As the bird trade in Australia continues to be an emerging source of new invasive species, continued monitoring of pet sales is crucial for early prevention and effective biosecurity (Vall-llosera and Cassey 2017b). Here, we have found that a number of characteristics which influence species abundance in trade are also characteristics associated with successful invasive species.
In our study, the price of a given bird species was a correlate of abudance, where cheaper species were more abundant. This has implications for biosecurity, as price is also a major correlate for species escapes from captivity in Australia (Vall-llosera and Cassey 2017b). Simultaneously, cheaper species are also correlated with captive escapes which, combined with the higher abundances in trade identified here, increases the propagule pressure of alien species in trade (Cassey et al. 2018). The two most commonly traded alien parrots, the rose-ringed parakeet (Psittacula krameri) and the green-cheeked parakeet (Pyrrhura molinae), and two of the most commonly traded alien songbirds, the domestic canary (Serinus canaria) and European goldfinches (Carduelis carduelis) are identified as extreme biosecurity risks (Australian Intergovernmental Environment & Invasives Committee 2018). In particular, wild incursions of rose-ringed parakeets are increasing in Australia due to increasing pet trade, and are likely to establish as an invasive species without ongoing control (Vall-llosera et al. 2016). This species possesses the most important correlates with abundance for parrots: it is relatively cheap (approximately AUD$120), is easy to care for (often sold for “beginners”), has a wide variety of songs and is available in many colour mutations. This is consistent with other studies where the characteristics which make a species popular in trade also makes them successful invasive species (Gippet and Bertelsmeier 2021). Concerningly, 81% of alien parrot species and 85% of alien songbird species listed as an extreme biosecurity risk on the Australian List of Threat Categories of Non-indigenous Vertebrates, which suggests that these species should not be kept unless there is sufficient management to reduce the potential of these species to establish. With the current lack of regulation around bird-keeping in Australia, we recommend these extreme risk species be closely monitored by environmental agencies and birdkeeping societies to reduce their potential for establishing invasive populations.
Although there were more alien species present in trade, native species were sold in far greater abundances. For native species, it is likely that many of these species are being sold outside of their native ranges, potentially in large numbers, which is an unexplored biosecurity risk. For example, the third most abundant native parrot species, the rainbow lorikeet (Trichoglossus haematodus), is native to the eastern coast of Australia, but has established invasive populations in Western Australia and Tasmania due to their popularity in trade (Chapman, 2005; Robinson 2020). With the high number of native species sold, there is potential for another ‘domestic non-native’ species to establish.
Most species characteristics we explored differed in their importance between parrot and songbird abundance. The most influential species characteristic on parrot abundance in trade was song complexity, the measure of how attractive people find the species’ call measured by unique uploads to a birdcall website (Xeno-Canto.org). While this may be a traditionally desirable characteristic in parrots due to their association with repeating phrases, it may also be driven by modern media, as videos of parrots mimicing songs and speech are popular online (e.g. Moloney et al. 2021; Siriwat et al. 2020). We also found that species which are easy to care for (i.e., “beginner” species) were more popular, which encapsulates species that are hardy, have generalist diets and do not need specialist housing or attention. Characteristics associated with easy care are also correlates of successful invasive species; where they are able to readily adapt to new environments (Vall-llosera and Sol D, 2009).
Alternately, songbird trade abundance is driven by characteristics more associated with aviary birds, where cheaper, more colourful species were more abundant. Plumage colour (here, for the “natural” colour, not colour mutations) was the next most important factor explaining abundance after median price; however, the direction of which is unclear. Similarly, the presence of colour mutations had some effect on species’ abundance, where pet owners may wish to have unusual colours or forms, collect a variety of different mutations, or attempt to breed new mutations. However, this might not have a direct influence on abundance, as the more a species is captive bred, the higher the chance of a colour mutation arising (Chan et al. 2021). The most abundant native and alien species, zebra finches (Taeniopygia guttata) and domestic canaries (Serinus canaria) respectively, have some of the most diverse colour mutations available, and are highly desired for use in birdkeeping competitions.
Interestingly, body mass was not identified as a significant correlate for either order, which has previously been identifed as a major driver of species abundance in international bird trade (Romero-Vidal et al. 2020; Siriwat and Nijman 2020). While being potentially more attractive, this may counteracted by their higher ongoing costs to keep. Annual fecundity did not correlate with species abundance for parrots, and very weakly for songbirds, indicating that the abundance in trade is not necessarily influenced by reproductive output (i.e., supply), but by consumer demand for certain species and its species characteristics.
Despite the high relative importance of median price on species abundance, it was not an appropriate proxy for abundance in Australian online trade. The correlation is far too weak and varied to make assumptions on the abundance of a single species. We recommend that the median price on its own is not used as a proxy for Australian online trade; however, the combination of other covariates, such as the influential species characteristics identified here, can be used to predict abundances.
While we quantified biological and economic correlates driving the abundances of traded bird species, it is important that this information is combined with consumer behaviour research on the reasons why people purchase pet birds. Biological species characteristics do not affect trade on their own, and where additional factors such as cultural preference or legislation may change the abundance of a species in trade. For example, wildlife trade is heavily influenced by regulation, which we did not consider in our analyses. While domestic bird trade in Australia is relatively unregulated, some jurisdictions require the pet owner to have a permit to keep or wild harvest a species, and others do not (Woolnough et al. 2020). This is likely to greatly influence the abundance of a species in trade, for both native and alien species, as species which require a permit would be available to a smaller market (Toomes et al. 2022). We found evidence of two illegally wild-caught individuals (Australian ringneck: Barnardius zonarius; and pink cockatoo: Lophochroa leadbeateri), which may be an indication of laws sufficiently protecting wild populations, or sellers purposefully concealing their origins. More detail on the trade or cultural factors which make a bird more likely to be purchased and kept as a pet, such as using surveys or choice experiments, would assist in further explaining trends in the bird pet trade (Krishna et al. 2019; Marshall et al. 2020). These, in combination with our findings, may assist in identifying key species requiring greater regulation to manage biosecurity and conservation priorities.
In conclusion, characteristics that correlate with a species’ abundance varied between songbirds and parrots, differed from international markets, and were synonymous with characteristics which predict successful invasive species. With actual abundance counts of species, and an understanding of drivers behind the online trade, this information will continue to assist policy makers and birdkeeping societies in identifying invasion risks of alien species and further aid in biosecurity efforts.