Increased plumage darkness of Shiny Cowbird females in the subtropics: an adaptation to bacterial degradation?

MAHLER, B.; LOPEZ, N. I.; DI GIACOMO, A. G.; REBOREDA, J. C.

Campos do Jordao, Brasil

Congreso; 25th International Ornithological Congress; 2010

International Ornithological Society

The shiny cowbird (Molothrus bonariensis) is a sexually dichromatic species, in which males are black blue iridescent and females are dull brown. However, in some subtropical areas of its distribution females show a plumage polymorphism that ranges from dull brown to dark brown and black. Plumage melanization has been shown to protect feathers from bacterial degradation, decreasing the effects of harmful bacterial activity and thus plumage damage. In this study, we tested whether bacterial feather-degrading activity is acting as the selective force to increase darkness in female shiny cowbird’s plumage. First, we compared differences in feather-degrading activity among bacterial strains isolated from subtropical (more humid) and temperate (less humid) zones where dark females are present and absent, respectively. Second, we studied the degradation of female feathers belonging to different color morphs when exposed to Bacillus licheniformis and to bacterial strains isolated from subtropical and temperate zones. We did not find differences in bacterial activity between locations or differences in susceptibility to bacterial degradation between female’s brown feathers and female’s darker feathers. These results suggest that female plumage polymorphism in shiny cowbirds did not arise as a defense to bacterial feather-degrading activity but has probably been driven by a different selective force.