Differential wear of feathers in the polymorphic New Zealand Fantail (Rhipidura fuliginosa)—a selective advantage of melanism?

Abstract

The New Zealand Fantail (Rhipidura fuliginosa) has two discrete colour morphs. The plumage of the black morph is almost entirely melanised whereas the pied morph is only partially melanised. This difference is most striking in the tail, with the rectrices of the black morph being black whereas all but the central pair of rectrices in the pied morph are white. To understand the selective advantages of differences in melanisation between the two morphs, we compared rates of missing or broken rectrices, and the degree of wear along the edges of the vanes. Using both museum skins and live-caught birds, we found no difference in the rate of missing feathers between morphs but rates of breakage and wear of feathers were significantly higher among pied-morph Fantails. The difference was most prominent late in the breeding season when activity levels are expected to be highest. By comparing homologous feathers differing only in level of melanisation, our results support the hypothesis that feathers containing melanin exhibit increased resistance to breakage and wear. Although this suggests that black-morph Fantails obtain an advantage in reduced wear of feathers, the benefit that white feathers provide to pied-morph Fantails to counter increased costs of feather wear is not clear.