Do conditions on the Mexican moulting grounds influence carotenoid content and composition of Bullock's oriole (Icterus bullockii) feathers?

The Bullock’s oriole (Icterus bullockii) has an interesting moult migration strategy, in which it stops mid-way to the wintering grounds, in the Mexican monsoon region to replace feathers (moult) before continuing migration south to overwinter. Moult-migrants, such as Bullock’s orioles, undergo such a strategy because food resources are more abundant in the monsoon region during early fall than on the breeding grounds, allowing better conditions for moult. During this time, Bullock’s orioles produce bright orange/yellow carotenoid-based plumage. Carotenoid colouration is an honest signal of individual quality and is important in social interactions and female mate choice across taxa. Here, we asked whether diet and habitat quality during moult (inferred by stable isotopes) were associated with feather carotenoid content and composition. Liquid chromatography mass spectrometry (LC-MS) was used to determine carotenoid concentrations in the breast feathers of after second year males of a Bullock’s oriole population breeding in British Columbia. Feathers were also analyzed for carbon (δ13C) and nitrogen (δ15N) stable isotopes to infer habitat conditions during moult. We also used reflectance spectrometry to calculate feather hue, red chroma, and brightness. Perhaps surprisingly, I found no direct relationship between features of colouration (as measured by reflectance spectrometry) and carotenoid concentration or composition. There was, however, a positive relationship between δ15N and concentration of lutein, as well as a negative relationship between δ15N and proportion of canthaxanthin. These results suggest that feeding at a higher trophic level during moult –perhaps having a diet high in insects –allows for a greater amount of lutein to be deposited in feathers. Feeding more on insects may also indicate a lack of β-carotene or dietary canthaxanthin, or an inability to effectively metabolize β-carotene to canthaxanthin in that environment, possibly due to stress. This study highlights the importance of moulting in highquality habitats for species that exhibit carotenoid-based colouration. A moult migration strategy may have evolved to take advantage of superior moult conditions in the Mexican monsoon region, which can directly influence the ability to obtain bright colouration, critical for inter- and intra-sexual interactions.