Seasonal changes in blue tit crown color: do they signal individual quality?

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Abstract
Plumage coloration is generally perceived as a static trait and therefore not a good indicator of current condition. However, changing of feather colors after molt does occur and may have important implications for signal function and sexual selection. We studied longitudinal changes in blue tit (Parus caeruleus) crown ultraviolet (UV)/blue color, a sexually selected trait, by repeatedly measuring the same individuals between early winter and late spring. Whereas crown UV reflectance (UV chroma and hue) decreased dramatically over time, brightness and saturation did not show consistent patterns of change. The magnitude of the decline in coloration exceeded sexual and age dichromatism in hue and UV chroma, respectively. Hence, seasonal color changes could have strong effects on blue tit sexual signaling. Between-individual variation in the decline in UV coloration was large and related to attributes of male, but not female, quality, such as size and condition. Thus, conspecifics could potentially gain information about male phenotypic quality by assessing color change over the year. However, the degree of decline in male UV color did not affect breeding success because neither the number of within-pair nor the number of extrapair offspring produced correlated with changes in crown color. Seasonal changes in the expression of plumage coloration are probably widespread, and maintaining plumage coloration could thus constitute an additional honesty-enforcing mechanism after molt is completed.