52 bird species are recorded as being introduced to mainland Australia (Duncan et al. 2001), with its large and diverse native avifauna, compared with the introduction of over 120 species to nearby New Zealand (Duncan et al. 2004), with its smaller and less diverse avifauna that suffered a major extinction event following European arrival. With fewer species, European colonists may have perceived island avifaunas as being particularly impoverished and in need of “improvement”.


4. What determines establishment success?


Once a species has been successfully transported and introduced, and thus passed through the first two steps on the introduction pathway, it finds itself at liberty in a novel environment. The next challenge it must overcome is to establish a self-­sustaining population. The historical record shows that the chances of this are roughly 50:50 (Cassey 2002b). The obvious question then is what is it that distinguishes those cases that result in successful establishment from those that end in failure? Producing an answer to this question is complicated by the range of factors that could potentially influence establishment success.
For example, success may be more likely for species with high reproductive rates, which can rapidly build up populations to levels beyond which extinction is unlikely. Such species are typically small-bodied and short-lived. Alternatively, large body size and longevity may buffer a species against environmental fluctuations that could rapidly extirpate populations of less robust exotics. Clearly, the environment itself may also play an important role, with more stable and benign climates being perhaps more accommodating to introductions. Such climates may also have more stable and abundant food resources for the introduced species to exploit. However, in many cases they are also likely to house more potential competitors for those resources, and more predators ready to exploit the introduced forms themselves. Success in the face of these interactions could depend on how adaptable and flexible individuals of the exotic species are, and hence on their intelligence or capacity to innovate. Any of these factors could also be affected by the circumstances of each separate introduction. If only a few individuals were released (or mainly males), in poor condition (perhaps after a long sea journey from their homeland), or at a disadvantageous time of year, then any advantages bestowed by features of a species’ biology or the location of introduction may count for little. On the other hand, carefully planned and supported introductions might succeed regardless. Thus, the idiosyncrasies of individual introduction events could mask the influence of other factors in determining establishment success.
This last point highlights a significant problem in any attempt to untangle the factors influencing the establishment of exotic species. As if the number of such factors were not enough, there is good reason to believe that many effects will be confounded, making the influence of individual characteristics harder to elucidate. For example, species that are commoner in Britain tended to be introduced in greater numbers to New Zealand (Blackburn & Duncan 2001a). Species that are common in Britain also possess traits that distinguish them from less common species—they tend to be passerines, small-bodied, and preferentially use certain types of niche (Nee et al. 1991, Gregory & Gaston 2000). It follows that for species introduced from Britain, characteristics of the introduced species will be confounded by the number of individuals introduced. If success is related to one of these factors, it is likely to be related to both. Separating the influence of confounded factors requires careful statistical analysis (using multivariate techniques).
Unfortunately, the issue of confounded factors is not the only significant problem that impedes the analysis of establishment success. The reader will recall that some species such as the House Sparrow (Passer domesticus) have been introduced to many exotic locations, while some locations such as Hawaii have received many exotic introductions. If some species are better at establishing than others, or if it is easier to establish at some locations than others, then the outcome of introductions of the same species, or of introductions to the same location, will be correlated. This means that each introduction event is unlikely to represent an independent piece of evidence for the influence of a factor on establishment success, because we would expect similar outcomes (either success or failure) for all species introduced to the same location, or for all introductions of the same species regardless of location. This violates a core assumption of standard statistical tests, which assume that every data point analysed provides an independent piece of information for the test. When data points are not independent, statistical tests are more likely to show that factors are a significant influence on establishment success when actually they are not.
Let us consider, for example, bird introductions to Hawaii and New Zealand. A total of 142 bird species were introduced to the Hawaiian Islands (Moulton et al. 2001), where 56% of introduction events succeeded in establishing viable populations; in turn, 120 species were introduced to New Zealand and associated islands (Duncan et al. 2005), where only 35% of introduction events succeeded. It would be easy to conclude that tropical islands are easier to invade than temperate islands, and that we have over 200 independent data points (introduction events) to support this view. In fact, this is akin to concluding that women are taller than men by only comparing adult Swedish women and adult Pygmy men. We only really have two independent data points (the archipelagos), which is certainly not enough from which to draw any firm conclusions. The best we can do is conclude that it has been easier for birds to establish in the Hawaiian Islands than in New Zealand, which may be because of their different latitudes, but could result from any of the many other differences between them. Indeed, even that conclusion could be erroneous: instead, the species introduced to Hawaii might simply be better invaders than those introduced to New Zealand, or introduced in larger numbers, and so on. If we wanted to draw conclusions about establishment success in tropical versus temperate regions, we would have to bolster our data with introductions to other regions. Even then, if the majority of tropical introductions were to the Hawaiian Islands, these data might tell us more about Hawaii specifically than the tropics generally. In practice, therefore, we would also need to employ statistical techniques that can allow for confounding variables and incorporate information on non-independence, and so produce unbiased estimates of the effects of different factors; examples include generalized least squares, linear mixed models, generalized linear mixed models, and generalized estimating equations (Diggle et al. 1994, Goldstein 1995). Such techniques are only now being applied to the analysis of establishment success in birds (e.g. Blackburn & Duncan 2001b, Cassey, Blackburn, Sol et al. 2004).
Problems such as those just described with historical data have led some to conclude that establishment success is likely to be inherently unpredictable (Ehrlich 1989, Gilpin 1990, Williamson 1996). Indeed, early studies showed a notable lack of success in identifying any consistent patterns in establishment (Williamson 1996). Never­theless, major goals of invasion ecology are to identify factors that have some potential for explaining the outcome of introduction events, and to identify which are most important and why (Williamson 1999). Despite their inherent problems, historical data on birds represent probably the best opportunity for understanding establishment processes for real introductions. That said, the problems described above mean that the results of many earlier studies must be treated with caution (Duncan et al. 2003). This needs to be kept in mind through what follows.
In order to make it easier to navigate through the tangle of factors that might potentially affect establishment success, we classify each factor into one of three categories: characteristics of the species introduced, such as their population growth rate or body size; features of the introduction location, such as its climate; and factors associated with, and often unique to, each introduction event, such as the number of individuals released. These are termed species-, location- and event-level factors respectively (Blackburn & Duncan 2001a; cf. Williamson 1999, 2001). The characteristics of each species and location ought to remain reasonably constant for all introductions concerning them. On the other hand, event-level characteristics seem very likely to vary idiosyncratically from one event to the next. Since the outcome of introduction events also seems to be idiosyncratic, this has led some authorities to conclude that event-level factors may be the key determinants of success (e.g. Williamson 1996, Duncan et al. 2003). Hence, it is with these that we begin our review of the causes of establishment success. In what follows, we review the findings of the wide range of studies that have explored the factors in each category thought most likely to be determining establishment success. We finish by synthesizing these findings to highlight what we think these studies really tell us.


