Plumage dichromatism of birds predicts introduction success in New Zealand

1. Reintroduction of plants and animals has become a common practice in the attempt to establish, re-establish or augment endangered populations. The fate of these translocation programmes is likely to depend, on the one hand, on several ecological attributes of the introduced species, on the other hand on introduction effort. 2. In a recent paper, Veltman, Nee & Crawley (1996) reported the correlates of introduction success of exotic birds released by humans in New Zealand before 1907: introduction effort (minimum number of release events and minimum number of released propagules) accounted for most of the variance in introduction success. 3. One of the factors potentially affecting the establishment of new populations concerns the intensity of sexual selection. Sexually selected species may be more vulnerable to extinction risks for several reasons (they may be more sensitive to both environmental and demographic stochasticity) and therefore have lower introduction success when compared to nonsexually selected species. 4. In the present study the data set reported by Veltman et al. (1996) was used to address the particular issue concerning sexual selection and introduction success. Each bird species was scored as monochromatic or dichromatic; plumage dichromatism is supposed to have evolved under sexual selection pressures. These scores were then correlated to the fate of introduction. 5. Plumage dichromatism was found to be, as expected, a significant predictor of introduction success, after removing the potential confounding effect of introduction effort. Dichromatic species had lower introduction success than monochromatic species. This pattern holds true for nonpasserines and passerines, although the correlation between plumage dichromatism and introduction success was not significant for the latter. 6. These results indicate that dichromatic species may experience reduced chances of establishing new populations when compared to monochromatic species. The presumed mechanisms involved in this phenomenon range from a reduced ability to adapt to a novel environment, to an increase in the risks of extinction through environmental and demographic stochasticity.