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

Examinations of variation in plumage dichromatism in birds have focuse d on male plumage brightness and largely neglected variation in female plumage brightness. Nest predation previously was concluded to constr ain male brightness and thereby reduce dimorphism in ground-nesting bi rds based on an incorrect assumption that nest predation is greater fo r ground nests. Correlations of plumage brightness and dichromatism wi th nest predation have never been tested directly and we do so here wi th data for warblers (Parulinae) and finches (Cardurlinae). We show th at 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 ma les, and one in which variation in female plumage brightness was negat ively correlated with nest predation. These results suggest that nest predation may place greater constraints on female than male plumage br ightness, at least in taxa where only females incubate eggs and brood young. These results also show that female plumage patterns vary at le ast partly independently of male patterns and emphasize the need to in clude 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 relat ionship between plumage dimorphism and nest predation was positive rat her than negative as previously argued.