a. The idiosyncrasies of individual introductions: event-level effects


Introduction effort

Most exotic species are released in low numbers and so start with small founding populations. Indeed, 49% of bird introductions for which the appropriate information exists involved the release of fewer than 50 individuals (Cassey, Blackburn, Sol et al. 2004). IUCN categorize bird species with fewer than 50 individuals left in the wild as Critically Endangered, or Critical. In quantitative terms, this means that they have more than a 50% chance of becoming extinct in the next 10 years (or three generations), even if their populations are not currently in decline (Stattersfield & Capper 2000). These are species living in their native environment, to which they have presumably had plenty of time to become well adapted. Their small populations are highly prone to extinction for a variety of reasons, including the effects of environmental fluctuations (e.g. successive harsh winters or long periods of drought), natural disasters (e.g. hurricanes or volcanic eruptions), demographic stochasticity (random variations in the sex ratio of offspring can leave a population with few females), genetic stochasticity (reduced genetic variation can lower a population’s adaptability, or increase the chances of inbreeding depression) and problems in finding mates at low densities. The probability of extinction for an equivalent-sized population of an exotic species that must additionally face the challenges of a new environment is presumably higher still. It is not surprising therefore that many introductions fail.

A straightforward explanation for the success of some introductions is that they involved the release of a greater number of individuals and so were more likely to escape the threats facing small founding populations. That is, establishment success increases with greater introduction effort (Cassey, Blackburn, Sol et al. 2004), or “propagule pressure” (Williamson 1996). Because information on release sizes has been recorded in many cases, it is possible to use historical bird introductions to test this hypothesis. All studies that have addressed this question so far have shown that species introduced with greater effort (typically measured as the total number of individuals released) have a higher probability of establishment (e.g. Dawson 1984 cited in Williamson 1996; Newsome & Noble 1986, Pimm 1991, Duncan 1997, Green 1997, Cassey, Blackburn, Sol et al. 2004). The same is true for re-introductions (Griffith et al. 1989, Wolf et al. 1998). Most studies show that greater introduction effort increases establishment success without specifically addressing the mechanism by which this increase is brought about, although Legendre et al. (1999) present evidence from birds introduced to New Zealand that it is at least in part due to a reduction in the effect of demographic stochasticity.
Introduction effort may influence establishment success not only through the number of individuals released, but also through the number of release attempts. A greater number of separate releases may improve the chances that at least some individuals encounter favourable conditions for population growth, while if different releases are from different locations, the probability that favourable habitat is encountered will also be increased (Crawley 1986). Veltman et al. (1996) reported that species successfully introduced to New Zealand were released at more localities than unsuccessful species, which supports the latter idea, although the number of localities and the total number of individuals released were correlated in their data. Griffith et al. (1989) found that re-introductions involving releases over a greater number of years had a greater probability of establishing, independent of the number of individuals released. This supports the suggestion that multiple releases are more likely to encounter favourable conditions for growth, because, unlike introductions, re-introductions are likely to be made into suitable habitat.
There are notable exceptions to the general pattern that greater effort leads to greater establishment success. Despite the introduction of over half a million Common Quail (Coturnix coturnix) to more than 30 states in North America between 1875 and 1958 (Bump 1970), no introduction succeeded (Lever 1987). Clearly, factors other than effort can determine the fate of introductions.


Environmental matching

We have mentioned that the survival probability of a small population of an exotic species is likely to be reduced by the additional challenges posed by a novel environment. It follows that the survival probability ought to be higher if a species is not so challenged because the climate and physical environment at the location of introduction matches that to which it is already adapted (Brown 1989, Williamson 1996). We classify environmental matching as an event-level factor here because it is a feature of neither the species nor the environment alone, but is defined by the interaction between the two. Two environmental matches will only be the same if they relate to two introductions of a given species to a given location, but as such multiple releases are defined as a single event, the environmental match is thus a feature unique to each introduction event.

A range of results supports the idea that a closer environmental match improves the chance that an exotic bird species will successfully establish. Regions at similar latitudes or within the same biogeographical region are more likely to be similar in climatic and habitat conditions, and avian introductions are more likely to succeed when the difference between a species’ latitude of origin and its latitude of introduction is small, and when species are introduced to locations within their native biogeographical region (Blackburn & Duncan 2001b, Cassey 2003). Success is also more likely when climatic conditions in the species’ native range are similar to those of the introduction location (Duncan et al. 2001). Also supporting the importance of environmental matching is the finding that species re-introduced into the core of their original range had greater establishment success than species released in the peri­phery (Wolf et al. 1998). It is often argued that conditions at the core of a species’ distribution will be more suitable for it than those at the edge (Brown 1984), although this is actually far from universally true (Gaston 2003).
Consistent with the importance of environmental matching is the finding that ­establishment success is often positively related to geographical range size across species (Moulton & Pimm 1986, Blackburn & Duncan 2001a, Duncan et al. 2001). Large geographical ranges typically span a wide range of environmental conditions, suggesting that the species that attain them may have broad environmental tolerances. Alternatively, wide-ranging species may utilize environments that are themselves widespread (Gaston 1994). Either way, wide-ranging species may be more likely to encounter suitable conditions following release at an exotic location, increasing their probability of establishment (for a related argument, see Ehrlich 1989).
The idea that a close match between the native and exotic environment may improve an introduced species’ chances of establishing makes no assumptions about the quality of the environment itself. Yet, it seems intuitive that different environments ought to differ in terms of the ease with which exotic species can make themselves at home. Just as a life on a tropical island may seem more desirable to many people than one on a temperate island, might it not also seem so to introduced birds? Certainly, Hawaii is well stocked with them—over 70 species on recent estimates. Thus, we now go on to examine the evidence that some regions are more amenable than others to the establishment of exotic birds.


b. Why some regions house more exotics than others: location-level effects


Natural features

There are many features of the environment that might determine whether or not a location is hospitable for an exotic species, but these can be broadly divided into three groups (Shea & Chesson 2002, Duncan et al. 2003).

