Energetic constraints on expression of carotenoid‐based plumage coloration

Carotenoid pigments are used by many bird species as feather colorants, creating brilliant yellow, orange, and red plumage displays. Such carotenoid‐based plumage coloration has been shown to function as an honest signal that is used in female mate choice. Despite recent interest in carotenoid‐based ornamental traits, the basis for individual variation in expression of carotenoid‐based plumage coloration remains incompletely understood. I tested the hypothesis that, independent of carotenoid access, food stress during molt would cause reduced expression of carotenoid pigmentation. I fed molting male House Finches Carpodacus mexicanus seed diets supplemented with either the red carotenoid pigment canthaxanthin or the yellow/orange carotenoid pigment β‐cryptoxanthin (in the form of tangerine juice). Within each diet treatment, one group of males was given restricted food access and the other group was given unrestricted food access. Carotenoid supplements were placed in water so carotenoid access was controlled independent of food access. The results indicated a strong effect of both carotenoid access and food access on color display. Some males in the β‐cryptoxanthin‐supplemented group grew red plumage, suggesting that they can metabolically modify yellow pigments into red pigments, but no bird supplemented with β‐cryptoxanthin grew plumage as red as birds supplemented with canthaxanthin. Males in the unrestricted food groups grew redder and more intensely pigmented plumage than males in the restricted food groups. These observations provide the best evidence to date of an energetic cost of carotenoid utilization in the generation of colorful plumage.