The pet trade is one of the primary pathways by which bird species continue to be globally transported (Westphal et al. 2008; McLardy and Burnett 2012). Although it is not the primary aim of the pet trade to introduce birds to new wild environments, pet bird species can become alien invaders if they are released or escape from captivity, and subsequently go on to establish a viable population (Hulme et al. 2008). The situation in Taiwan, where the pet trade is extremely active, suggests that this can easily happen. More than 300 alien bird species have been recorded in the bird trade in Taiwan in just three surveys, and 90 alien bird species have been recorded in the wild. At least 25 alien bird species breed in Taiwan, and 23 of these species have been recorded for sale in Taiwanese pet shops. Thus, the wildlife pet trade appears to be a major driver of the introduction and establishment of alien birds in Taiwan.

Previous studies have shown that the level of international trade and the volume of merchandise imports are positively correlated with the numbers of invasive species across countries (Westphal et al. 2008). This implies that one of the key determinants of the likelihood that a species is introduced is its availability for release or escape (Lee and Shieh 2005; Severinghaus 2007; Blackburn et al. 2015) and therefore species abundance in captivity is predicted to provide important information for understanding the invasion process. Our analysis revealed that, as expected, species abundance in captivity is a strong and consistent predictor of which pet shop species have been recorded at large in the wild. This result also holds for species abundance in captivity as recorded in Chi’s survey, two decades earlier, and has also been shown by Lee and Shieh (2005) to apply to estrildid finches for sale in Taiwan.

Species that have been available in the pet market for a long period are also much more likely to find their way into the wild, again reflecting the influence of availability on introduction success. As species recorded in all three surveys were much more likely to have been recorded in the wild (32 %) than those not (6 %). However, what is more surprising is that 28 of the 90 species recorded in the wild were not recorded for sale in Taiwanese pet shops by any of the three surveys, even though these surveys recorded 312 species for sale in total. This suggests either that species are making their way into the wild from other sources, or perhaps more likely, that there are many more species traded in pet shops than have been identified by the three surveys.

The influence of the availability of species for sale on occurrence in the wild is exacerbated in eastern countries like Taiwan by prayer animal release. This is a common practice in Eastern religions such as Buddhism and Taoism, whereby animals are bought only to be released into the wild for good karma (Severinghaus and Chi 1999). Prayer animal release is extremely active in Taiwan, where it has been estimated that 200 million animals are released annually (Environment and Animal Society of Taiwan 2009). Indeed, in 2014, a religious TV channel in Taiwan (Life TV) launched the aim of having 1000 international animal release events, to be carried out in the shortest possible time. These events were practiced across China, Hong Kong, Thailand, Singapore and Malaysia, but mainly in Taiwan (Hai-Tao 2014). Prayer release often involves large numbers of alien species (Severinghaus and Chi 1999), providing a clear link from trade to wild.

Interestingly, while propagule pressure is the most consistent predictor of establishment success in birds (Blackburn et al. 2009a), and other organisms (Lockwood et al. 2005; Hayes and Barry 2008), we did not find strong evidence of an effect of numbers for sale on establishment success for birds in Taiwan (Tables 5, 6). We do not have direct information on the numbers of bird released in Taiwan, but the positive effect of numbers for sale on introduction probability (Tables 2, 3) implies that species most commonly sold in Taiwan are more likely to escape or to be released; we would also therefore expect numbers for sale to predict establishment success. One possible reason for this finding is the origin of birds for sale. Carrete and Tella (2008) suggested that whether alien individuals are captive-bred or wild-caught is a key determinant of establishment success, with the latter more likely to establish viable populations. Differences in the origins of different species may affect the likelihood that escaped individuals would establish. Although we do not have direct information on species origin for all the species identified or sale, some established species, such as mynas and starlings, are mainly sourced from the wild. Species claimed to be captive-bred may be in fact wild-caught birds, as Taiwan is one of the main destinations of re-exported species through “wildbird laundering” by other Asian countries (Shepherd et al. 2012). Robust information on the sources of birds in trade would be useful to disentangle these effects.