First, and what is probably most attractive to us about the Hawaiian Islands, the climate there might be relatively benign. Typically, we think of benign climates as those without extremes of temperature, humidity or rainfall. However, as we explored in the previous section, the definition of “extreme” will depend on the conditions to which each species is best adapted, and so there is unlikely to be any universal standard. Nevertheless, we do know that most bird species are located at warm tropical latitudes where water is plentiful, and that the number of species tends to decrease as annual minimum temperatures or water availability decrease (e.g. Whittaker et al. 2003). This suggests that, on average, climates at lower latitudes ought to be more benign for most species. Islands may also be more benign, as the maritime influence on their climate tends to moderate temperature extremes and ensures rain, at least to windward.
Second, establishment should be favoured at locations where there are more available resources. The specific requirements of species mean that, once again, there is unlikely to be any universal standard for resource availability. Moreover, it is a major task to quantify these requirements for even one species. Nevertheless, similar logic to that in the previous paragraph leads us to suspect that resource availability should, in general, be higher at lower latitudes, where the greater diversity and abundance of plant and animal life suggests more opportunities for exotics. However, that same diversity also implies greater competition for those resources. Whether resource abundance or number of competitors is likely to be the dominant force in determining exotic establishment is unclear, but we can at least say that since both vary with latitude, so too should establishment success. A clearer prediction is that the lower species richness of birds on islands compared to proximal mainlands should lead to lower competition, and hence easier exotic establishment on islands.
It has also been suggested that since larger areas of habitat provide more resources in total, larger areas potentially could support more individuals of an exotic release. Since, as discussed in a previous section, larger populations are less likely to go extinct, establishment may be more likely in larger areas. Alternatively, however, establishment may be reduced in larger areas as these will tend to house more native competitors (both in total and in each local community: Rosenzweig 1995, Srivastava 1999, Gaston & Blackburn 2000). Moreover, if larger areas result in more dispersed founding populations, that may lower the probability of successful establishment regardless of the effect of competitors, because of the greater difficulty in finding mates.
Third, establishment should be favoured at locations where exotics are challenged by fewer enemies, be they predators, parasites or diseases. Once again, it is difficult to identify precisely which enemies will be, or even are, more important in preventing the establishment of any given species, so studies tend to assume that more species generally will result in more enemies specifically. Thus, enemies should make it harder for exotics to establish at lower latitudes and in larger areas, where species richness is higher, but easier on islands in comparison to mainlands (Elton 1958).
Tests of the underlying processes (e.g. the influence of interspecific competition) determining location-level variation in establishment success are difficult for historical bird data, as the direct causes for failure are inaccessible to investigation. Studies of the influence of location thus regularly focus on testing the predictions outlined above about the influence of latitude, insularity, area and species richness. Unfortunately, the three sets of location-level variables produce contrasting predictions with respect to relationships expected. All three predict that islands should be easier to invade than mainland regions. The influences of climate and resource availability suggest that it should be easier to establish at lower latitudes or where species richness is higher, while resource availability predicts better establishment in larger areas. However, the effects of competitors and enemies predict the reverse in each case.
In support of the island prediction, bird species introduced to mainland Australia had a higher failure rate than bird species introduced to Australia’s offshore islands (Newsome & Noble 1986). However, this result could be an artefact of differences in the establishment ability of the different sets of species introduced to mainland and islands. Additional support for the influence of insularity is hard to find. A study by Sol (2000) that took account of species identity in two independent island-mainland comparisons (New Zealand vs. Australia, and Hawaiian Islands vs. USA mainland) yielded no evidence that islands were easier to invade. This result was subsequently generalized in two global analyses of bird introductions, neither of which could find a relationship between establishment success and whether the introduction was to a mainland or island location (Blackburn & Duncan 2001a, Cassey 2003). The high proportions of exotic bird species found on islands appear to be a consequence of the many attempts to introduce birds to islands (see Section 3c), rather than any inherent feature of islands that make them easier to invade.
Latitude and habitat area are no better at predicting establishment success. Blackburn and Duncan (2001b) failed to find any relationship for birds between success and latitude of introduction in their global study. Case (1996) found a weak positive correlation between island area and the number of bird species introduced, but no evidence that area also influenced establishment. Smallwood (1994) similarly found no evidence that smaller Californian nature reserves were any harder for birds to invade than larger reserves.
Bird introductions also provide little evidence for the hypothesis (Elton 1958) that species richness influences establishment success. Case (1996) showed that the number of established bird species does not vary significantly with the richness of the native avifauna, nor indeed with the variety of potential mammalian predators. There is no difference in the establishment probability of birds introduced to Australia versus New Zealand, even though the former has more natives. These studies suggest that resident species richness is not important in determining the outcome of bird introductions. In fact, competition between native and exotic birds is probably unlikely to regulate exotic establishment because most aliens establish in highly modified habitats, such as farmland and urban areas, which are little used by native species (Diamond & Veitch 1981, Simberloff 1992, Smallwood 1994, Case 1996). In New Zealand, for example, it is rare, on the one hand, to see native bird species on farmland, other than recent colonists that appear to have moved in from Australia specifically as a consequence of the availability of such habitat, or, on the other hand, exotic bird species in native woodland, although Common Chaffinches (Fringilla coelebs), Common Blackbirds (Turdus merula) and Song Thrushes (Turdus philomelos) do appear to be moving in. While it is possible that these non-significant results for location are because opposing influences of climate/resource availability and competitors/enemies on establishment success cancel out, we suspect that they reflect the truth.


Fighting amongst themselves? Competition between exotics

Although competition between native and exotic birds is likely to be very limited because they tend to be segregated by different habitat requirements, that still leaves open the possibility that establishment success could be mediated by competition amongst the exotics, at those locations which have received multiple releases. This argument has been propounded in a suite of studies of different island exotic bird faunas by Michael Moulton and colleagues (Moulton & Pimm 1983, Moulton 1985, 1993, Moulton & Sanderson 1996, Moulton et al. 2001, Brooke et al. 1995). Moulton et al. present two types of evidence in support of this idea.

First, the success of passerine introductions to both Hawaii and St Helena declines with time as the number of exotic species accumulates. Moulton et al. argue that this is because each successive release faces competition from an ever-larger number of established species. Second, successfully established species at several locations overlap less in their morphological characteristics than would be expected if established species were a random selection of all species released; this is termed “morphological overdispersion”. Moulton et al. argue that this pattern arises because species are more likely to fail due to competition with other introduced species of similar morphology. Each of these conclusions has, however, been questioned.
For example, Duncan (1997) noted that although later introductions of passerine birds to New Zealand faced a greater number of already established species and were less likely to succeed, this relationship was confounded by introduction effort. Later introductions tended to be of fewer individuals and so were less likely to succeed for that reason. Similarly, Duncan and Blackburn (2002) showed that significant morpho­logical overdispersion found by Moulton et al. (2001) among gamebirds established in New Zealand could not have resulted from competition. The distribution of releases over the islands in both space and time meant that it was highly unlikely that individuals from any two gamebird introductions would ever have met to compete. Instead, overdispersion could be explained by the fact that gamebird species were released in lower numbers if a similar species was already established in New Zealand. Duncan et al. (2003) pointed out that it is not only birds that are introduced to exotic locations. Later bird releases in New Zealand would also have faced a greater diversity and abundance of introduced mammalian predators. The effects of competition and predation in preventing historical establishment would be difficult to distinguish. These results for New Zealand do not rule out the importance of interspecific competition among exotics in other introduced bird assemblages, but they do bring into question the evidence presented in support of this mechanism.
In summary, the evidence garnered to date from historical bird introductions suggests no consistent effect of the release location on exotic success or failure. This might be considered somewhat surprising in the light of our earlier conclusion that environmental matching does seem to matter. Clearly, the compatibility between an introduced species and the environment into which it is released is more important than features of the environment per se. Does this mean that the same is true for characteristics of the species? Next, we consider the evidence that certain traits enhance the establishment success of the species that possess them.


