Carotenoid-based plumage coloration and aggression during molt in male house finches

Birds often use colorful traits to mediate resource competitions, and typically individuals with bright or large patches of color are more competitive than or dominant to those with drab or small color patches. Male House Finches (Carpodacus mexicanus) are an exception, however, as drab males tend to be more aggressive than bright males during the breeding season and in winter. One hypothesis for this 'negatively correlated handicap' is that drab male house finches, being comparatively sexually unattractive and in poor health and condition, have more to gain by elevating aggression and increasing access to food and pursuit of (especially extra-pair) mates. It would seem important then to test this hypothesis during the period of molt, when birds are actively acquiring the foods and carotenoid pigments that make them colorful and a time in which there is a clear link between plumage color and nutrition/health in this species. We conducted two captive dominance experiments with male House Finches from the southwestern United States (their native range) to examine the relationship between carotenoid-based plumage coloration and aggression during molt. In the first experiment, where birds exhibited their natural, currently growing plumage color, we found that drab males were significantly more aggressive than bright males. However, when plumage colors were manipulated with art markers in a second experiment, color display was no longer significantly predictive of agonistic outcomes. These results suggest that: (1) drab male dominance is not an artifact in eastern US populations and instead is a conserved property of native and non-native House Finches, (2) like in our previous studies in winter, plumage color is correlated with dominance but does not serve as a visual signal of social status in this species, and (3) drab males should have a competitive advantage over access to important carotenoid-rich foods during molt, but apparently do not become as colorful because they do not adequately locate carotenoid-rich foods or do not use them efficiently for energetic or health reasons, which are perhaps exacerbated by elevated testosterone levels.