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Conditions on the Mexican moulting grounds influence feather colour and carotenoids in Bullock's orioles (Icterus bullockii)

20 Mar 2017


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.

Plumage colouration in birds plays critical roles in both intra- and intersexual communication and is often heralded as a classic example of an honest signal. Here, we used a combination of stable isotope analysis, reflectance spectrometry, and carotenoid analysis to ask whether plumage colouration and feather carotenoid content in Bullock's orioles, a western North American molt-migrant that interrupts fall migration to molt in the Mexican monsoon region, are influenced by conditions experienced during stopover moult. Our results reveal that birds with higher feather δ15N values acquired more colorful plumage but had feathers with lower carotenoid concentrations. Because moult diet/habitat during the Mexican monsoon region can influence feather colouration, this may reveal an important carryover effect, influencing inter- and intrasexual signaling in subsequent seasons. Photo credit: Rick Derevan.

Click here to view the full article which appeared in Ecology and Evolution