c. Why some birds are better invaders than others: species-level effects


Travel around the world and you will encounter a range of exotic bird species. After all, more than 200 have successfully established somewhere (Cassey 2002a). However, there are certain exotics that you will encounter again and again. Examples include the Common Myna (Acridotheres tristis), Common Starling, House Sparrow and Rock Dove (Long 1981). Indeed, it is difficult to think of some of these species as anything other than aliens. A colleague of ours told us about walking through a park in London on a first visit to England, inwardly bemoaning the presence of so many exotic birds. Having studied exotics around the world, it was with a sense of embarrassment that the realization dawned that the House Sparrows, Common Starlings, Common Chaffinches and Common Blackbirds were actually native! The widespread success of certain introduced birds suggests that some species may simply be good at establishing in novel environments. Whether species differ inherently in their probability of establishing, and, if so, what characteristics regulate this difference, are two questions that have long interested ecologists (Elton 1958, Mayr 1965, Ehrlich 1989).


Do species differ in their probability of establishing?

Simberloff and Boecklen (1991) noted what they termed an “all-or-none” (AON) pattern for bird introductions to the Hawaiian Islands. Each species tended to establish successfully or to fail repeatedly on all islands to which it was introduced. Mixed outcomes, where a species established on only some islands, were rare (although see Moulton 1993, Moulton & Sanderson 1996, 1999, Duncan 1997, 1999). The AON pattern suggests that some species are particularly good at establishing, and so succeed everywhere they are introduced, while other species are poor and so fail everywhere. That would imply in turn that characteristics of the species are key determinants of establishment success.

The observation of an AON pattern for introductions to Hawaii of itself provides little evidence for the existence of good and bad establishers. It may instead say more about the match between the Hawaiian environment and the requirements of the various species released. Conversely, the existence of good and bad establishers is not refuted by the objection (Moulton 1993, Moulton & Sanderson 1996, 1999, Duncan 1997, 1999) that most species do not follow a strict AON pattern, as the lists of successes and failures catalogued by Long (1981) and Lever (1987) both show. Release of a particularly poor invader at an especially amenable site, or vice versa, will muddy the statistics. The question is whether species do genuinely differ in establishment success, all else being equal. Blackburn and Duncan (2001b) addressed this question by fitting statistical models for establishment success in global bird introductions that controlled for the confounding effects of event-level and location-level variables. They found that each species tends to have a similar establishment outcome every time it is introduced, but that there are highly significant differences among species. In other words, some species genuinely seem to be better than others when it comes to exotic establishment. This leaves us with the question of why?


Why do species differ in their probability of establishing?

Bird species differ in a huge variety of characteristics, from size to colour, from physiology to behaviour. It seems reasonable to assume that amongst all these traits is one, or more, that varies in tandem with a species’ probability of establishing. If that is indeed the case, such traits may act in one of three ways: they may pre-adapt a species to the new environment; they may favour population increase from the low numbers that were released; or they may constrain establishment success (Blackburn & Duncan 2001b).

Observed relationships between establishment success and environmental matching suggest that it is particularly important that an introduced species finds the exotic location to suit its ecological requirements. The likelihood of this should be higher if the exotic has catholic tastes. Thus, we would expect generalist species with broad environmental tolerances to be better at establishing than specialists (Ehrlich 1989). Several studies have found evidence that birds with broader diets were more likely to establish viable populations following release (Mclain et al. 1999, Cassey 2001b). Similarly, Brooks (2001) found that introduced birds categorized as habitat specialists were less likely to establish successfully. However, generalism is not found to enhance success in every case (Veltman et al. 1996, Sol et al. 2002). One problem here may be that most measures of it are relatively crude. For example, dietary generalism is often assessed by dividing food into a few broad types (such as vegetation, fruit, invertebrates, etc.), and summing the number of types a species uses. It is often the case that fewer dietary records exist for rare or more geographically restricted species, which can make such species seem artificially specialized (Gaston 1994). Since more abundant species are usually released in larger numbers (Section 3b), generalism may thus be confounded with effort, whose importance in establishment success we have already seen.
The likelihood that an exotic species can establish successfully may also be higher if the species has the flexibility to change its behaviour in response to novel challenges posed by the new environment (Mayr 1965, Sol & Lefebvre 2000, Sol et al. 2002). Exploiting new food resources, surviving new climates or avoiding new enemies may all require the modification of a species’ existing repertoire, or even the invention of completely new forms of behaviour (Lefebvre et al. 2004). The ­hypothesis that behavioural flexibility enhances establishment (Mayr 1965) is supported by studies by Sol and colleagues (Sol & Lefebvre 2000, Sol et al. 2002), which show that established birds tend to have larger brains for their body size, and show more forms of innovative behaviour in their region of origin, than do failed species.
Most introduced species must secure a foothold in their new environment from a bridgehead of relatively few individuals. It is in these early stages that exotic birds are particularly vulnerable to the stochastic extinction processes that afflict small populations, and which so trouble conservation biologists (Section 4a). Thus, it has been suggested that species may be more likely to establish if, through rapid population growth, they can more quickly escape the risks posed by small population size (Moulton & Pimm 1986, Pimm 1991). Population growth rates are difficult to measure directly, so most tests of this idea utilize traits that are known to be related to them. Small body mass, short development time, multiple broods per season and large clutch size should all be indicative of a bird species with fast population growth.
However, it is also easy to think of reasons why species with these characteristics should also be less likely to establish. Small-bodied species with high rates of population growth also tend to have more variable population sizes. This could increase their risk of extinction following release because their population densities may be more likely to be pushed down to terminally low levels (Pimm 1991). If population variability is more influential than population growth rate, larger-bodied, longer-lived bird species, that have populations with slower rates of growth but that are more resistant to environmental fluctuations, would be expected to have a higher probability of establishment (Legendre et al. 1999, Forsyth & Duncan 2001).
Tests of these contradictory predictions for the influence of life-history characteristics of avian establishment success reflect them in their contradictory findings. For example, Cassey (2001b) found that birds with longer generation times were more likely to establish following introduction to New Zealand, whereas Griffith et al. (1989) reported higher establishment success for re-introduced species classified as “early breeders” than for those considered “late breeders”. Cassey (2001b) also showed that establishment in New Zealand was more likely for species with larger clutches, thus finding support for the influence of both “fast” and “slow” life-history traits on success (see also Green 1997). However, other studies find the reverse relationship for clutch size (Duncan et al. 2001), or no relationship at all (Veltman et al. 1996).
Results for body size are no less equivocal (e.g. Veltman et al. 1996, Green 1997, Sol & Lefebvre 2000, Blackburn & Duncan 2001b, Duncan et al. 2001), and opposing trends can even be found within a single study. Cassey (2001a) showed that small body size equated to higher establishment success across bird species. Yet, when he compared species within the same taxonomic family, he found that larger-bodied species were more likely to establish. A pattern like this could arise if, for example, a primary role in establishment for high population growth rate (giving the negative relationship across species) was modified amongst close relatives by resistance to environmental fluctuations. Species related by family tend to have similarly high or low population growth rates (Owens & Bennett 1995), and so variation in success amongst them may depend on their relative ability to weather hard times, giving the positive relationship within families.
Some traits may just be bad news for any species attempting to establish a foothold at an exotic location. One commonly cited example is migration (Kolar & Lodge 2001). If it is hard enough to respond to the challenges of one new environment, migrants have to deal with two; that is, if they can even locate a wintering ground, or indeed if there is a wintering ground for them to locate. The physiological costs of preparing for a migratory flight cannot help either. Certainly, migratory species released in New Zealand have been less likely to establish than non-migratory species (Veltman et al. 1996, Duncan et al. 2003). However, even migration does not appear to be an unequivocal handicap, as Duncan et al. (2001) found no evidence that it hindered the establishment of bird species introduced to Australia.


