Are introduced and re-introduced species comparable? A case using birds

Knowledge of the causes of success or failure in introductions may provide information to help improve the chances that re-introductions succeed. However, the utility of this information will be influenced by the comparability of sets of introduced and re-introduced species. Here, we assess the extent of this similarity using recent historical data on introduced and re-introduced birds. Introductions are less often threatened than expected and have larger ranges and earlier ages at maturity than re-introductions. Introductions also generally have 'weedy' life-history characteristics relative to family-typical values, whereas those of re-introductions more closely conform to these typical values. However, introductions and re-introductions do not differ in migratory behaviour, mating system or dichromatism, nor do they show statistically consistent differences in body mass, incubation time or annual fecundity. The taxonomic composition of the lists of introductions and re-introductions is similar, with more species than expected on both lists. Overall, these results suggest that the characteristics of introductions and re-introductions are sufficiently similar that conclusions from studies of the former may reasonably be used to inform studies of the latter.