Offspring plumage coloration as a condition-dependent signal in the blue tit

Abstract

In many species, offspring display conspicuous coloration already early in life, even though they might be very vulnerable to predation at this stage. However, most attention has been drawn to the conspicuous plumage displayed by adult individuals in a sexual context, while other signaling functions have been explored much less. Here, we investigated whether the yellow breast plumage of blue tit (Cyanistes caeruleus) nestlings shows patterns of condition dependence and hence signals individual quality, as has been described for adult birds. During three consecutive breeding seasons, we, therefore, explored the association between nestling body mass and three color components of the yellow breast plumage (i.e., UV chroma, carotenoid chroma, and total brightness), considering both within and among nest effects. Variation in carotenoid chroma was not related to body mass. However, UV chroma and total brightness varied with body mass on an among-nest level, suggesting that they might signal aspects of genetic quality or parental rearing capacity. Interestingly, we also found a within-nest effect of body mass on total brightness, suggesting that this is a good candidate for a condition-dependent signal within the family. Thus, other family members could rely on brightness to adjust their behavioral strategies, such as feeding behavior in parents. Our study thus reveals that certain color components of the yellow breast plumage might signal different aspects of offspring quality, and they might have a correlated signaling value across life-history stages.

Keywords: UV coloration; carotenoids; condition dependence; honest signaling; nestling coloration; quality; sex differences.

FIGURE 1

Correlations between yellow breast feather color parameters (mean nest values) measured in blue tit nestlings: (a) UV chroma versus total brightness, (b) carotenoid chroma versus total brightness, and (c) UV chroma versus carotenoid chroma.

FIGURE 2

Among‐nest effect of body mass (i.e., average body mass of the brood) on (a) UV chroma, (b) carotenoid chroma, and (c) total brightness of yellow breast feathers measured in blue tit nestlings. Regression lines and ±95% confidence intervals (blue shaded area) are shown.

FIGURE 3

Within‐nest effect of body mass (i.e., individual deviation from the brood average) on (a) UV chroma, (b) carotenoid chroma, (c) total brightness of yellow breast feathers measured in blue tit nestlings. Regression lines and ±95% confidence intervals (blue shaded area) are shown.