The number of birds for sale is negatively correlated to price, while cheaper bird species are more likely to be purchased in abundance for release in religious events (Su et al. 2015), and we did find some suggestion of a negative effect of price on establishment success (Table 4). Nevertheless, this effect was weak and inconsistent (Tables 5, 6). The lack of an effect of numbers on establishment success might reflect the poor condition of prayer release birds. These birds have high mortality before they are released. The released birds further suffer 1 % mortality, and up to 35 % of birds with bad condition that can only perform short flights, in the period immediately after release (Hsu 2008; Environment and Animal Society of Taiwan 2009).

Based on our survey, introduced species are likely to have more attractive songs (Table 2 and Figs. 2, 3). Our prediction was that species with higher song attractiveness would be more valuable to their owners, and hence would be better secured from escaping. However, the univariate relationship between introduction success and song attractiveness is not a simple consequence of co-linearity with price, as this relationship is also recovered in multivariate models. In fact, Su et al. (2015) showed a negative correlation between song attractiveness and a species’ price for sale in pet shops. One possibility is that the result is a consequence of the popular cultural activity of bird singing competitions in Taiwan. Bird owners often bring their “best singer” individuals to competitions, which are mostly held outdoors, thereby increasing the chance that such species might escape into the wild. We found that in some circumstances, songbirds can also be released deliberately by owners, for example, if purchased birds do not sing as well as expected. This would potentially increase the likelihood that such species would be released, and thus observed in the wild, producing the positive relationship between introduction probability and song attractiveness that we observed.

Our analyses revealed no influence of body mass on introduction success in Taiwan. However, it is the most consistent predictor of which introduced species are subsequently recorded as breeding in the wild; this is true for analyses based on data both from our survey and Chi’s survey (Tables 5, 6). Some studies have suggested that species with faster population growth are more likely to succeed in establishing, because such populations can rapidly escape the dangers of small population size (Moulton and Pimm 1986; Cassey 2002). In contrast, other studies have suggested that these species are likely to fail in establishment because they face higher stochastic demographic variance than those with lower growth rate (Sæther et al. 2004; Blackburn et al. 2009a). Comparative studies of the relationship between establishment success and life history also suggest that species with slower population growth rates are more likely to succeed in establishing (Blackburn et al. 2009b). Species with slow population growth rate tend to have longer life-span, low rates of reproduction and larger body mass (Blackburn et al. 2009a), and may also be species that prioritise future over current reproduction (Sol et al. 2012). Our finding of a positive relationship between body mass and establishment success supports the latter view: features associated with slow population growth rates are more likely to determine whether or not introduced species succeed in establishing in Taiwan, following introduction. Such species may be more likely to establish because they are less susceptible to the negative effects of demographic and environmental stochasticity on small founding populations (Sæther et al. 2004; Blackburn et al. 2009a).

Our analyses are potentially affected by the difficulty of determining, for many alien species, whether individuals recorded in the wild derive from captivity or have occurred naturally as vagrants. There are 127 species identified as vagrants in the checklist of the birds of Taiwan (Ding et al. 2012), which is more than the numbers of alien species recorded in the wild since 1980 in our data. It is possible that some of the individuals of these species originated in captivity, or vice versa, that some of the species recorded as alien introductions are actually vagrants. We took the conservative line that all individuals of species that (Ding et al. 2012) listed as vagrant in Taiwan were actually vagrants. Therefore, the numbers of alien species recorded in the wild is perhaps likely to be underestimated. Vagrants are more likely to come from regions in geographic proximity to Taiwan, but we do not think that this would alter our conclusions. For example, our survey showed that species from nearby geographic regions tend to be sold in larger numbers in pet shops, while species sold in larger numbers are more likely to be introduced (Table 2). Therefore, excluding vagrants that are actually aliens is likely to exclude species more abundant in shops, and hence bias our results away from the results we found.

In conclusion, our analyses combining records on alien bird species found in the wild in Taiwan with surveys of the pet bird trade spanning 20 years reveal that introduction success is dependent primarily on a species availability for release or escape. This is likely to be because species sold in large numbers are also liberated in large numbers, especially as a result of the practice of prayer animal release. However, this does not translate into an effect of numbers on establishment success, perhaps because many of those birds released are in poor condition. Rather, establishment success is greater for species that theory suggests will be less susceptible to the negative effects of demographic and environmental stochasticity on small founding populations. Thus, the pet trade influences bird invasions in Taiwan by determining which species are exposed to novel environments, but which of those introduced species goes on to establish may depend more on their intrinsic life histories (Sol et al. 2012; Cassey et al. 2014).