Effects of nestling condition on UV plumage traits in blue tits: an experimental approach

Intraspecific sexual and social communications are among the most important factors shaping costly color traits in birds. Condition capture models assume that only animals in superior condition can develop and maintain a colorful plumage. Although there is good evidence that carotenoid-based components of plumage colors show condition dependence, the situation is more controversial with the underlying UV-reflecting structural component. We conducted a brood size manipulation in blue tits (Parus caeruleus) to investigate condition-dependent effects on plumage coloration in male and female offspring. Carotenoid chroma and UV reflectance of the yellow breast plumage showed condition-dependent expression in male and female fledglings. However, only males that were raised in reduced broods had higher UV reflectance in the UV/blue tail feathers, whereas female tail coloration did not differ between treatments. Our data suggest that there is a sex-specific effect on the blue but not the yellow plumage and that this is related to differences in the signaling function of both plumage traits. Although sexual selection may already act on male nestlings to develop colorful tail feathers for the next breeding season, the UV/yellow breast feathers are molted during the postjuvenile molt, and their signaling value is likely to be important for both sexes during the extended postfledgling phase.