Sexual ornaments and reproductive behaviors represent life history investments, whose expression trades off against other allocation options. Thus, levels of sexual ornamentation may not always positively correlate with metrics of condition, performance, and fitness. Further, diversity in allocation strategy may select for multifaceted sexual displays in which some ornaments signal allocation strategy, whereas others signal individual quality. In this dissertation, I use the yellow warbler (Setophaga petechia) as a model species to explore whether carotenoid- and phaeomelanin-based sexual pigmentation play distinct or complementary roles in signaling individual condition, oxidative stress, and condition-dependent parental capacity, versus reproductive allocation strategy. Past research has suggested that carotenoid-based pigmentation is highly condition-dependent and linked to antioxidant defenses, whereas melanin-based pigmentation signals differences in reproductive strategy. However, others have suggested that this dichotomous view of the two pigment types is overly simplistic.
I performed an intensive field study to achieve my research objectives. I quantified pigmentation using reflectance spectrometry and digital photography, used feather quality to indirectly quantify condition at molt, assayed oxidative stress across two populations, and video-recorded nests to measure reproductive behavior. Finally, I employed microsatellite-based paternity analysis to quantify reproductive success.
This research yielded the following important conclusions. First, the condition-dependence and overall expression of carotenoid pigmentation was age-dependent, whereas melanin pigmentation displayed age-independent expression and correlated with condition metrics across age classes. These results suggest that multifaceted sexual displays may maintain reliable signaling of individual quality across age classes, suggesting an underexplored age-dependent mechanism for maintenance of multifaceted displays. Second, contrary to predictions, carotenoid-based pigmentation was not more strongly associated with condition metrics than melanin-based pigmentation in yellow warblers. Rather, in males, melanin-based pigmentation was more consistently linked to condition at molt, good oxidative status, and paternal performance, although both types of pigmentation affected patterns of extrapair paternity. Thus, results are contrary to dichotomous signaling functions of carotenoid- versus melanin-based pigmentation. Finally, highly melanic male warblers adjusted patterns of mating and paternal effort across the nesting cycle, suggesting that behavioral flexibility may be an underappreciated mechanism by which highly ornamented males ameliorate reproductive tradeoffs between mating and paternal performance.
