Abstract
Carotenoid‐based plumage coloration plays a critical role for both inter‐ and intrasexual communication. Habitat and diet during molt can have important consequences for the development of the ornamental signals used in these contexts. When molt occurs away from the breeding grounds (e.g., pre‐alternate molt on the wintering grounds, or stopover molt), discerning the influence of habitat and diet can be particularly important, as these effects may result in important carryover effects that influence territory acquisition or mate choice in subsequent seasons. Several species of songbirds in western North America, including the Bullock's oriole (Icterus bullockii), migrate from the breeding grounds to undergo a complete prebasic (post‐breeding) molt at a stopover site in the region affected by the Mexican monsoon climate pattern. This strategy appears to have evolved several times independently in response to the harsh, food‐limited late‐summer conditions in the arid West, which contrast strongly with the high productivity driven by heavy rains that is characteristic of the Mexican monsoon region. Within this region, individuals may be able to optimize plumage coloration by molting in favourable areas characterized by high resource abundance. We used stable isotope analysis (δ13C, δ15N) to ask whether the diet and molt habitat/location of Bullock's orioles influenced their expression of carotenoid‐based plumage coloration as well as plumage carotenoid content and composition. Bullock's orioles with lower feather δ15N values acquired more colorful plumage (orange‐shifted hue) but had feathers with lower total carotenoid concentration, lower zeaxanthin concentration, and marginally lower canthaxanthin and lutein concentration. Examining factors occurring throughout the annual cycle are critical for understanding evolutionary and ecological processes. Here, we demonstrate that conditions experienced during a stopover molt, occurring hundreds to thousands of kilometers from the breeding grounds, influence the production of ornamental plumage coloration, which may carryover to influence inter‐ and intrasexual signaling in subsequent seasons.
Highlights
Sexual selection is a strong evolutionary force that has been instrumental in shaping the elaboration of ornamental traits and driving genetic divergence and speciation across a range of taxa
We asked whether carotenoid concentration or the proportion of canthaxanthin, lutein, and zeaxanthin in the feathers was predicted by feather isotope values. δ13C was not associated with any measure of carotenoid concentration or composition; δ15N was significantly associated with total carotenoid composition and zeaxanthin
This study is the first to demonstrate that conditions experienced during molt stopover, occurring hundreds to thousands of kilometers from the breeding grounds, correlate with the acquisition of an important ornamental signal in a songbird
Summary
Sexual selection is a strong evolutionary force that has been instrumental in shaping the elaboration of ornamental traits and driving genetic divergence and speciation across a range of taxa. In American redstarts (Setophaga ruticilla), males from more northern latitudes express higher red chroma values, which likely reflects higher feather carotenoid content (Norris, Marra, Kyser, Ratcliffe, & Montgomerie, 2007); as for house finches, this geographic variation in plumage coloration was attributed to regional variation in diet and carotenoid availability These studies demonstrate that molt location and habitat can play an important role in the acquisition of plumage coloration. The authors showed that there was extensive variation in feather hydrogen isotopes, indicating that molt might be occurring across a relatively large geographic area or range of habitats within the Mexican monsoon region This finding is consistent with low reported molt location fidelity in Bullock’s orioles; Pyle et al (2009) suggest that molt-migrants, including Bullock’s orioles, track resources during the molt period, which implies that individuals may be able to optimize plumage coloration by selecting high-q uality molt locations (i.e., areas where food, and perhaps even carotenoid-rich food, is abundant). The relationship between feather carotenoids and δ15N could be either positive or negative, depending on the relative richness of dietary carotenoids and the specific biome in which the birds molt
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