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

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.

A tail of plumage colouration: disentangling geographic, seasonal and dietary effects on plumage colour in a migratory songbird

“…In a study of Bullock's orioles, Sparrow et al (2017) found that feather δ 15 N values were associated with feather colouration and feather carotenoid content. The authors found that while δ 15 N values were positively associated with overall carotenoid concentration, birds with more orange-shifted hue had feathers with lower δ 15 N values.…”

“…A recent study on Bullock's orioles Icterus bullockii demonstrated that feather δ 15 N values were directly associated with feather carotenoid content and hue (Sparrow et al 2017). In the case of Bullock's orioles, birds with lower feather δ 15 N values expressed more orange‐shifted hue.…”

“…In the case of Bullock's orioles, birds with lower feather δ 15 N values expressed more orange‐shifted hue. Because Bullock's orioles rely on both insects and fruit during moult, the authors suggested that individuals with a relatively higher amount of fruit in their diet (or those moulting in habitats that resulted in lower δ 15 N values) may have been in better condition and better able to metabolically convert dietary carotenoids to red or orange keto‐carotenoids (McGraw 2006, Sparrow et al 2017, Weaver et al 2018).…”

A tail of plumage colouration: disentangling geographic, seasonal, and dietary effects on plumage colour in a migratory songbird

“…A recent study on Bullock’s orioles ( Icterus bullockii ) demonstrated that feather δ 15 N values were directly associated with feather carotenoid content and hue (Sparrow et al 2017). In the case of Bullock’s orioles, birds with lower feather δ 15 N values expressed more orange-shifted hue.…”

“…In the case of Bullock’s orioles, birds with lower feather δ 15 N values expressed more orange-shifted hue. Because Bullock’s orioles rely on both insects and fruit during moult, the authors suggested that individuals with a relatively higher amount of fruit in their diet (or those moulting in habitats that resulted in lower δ 15 N values) may have been in better condition and better able to metabolically convert dietary carotenoids to red or orange keto-carotenoids (McGraw 2006; Sparrow et al 2017; Weaver et al 2018).…”