d. Synthesis and future directions


Readers arriving at this point may by now be thoroughly confused by the variety of event-, location- and species-level characteristics that do or do not influence establishment success, or that in some cases seem to do both. They might recall the comment near the start of this section that some people have considered establishment success to be inherently unpredictable, and have some sympathy with this view. However, we are not so pessimistic. Recent studies that apply modern multivariate statistical techniques simultaneously to assess the influence of multiple hypotheses, while accounting for the confounding effects of non-independence among events have, to our minds, greatly clarified the likely causes of establishment success.
An important advance towards this clarification has been provided by analyses that quantify where in the taxonomic hierarchy variation in establishment success is located (Blackburn & Duncan 2001b, Cassey 2002b, Sol et al. 2002). What these studies show is that species tend to have similar establishment outcomes for all their introduction events. In other words, if you want to predict the likely success of a bird release, a good place to start is by checking on its past history of success. However, these studies also show that higher taxonomic levels have much less explanatory power to predict establishment success. In other words, the past introduction history of one species of sparrow tells you very little about the likely success or failure of another sparrow species. This has two significant implications for our understanding of establishment success.
First of all, the fact that related species do not exhibit similar levels of introduction success means that success will not be well predicted by any characteristic that species do share (Blackburn & Duncan 2001b). This includes most of the life-history traits related to population growth rates, such as body size, clutch size and longevity. It also suggests that location-level characteristics are also unlikely to be good candidates for predicting success unless it is true that certain species tend to be introduced to certain environments. However, in that special case, variation in success would probably be due to variation in environmental matching, which is an event-level characteristic. Thus, the pattern of variation in establishment amongst species tells us why neither species-level nor location-level effects have been found to be good or consistent predictors of success.
The second implication is that what does influence establishment success must be a characteristic (or suite thereof) that does vary among closely related species. This suggests two broad classes of candidate (Blackburn & Duncan 2001b). The first class includes those species-level variables in which relatives do often differ markedly. Obvious candidates include abundance, geographical range size, or whatever factors (currently unknown) in turn underlie their variation. The second class of candidates is event-level effects, and in particular introduction effort. These effects are well known to be idiosyncratic, but that idiosyncrasy could be patterned such that different introduction events involving the same species share event-level characteristics. If so, then that would explain both the distribution of success across species, and the cause of that success (Duncan et al. 2003).
Cassey, Blackburn, Sol et al. (2004) have gathered information on effort for more than 800 bird introductions worldwide. They found that variation in effort was distributed across the taxonomic hierarchy in the same way as establishment success. In other words, species tend to be released in broadly similar numbers in all their introduction events, but those numbers differ on average between even related species. Blackburn and Duncan (2001b) showed why this could happen. Species that are more abundant or widespread in their native ranges have tended to be released in larger numbers, presumably because common species are more likely to be caught in large numbers. Moreover, Cassey, Blackburn, Sol et al. (2004) showed that many of the characteristics previously suggested to explain establishment success (including annual fecundity, body mass, migratory tendency, geographical range size, dietary generalism, and whether or not the release is on an island) are also correlated with effort. Recall the similar effects described above, for example the confounding of overdispersion and effort for Galliformes introduced to New Zealand (Moulton et al. 2001, Duncan & Blackburn 2002). When all the aforementioned characteristics were analysed together using multivariate statistics, only effort and habitat generalism were found to explain independent variation in establishment success (Cassey, Blackburn, Sol et al. 2004).
Thus, introduction effort currently seems our best candidate to explain variation in establishment success (Duncan et al. 2003, Cassey, Blackburn, Sol et al. 2004). Effort differs among species because species differ in their abundance and range size, and hence in their availability for capture, transport and release. However, effort is not the whole story. Cassey, Blackburn, Sol et al. (2004) also showed that average success and average introduction effort are not correlated across regions, and neither are they correlated across taxa. In other words, although increases in effort elevate success, they do not do so in a consistent manner across species. For example, similar average numbers of individuals released lead to the success of 70% of the introduction events in Sturnidae but only 30% in Muscicapidae, and 56% of the introduction events in Hawaii but only 35% in New Zealand (Cassey, Blackburn, Sol et al. 2004). This idiosyncratic taxonomic and regional variation in success reflects similar findings in other studies (Blackburn & Duncan 2001b, Duncan et al. 2001, Moulton et al. 2001): for example, that introduced gamebirds (Galliformes) were significantly less likely to establish in Australia than species of other orders. These as yet unexplained patterns of taxonomic and regional variation in establishment success are likely to be important topics for subsequent investigation.


