Biological invasion is a global problem with large negative impacts on ecosystems and human societies. When a species is introduced, individuals will first have to pass through the invasion stages of uptake and transport, before actual introduction in a non-native range. Selection is predicted to act during these earliest stages of biological invasion, potentially influencing the invasiveness and/or impact of introduced populations. Despite this potential impact of pre-introduction selection, empirical tests are virtually lacking. To test the hypothesis of pre-introduction selection, we followed the fate of individuals during capture, initial acclimation, and captivity in two bird species with several invasive populations originating from the international trade in wild-caught pets (the weavers Ploceus melanocephalus and Euplectes afer). We confirm that pre-introduction selection acts on a wide range of physiological, morphological, behavioral and demographic traits (incl. sex, age, size of body/brain/bill, bill shape, body mass, corticosterone levels, and escape behavior); these are all traits which likely affect invasion success. Our study thus comprehensively demonstrates the existence of hitherto ignored selection acting before the actual introduction into non-native ranges. This could ultimately change the composition and functioning of introduced populations, and therefore warrants greater attention. More knowledge on pre-introduction selection also might provide novel targets for the management of invasive species, if pre-introduction filters can be adjusted to change the quality and/or quantity of individuals passing through such that invasion probability and/or impacts are reduced.

We tested whether pre-introduction selection occurs, when it acts, and on which phenotypic traits, by following the fate of a set of individuals previously characterized for various traits (that we a priori thought to be potentially important for invasion success and to be exposed to pre-introduction selection) across a number of potentially selective events: capture, initial acclimation, and captivity. Native individuals of P. melanocephalus and E. afer were caught by us using mist nets . We collected data for the phenotypic characterization of individuals (sex, age, morphometric and behavioral measures, and a feather to measure corticosterone levels). We also accompanied professional local bird trappers. They caught individuals using a traditional clap net lying on the ground baited with seeds and stuffed decoys to attract birds We characterized the phenotype of all these individuals in the same way as for the individuals caught by us and marked them with uniquely numbered plastic rings, after which they were handed back to the trappers. The first invasion filter of selective uptake was assessed by statistically comparing the traditionally caught birds with those caught by us using mist nets. We then monitored the survival of the marked individuals captured by local trappers and in their care, and tested statistically if survival was associated with any phenotypic characteristics.