SEXUAL DICHROMATISM IN BIRDS: IMPORTANCE OF NEST PREDATION AND NEST LOCATION FOR FEMALES VERSUS MALES.

Examinations of variation in plumage dichromatism in birds have focused on male plumage brightness and largely neglected variation in female plumage brightness. Nest predation previously was concluded to constrain male brightness and thereby reduce dimorphism in ground-nesting birds based on an incorrect assumption that nest predation is greater for ground nests. Correlations of plumage brightness and dichromatism with nest predation have never been tested directly and we do so here with data for warblers (Parulinae) and finches (Carduelinae). We show that male plumage brightness varies among nest heights, but in a pattern that is not correlated with nest predation. Female plumage brightness also varies among nest heights, but in a pattern that differs from males, and one in which variation in female plumage brightness was negatively correlated with nest predation. These results suggest that nest predation may place greater constraints on female than male plumage brightness, at least in taxa where only females incubate eggs and brood young. These results also show that female plumage patterns vary at least partly independently of male patterns and emphasize the need to include consideration of both female and male plumage variation in tests of plumage dimorphism. Plumage dimorphism differs between ground and off-ground nesters as previously described and, if anything, the relationship between plumage dimorphism and nest predation was positive rather than negative as previously argued.