5. Spread following establishment


When the Common Starling was introduced in New York’s Central Park, it took around a decade for the species to establish itself. Another decade passed before the species began slowly to spread, but then around a decade after that the rate of spread increased (Figure 4) so that within 80 years of release starlings had advanced right across the continent to the Pacific coast (Shigesada & Kawasaki 1997). At about the same time as the Common Starling was first introduced, a related species, the Crested Myna (Acridotheres cristatellus), was released and became established on the opposite side of the continent, in Vancouver. The myna initially became reasonably numerous but never spread, remaining confined to the environs of its initial release area. Spread is now unlikely, as the species has declined to low numbers (Duncan et al. 2003).
Birds probably have the greatest powers of dispersal of any organism. Individuals can cross continents in a matter of hours. Migrants of some species, such as the Arctic Tern (Sterna paradisaea) or the Sooty Shearwater (Puffinus griseus) undertake annual migrations that cover thousands of kilometres. Individuals of even relatively sedentary species can disperse over tens or hundreds of kilometres. Given that, and that a species has established a self-sustaining population at a location, why is it that only some species spread out from that bridgehead to invade the exotic location? Or more generally, why do species differ in the extent of their final distribution at any given location? How is that spread achieved? And why do species differ in how fast they spread? We address the last two questions first.


a. How do species spread? Pattern and rate


To understand how spread is achieved, it is necessary first to know the pattern of spread that needs to be explained. Data that allow us to plot the pattern of range expansion over time, such as those for the Common Starling, are relatively rare for birds. However, plots equivalent to Figure 4 also exist for the House Finch (Carpodacus mexicanus) in eastern North America (Shigesada & Kawasaki 1997); the Egyptian Goose (Alopochen aegyptiaca) in the Netherlands (Lensink 1998); the Common Waxbill (Estrilda astrild) in Portugal (Silva et al. 2002); and for the natural range increase of the Eurasian Collared Dove (Streptopelia decaocto) in Europe (Hengeveld 1989). In each case, spread shows the biphasic pattern illustrated in Figure 4, with an initial period of relatively slow increase followed by a higher but constant rate of spread. For the waxbill, the curve has a third phase as the growth rate slows, probably because all sites available for the species have been occupied. Spread in the initial growth phase has been calculated as 1·16 km/yr for the Egyptian Goose; 3·5 km/yr for the House Finch; and 11·2 km/yr for the Common Starling. These values increase to 4·59, 20·7 and 51·2 km/yr, respectively, in the faster second phase of spread. These biphasic growth curves are termed ‘type II’ (Shigesada & Kawasaki 1997), to distinguish them from type I curves, which show a single constant rate of range increase, and type III curves, where an initial linear rate of slow growth is followed by a period of growth at a faster but continually increasing rate. So, what do these type II curves tell us about how bird ranges expand? An answer can be obtained by examining the properties for different mathematical models of range expansion.
One simple way of thinking about an expanding range is as a growing population that is expanding the area it occupies by diffusion of individuals across the landscape. This is the approach taken in a classic paper by Skellam (1951). Skellam was able to show that if the invading population is growing exponentially, if the landscape is uniform, and if the diffusion of individuals across the landscape follows a random walk (i.e. they move randomly in a way that is similar mathematically to the well-known Brownian motion of smoke particles in air), then the invasion front advances at a constant velocity. In other words, the rate of range expansion is such that the points in a plot of radial distance versus time (e.g. Figure 4) fall on a straight line. Although the model assumes exponential growth, and hence that the population size grows without check to infinite size, the same prediction is obtained if the more realistic logistic pattern of population growth is assumed, where populations reach some finite carrying capacity. This is because diffusing populations grow by expansion at the range edge, where they will not have reached carrying capacity, and so probably are growing in an exponential manner. Skellam’s model is a useful start, as some invading populations do follow this type I pattern of range expansion (Shigesada & Kawasaki 1997). Unfortunately, bird invaders are not among them.
Skellam’s model assumes that ranges grow only by the random diffusion of individuals across the environment. One alternative is to assume “stratified diffusion”, where spread occurs both by random diffusion of individuals from the core population, but also by occasional long-distance dispersal events. This process can be simulated by what is termed a “ coalescing colony” model: the name speaks for itself. Under this model, the range initially expands through random diffusion at the wave front, as in Skellam’s model, and so grows at a constant but slow rate. However, the population occasionally produces long-distance dispersers that go on to found new colonies, which then also start to expand. When these new colonies meet the expanding front of the core range, they are absorbed into it. When this first happens it results in an increase in the rate at which the core range expands, and this is maintained as subsequent colonies are encountered and absorbed (Shigesada & Kawasaki 1997). When the rate at which long-distance dispersers are produced is proportional to the radius (or circumference) of the core range (rather than its area), the change in radial distance over time follows the type II curve.
The coalescing colony model has two attractive features. First, it generates the biphasic pattern of range expansion over time characteristic of birds (e.g. Figure 4). Second, it models a pattern of dispersal that bird invaders appear to follow. For example, Hengeveld (1988) showed that the distribution of dispersal distances of individual Eurasian Collared Doves included both long-distance and short-distance events as the species spread across Europe. Several of the exotic bird species introduced to the main islands of New Zealand have colonized outlying islands, which in many cases involve dispersal over several hundred kilometres of ocean. Of course, these colonies will not coalesce with the core populations, but they are indicative of long-distance dispersal events. More generally, ringing returns from native British birds show that most individuals of most species disperse relatively short distances (under 5 km), both as juveniles and adults, but a few individuals travel long distances (over 100 km; Paradis et al. 1998). The spread of the House Finch following its introduction to the eastern USA has involved both neighbourhood diffusion and jump dispersal events, the latter resulting in small satellite populations that eventually coalesced with the core part of the range (Shigesada & Kawasaki 1997). These satellites seem to develop preferentially in good-quality habitat (Gammon & Mauer 2002).
Exploring the features of different models for spreading populations allows us to identify processes that can, and also those that cannot, explain the patterns of range expansion we see for invasive birds. Nevertheless, while these models allow us to examine factors affecting the rate of expansion in a few typically widespread exotic species, no study has yet quantified or examined the reasons underlying interspecific variation in rate of spread.


