Abstract
Dietary access to carotenoids is expected to determine the strength of carotenoid-based signal expression and potentially to maintain signal honesty. Species that display carotenoid-based yellow, orange, or red plumage are therefore expected to forage selectively for carotenoid-rich foods when they are depositing these pigments during molt, but whether they actually do so is unknown. We set out to address this in the hihi (Notiomystis cincta), a New Zealand passerine where males, but not females, display yellow carotenoid-based plumage. We measured circulating carotenoid concentrations in male and female hihi during breeding and molt, determined the nutritional content of common foods in the hihi diet, and conducted feeding observations of male and female hihi during molt. We found that although male and female hihi do not differ significantly in plasma carotenoid concentration, male hihi have a greater proportion of carotenoid-rich foods in their diet than do females. This is a consequence of a greater fruit and lower invertebrate intake than females and an avoidance of low-carotenoid content fruit. By combining behavioral observations with quantification of circulating carotenoids, we present evidence that colorful birds forage to maximize carotenoid intake, a conclusion we would not have drawn had we examined plasma carotenoids alone.
Highlights
The brightly colorful, showy plumage displayed by males of many bird species has always fascinated biologists, forming the cornerstone of research on sexual selection and contributing to the development of theory on honest signals (Andersson 1994; Hill and McGraw 2004)
Plasma carotenoid concentration tended to be greater for males than females during the breeding season, and greater for females than males during molt (15.69 ± 1.47 for females vs. 14.38 ± 0.85 for males; Figure 1), the interaction between sex and season was not significant (t = −1.24, P = 0.22)
Plasma carotenoid concentration was significantly greater during molt than during the breeding season
Summary
The brightly colorful, showy plumage displayed by males of many bird species has always fascinated biologists, forming the cornerstone of research on sexual selection and contributing to the development of theory on honest signals (Andersson 1994; Hill and McGraw 2004). Carotenoids have antioxidant and immunostimulant functions, suggesting an allocation trade-off where only the healthiest individuals can afford simultaneously to invest carotenoids in both pigmentation and selfmaintenance (Lozano 1994; von Schantz et al 1999). These are not mutually exclusive hypotheses, because healthy birds might be the more capable individuals at accessing carotenoids when they are limited in the environment. For birds that deposit dietary carotenoids unmodified into their integuments (e.g., lutein, zeaxanthin; the most common carotenoids used in pigmentation of yellow colors; McGraw 2006), the link between environmental carotenoid access and diet is predicted to be strong
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