b. Determinants of final distribution


Although no study has considered the reasons behind interspecific variation in the rate of spread, three studies have examined why different introduced species end up with different final range sizes at the exotic location (Duncan, Blackburn & Veltman 1999, Duncan, Bomford et al. 2001,Williamson 2001). These studies concern the exotic bird assemblages of Australia and New Zealand. We highlight three factors that these studies find to be consistently related to exotic range size.
First, exotic species have larger ranges if the exotic location has larger areas of suitable habitat or environmental conditions. In New Zealand, the species with the largest geographical ranges were those whose preferred habitat was most widespread, specifically species that use extensive human-modified habitats such as farmland. On the larger and more climatically varied continent of Australia, exotic species with larger geographical ranges were those with a greater area of more climatically suitable habitat available. This variable explained more than two-thirds of the variation in final range size amongst these species.
Second, exotic species have larger ranges in the exotic location if they have larger native geographical range sizes. This pattern could arise because species with larger native ranges may have broader environmental tolerances or use more widespread resources. This might not only enhance establishment, as we discussed in Section 4c, but also allow a greater extent of spread across the exotic environment.
An alternative possibility is that the range-size effect could arise from the general trend for more widespread species to be introduced in larger numbers (see Section 4d). Species with large founding populations might get a head-start in establishment that allows them to appropriate a greater proportion of the resources available for exotic invaders in the new environment. In practice, this initial advantage could have compounded itself—those species initially able to capture a greater share of resources would have had faster population growth and rate of spread, allowing them further to pre-empt resources at newly colonized sites as their ranges expanded. Two pieces of evidence are consistent with this view for exotic bird species in New Zealand. First, species introduced with greater effort were more likely to spread and achieve a larger geographical range size (Duncan et al. 1999). Second, the relationship between range size and effort was most pronounced among closely related species. Related species are more likely to compete for similar resources, and so it is amongst these species that we would expect any head-start given by variation in introduction effort to be most important (Duncan et al. 1999). This explanation implies that competition may play a role in limiting the range sizes of established species, even though it does not appear to influence establishment success (see Section 4b). Nevertheless, it requires further testing to establish whether the relationship between introduction effort and exotic range size holds beyond the single example of New Zealand. It does not hold for Australia (Duncan et al. 2001).
Third, the geographical range size of exotic species in New Zealand and Australia relate consistently to several life-history traits (Duncan, Blackburn & Veltman 1999, Cassey 2001b, Duncan, Bomford et al. 2001, but see Williamson 2001). More widespread species tend to be those with life-history traits associated with high population growth rates (see Section 4c). The reason for these relationships may be similar to that posited to explain why such species should be more likely to establish viable populations following release: they may be less vulnerable to local extinction following colonization of unoccupied sites (Gaston 1988).
Whatever the reasons for the relationships just described, they suggest that the extent of spread by exotic invaders may depend on both characteristics of the species and of the location. This is in contrast to the likelihood of establishment in the first place, which seems to be primarily determined by idiosyncratic characteristics of the specific introduction event. Thus, location- and species-level effects may not determine which species make it into the exotic environment, but may be much more important in determining which species can spread across it.


6. Impact of avian introductions


Introduced species are organisms that have not evolved over time with the recipient community, so once established they have the potential to produce significant ecological changes. The importance of their impact will depend on a combination of the abundance, distribution and impact per individual of the invader (Parker et al. 1999). In the region of introduction, invaders are often released from the factors that could have limited their abundance and distribution, such as competitors, predators, pathogens or parasites (Torchin et al. 2003). This may allow them to grow in excess, spread over wide regions and become pests. Native species, on the other hand, may not possess mechanisms to respond to novel competitors, predators, pathogens or parasites, making them more vulnerable to invaders. Despite this, ecologists estimate that only a minority of species that are introduced cause major impact on the recipient community (Williamson 1996). The reasons why some species have greater impact than others remain largely unknown, and to uncover them should be a major goal of future research.
Birds have caused, and are still causing, a variety of ecological impacts in the regions where they have been introduced. Surprisingly, however, the impact of bird invaders has been largely under-appreciated by ecologists, especially when compared with that caused by other vertebrates (see e.g. Ebenhard 1988, Lever 1994). Most avian invaders occur in human-modified habitats rather than in pristine habitats (Case 1996), so their ecological impact is often assumed to be relatively unimportant. As a result, little effort has been expended in evaluating it rigorously. However, if we do not measure the impact of non-indigenous birds, we will never know the real magnitude of the problem. In addition, introduced birds not only cause ecological impacts but they also impose substantial economic costs to human societies (Lever 1994). Although our understanding of the quantitative ecological and economic impact of birds is currently poor, we have examples of several major effects of avian invaders on native individuals, populations, genetic pools and communities, as well as those on human societies.


a. Individual and population effects


Non-indigenous species may predate indigenous species, limit their access to resources through competition and transmit parasites and pathogens to them, all of which may negatively affect the native populations. Predation is perhaps the strongest impact of introduced vertebrates. This is clearly exemplified by the devastating effect of rats and cats introduced into islands that lacked native predators (Courchamp et al. 2003, Blackburn & Gaston 2005). In birds, predators are uncommon among the species that have ever been introduced, but a negative impact through predation has nonetheless been documented. For example, the Pacific Marsh-harrier (Circus approximans), introduced into Tahiti in 1880 to control introduced rats, is one of the causes of the imminent threat of extinction faced by the Polynesian Imperial-pigeon (Ducula ­aurorae) (Thibault, 1988).
Introduced birds may also affect native birds through competition for limited resources. In North America, for example, Common Starlings have been found to usurp nests of several North American hole-nesting species (Ingold 1996, 1998). Competition for either nesting sites or food supplies with natives has been suggested for over 20 introduced bird species (Lever 1994). However, there are surprisingly few instances in which competition with introduced species causes a decline in number of resident species (Davis 2003), so whether or not competition negatively affects entire populations remains debatable. In the case of starlings, a recent comparison of densities of native cavity-nesting species before and after invasion of sites by the starling revealed that only five out of 27 species examined exhibited significant negative effects potentially attributable to starlings (Koenig 2002). Moreover, a further five species increased following starling colonization, including the Red-bellied Woodpecker (Melanerpes carolinus), one of the only species for which there was good evidence of a high frequency of nest usurpation by starlings. As Koenig (2002) points out “Despite their rapid spread, striking abundance, and aggressive nature, starlings appear thus far to have had little negative affect on the native cavity-nesting bird species with which they are known to interfere”. It remains to be investigated if this conclusion also holds for the other introduced species suggested to compete with native species.
Introduced birds may also affect individuals and populations of native species through the transmission of parasites and pathogens. A well-studied example is the avian malaria that is strongly implicated in declines of native passerine birds in ­Hawaii (van Riper et al. 1986). Malaria was introduced to this island through exotic birds kept by settlers, and its transmission was facilitated by the subsequent introduction of the southern house mosquito (Culex quinquefasciatus). Native birds were not resistant to malaria and succumbed rapidly to its detrimental effects. Avian malaria has contributed to the extinction of at least 10 native species and threatens many more.


b. Genetic effects


Closely-related species sometimes interbreed and produce fertile hybrids. This pheno­menon is relatively rare in nature, where interbreeding is usually avoided by the action of geographical, ecological, behavioural or morphological barriers. Yet, it may be facilitated when a species is introduced in a region that contains a close relative (Rhymer & Simberloff 1996). Given that alien and native species were previously separated by geographical barriers, they may have been less likely to have evolved ecological, behavioural or morphological mechanisms to avoid interbreeding.
Hybridization is a conservation issue because it may alter the genetic structure and reduce the fitness of native populations (Mooney & Cleland 2001). A number of cases corroborate the importance of this phenomenon in birds. In Europe, the long-term survival of the Endangered native White-headed Duck (Oxyura leucocephala) is threatened by hybridization with the American Ruddy Duck (Oxyura jamaicensis), an introduced species currently expanding on the European continent. Likewise, hybridization with introduced Mallards (Anas platyrhynchos) has been related to the population decline of the Pacific Black Duck (Anas superciliosa) (Rhymer & Simberloff 1996). Whilst hybridization is particularly common in ducks, it is also observed in other taxonomic groups. The nominate race of the Madagascar Turtle-dove (Streptopelia picturata), introduced to the Seychelles from Madagascar, has so massively hybridized with the local race rostrata that a hybrid form resembling the invaders now lives in the Seychelles.


c. Community effects


Introduced birds have the potential to alter the composition of the recipient community through a variety of mechanisms, although evidence for such an impact is only available for a few case-studies. One of these is the dispersal of seeds from the ornamental plant Lantana by Common Mynas introduced in the Hawaiian Islands (Lever 1994). This plant was introduced from Mexico in 1958 to local gardens, where it serves as a major food supply for Common Mynas. Thanks to subsequent seed dispersal by mynas, the plant has become an agricultural problem in the islands.
A second example of the impact of a bird invader on the community structure comes from Mute Swans (Cygnus olor) introduced in the USA: experimental exclosures have shown that grazing by swans affects the diversity and density of aquatic plants (Cobb & Harlin 1980). This is one of the few cases where the impact of a bird has been evaluated experimentally.


d. Effects to human societies


Invaders are widely recognized as the cause of significant economic and health problems to human societies, although the actual magnitude of these impacts has rarely been quantified. Perhaps the most common economic impact associated with introduced birds is the damage caused to agricultural crops. At least 37 naturalized species from four taxonomic orders have been reported to affect crops significantly (Lever 1994). In Australia, for example, the Common Starling is a pest of soft fruits and cereals, causing the loss of 10–15% of certain crops. In general, species that are considered to be pests in their natural range also become pests in the places where they are introduced (Lever 1994), a pattern with which the Common Starling fits well. However, the reverse is not always true. Species that produce little damage in their natural range may become pests when introduced in places where the natural factors that limit their abundance and distribution are absent. The Ring-necked Pheasant (Phasianus colchicus), for example, is regarded as harmless in its native range in the Middle East and Far East, but causes significant losses to the agriculture in many of the regions where it has been introduced (Lever 1994).
Introduced birds may also act as a reservoir of severe diseases that affect humans and domestic animals. Feral pigeons, descended from Rock Doves introduced in many regions of the world, carry diseases such as salmonellosis, tuberculosis, ornithosis and Newcastle disease (Johnston & Janiga 1995). Whilst there is no direct evidence that the diseases have been passed from pigeons to humans, the risk cannot be discounted.
The literature also reports a variety of additional problems that avian invaders cause on human activities. In Britain, for example, the introduced population of Canada Geese (Branta canadensis) not only is regarded as causing significant agriculture damage, but also has been implicated in damage to amenity sites, eutrophication of water-bodies, and as a threat to aircraft safety (Wattola et al. 1996).


7. Solutions to the problem of avian invaders


The seriousness of the impacts of avian invaders should not be underestimated, given how unpredictable their effects are. Future exotic introductions should best be avoided, and strategies to deal with species that have already been introduced urgently need to be implemented. The solution to the problem of invaders will be difficult, but not impossible. Five types of measure appear to be priorities (Genovesi 2000, Myers et al. 2000, Genovesi & Shine 2002):

1.   Research. This is usually the first cry of the researcher, but with good reason. Research is necessary to help prevent new invaders, to evaluate the ecological, economic and health problems the current invaders generate, and to develop efficient measures to help mitigate these problems. For decision-taking with respect to these issues, it is particularly important that we improve our ability to predict the consequences of introducing a given alien species. Good progress has been made in the last decades in our predictive power, as we hope to have demonstrated throughout this review, but important aspects remain to be investigated. Predicting the impact of introduced species is perhaps one of the major unresolved issues in birds, and in other organisms as well (Ricciardi & Rasmussen 1998, Parker et al. 1999). Assuming that we can define and measure impacts, we can, in principle, apply comparative methods to identify factors that distinguish introductions with little effect from those with large effect, as has been done for other invasion transitions. This information can then be used in risk-assessment models.

2.   Monitoring. The development of monitoring programmes for alien birds is of key importance for the identification of alien introduction and invasion pathways, the early detection of invaders, and the assessment of alien threats. Given that birds have good dispersal abilities and may easily reach distant regions, this information should be made available to other countries through regional and global information systems (Ricciardi et al. 2000). Intensive monitoring is particularly feasible in birds, as birdwatching is a widespread activity in many regions of the world.
3.   Legislation. Laws and regulations may serve to encourage countries to design management strategies, to prevent the import of high-risk species, to prohibit deliberate introductions and to force captive collections to implement measures that reduce the chance of escapes of non-native species. For example, because of the ecological problems associated with Ruddy Ducks, the UK government has, since 1995, prohibited trade in the species without licence (Hughes 1996). States are a potential source of avian invaders for other states, and hence they should take appropriate co-operative actions to minimize risks. The Bern Convention, for example, specifically states that the contracting parties must undertake strict control to prevent the introduction of non-native species.
4.   Mitigating impact. The design of control strategies to limit or eradicate high-risk non-indigenous species is the only solution once a species is established. There exist a variety of methods to control or eradicate birds (Genovesi 2000), including trapping, population sterilization, behavioural management and biological control. The decision to use one or another method will critically depend on the characteristics of the introduced population and the impact it produces. Although vertebrates are often difficult to control, success in reducing their population density and even reaching a complete eradication can be achieved in well-planned programmes (e.g. Myers et al. 2000). In the UK, for example, research into control programmes led to the conclusion that shooting was the most effective means to control the growing introduced population of Ruddy Ducks (http://www.unep-wcmc.org/aewa/eng/info.htm). After informing the public about the programme, and despite some controversy, a three-year trial started in the early 1990s to assess whether the eradication of the species was a feasible option. More than 2500 Ruddy Ducks were shot within this period, halving the population size. Based on these data, it was estimated that the UK Ruddy Duck population could be reduced to fewer than 175 birds in less than six years.
5.   Informing the public. The public needs to be aware of the serious problems that introduced species generate, so that no further irresponsible introductions are carried out. The support of the public is also important to influence political decisions and, as we have just seen, for the successful implementation of control strategies.


The growing frequency of non-indigenous species is threatening global biodiversity, ecosystem functioning, national economies and human health, making it urgent that we implement strategies to mitigate this impact. Fortunately, and despite early scepticism, our understanding of the invasion process has significantly improved during the last few years. This has broadened our ability to predict and prevent invasions, providing a good starting-point for the development of regional and global strategies against invaders. Although much work still remains to be done, we should be optimistic about the possibility that the problems posed by non-indigenous birds can be solved in the not too distant future.

Daniel Sol
Tim Blackburn

Phillip Cassey
Richard Duncan
Jordi Clavell
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