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Understanding the origin and persistence of phenotypic variation within and among populations is a major goal in evolutionary biology. However, the eagerness to find unadulterated explanatory models in combination with difficulties in publishing replicated studies may lead to severe underestimations of the complexity of selection patterns acting in nature. One striking example is variation in plumage coloration in birds, where the default adaptive explanation often is that brightly colored individuals signal superior quality across environmental conditions and therefore always should be favored by directional mate choice. Here, we review studies on the proximate determination and adaptive function of coloration traits in male pied flycatchers (Ficedula hypoleuca). From numerous studies, we can conclude that the dark male color phenotype is adapted to a typical northern climate and functions as a dominance signal in male-male competition over nesting sites, and that the browner phenotypes are favored by relaxed intraspecific competition with more dominant male collared flycatchers (Ficedula albicollis) in areas where the two species co-occur. However, the role of avoidance of hybridization in driving character displacement in plumage between these two species may not be as important as initially thought. The direction of female choice on male coloration in pied flycatchers is not simply as opposite in direction in sympatry and allopatry as traditionally expected, but varies also in relation to additional contexts such as climate variation. While some of the heterogeneity in the observed relationships between coloration and fitness probably indicate type 1 errors, we strongly argue that environmental heterogeneity and context-dependent selection play important roles in explaining plumage color variation in this species, which probably also is the case in many other species studied in less detail.

Carotenoid-based coloration occurs predominantly in adult birds, yet in some species from the family Paridae, this trait is also present at the nestling stage. One of the factors proposed to affect the expression of this trait in immature birds is hatching date. Here, using the avian tetrahedral colour space model, we examined the influence of hatching date on the breast carotenoid-based plumage coloration of the Blue TitCyanistes caeruleusnestlings. Because Blue Tits are sexually dichromatic, we also investigated the potential interaction between hatching date and sex that could arise from differences in condition dependence of this trait between males and females. We found a positive relationship betweenUVchroma of breast feathers and hatching date. The amount ofUVreflectance is thought to be negatively related to carotenoid content in feathers. The observed increase ofUVchroma through the breeding season might therefore be caused by a seasonal decline in the availability and quality of Lepidoptera larvae - the main source of carotenoids in food of the Tits. We also observed a sex difference in the relationship between brightness of breast feathers (achromatic, structural component) and hatching date, which in males was negative and in females not significant. Our study provides further evidence that the timing of breeding is related to the expression of nestling carotenoid-based coloration, a potentially meaningful element of offspring-parent communication, and suggests a sex-specific effect of hatching date on its structural component.

A standard approach to model how selection shapes phenotypic traits is the analysis of capture-recapture data relating trait variation to survival. Divergent selection, however, has never been analyzed by the capture-recapture approach. Most reported examples of differences between urban and nonurban animals reflect behavioral plasticity rather than divergent selection. The aim of this paper was to use a capture-recapture approach to test the hypothesis that divergent selection can also drive local adaptation in urban habitats. We focused on the size of the black breast stripe (i.e., tie width) of the great tit (Parus major), a sexual ornament used in mate choice. Urban great tits display smaller tie sizes than forest birds. Because tie size is mostly genetically determined, it could potentially respond to selection. We analyzed capture/recapture data of male great tits in Barcelona city (N = 171) and in a nearby (7 km) forest (N = 324) from 1992 to 2008 using MARK. When modelling recapture rate, we found it to be strongly influenced by tie width, so that both for urban and forest habitats, birds with smaller ties were more trap-shy and more cautious than their larger tied counterparts. When modelling survival, we found that survival prospects in forest great tits increased the larger their tie width (i.e., directional positive selection), but the reverse was found for urban birds, with individuals displaying smaller ties showing higher survival (i.e., directional negative selection). As melanin-based tie size seems to be related to personality, and both are heritable, results may be explained by cautious personalities being favored in urban environments. More importantly, our results show that divergent selection can be an important mechanism in local adaptation to urban habitats and that capture-recapture is a powerful tool to test it.

Only during the past decade have vision-system-neutral methods become common practice in studies of animal color signals. Consequently, much of the current knowledge on sexual selection is based directly or indirectly on human vision, which may or may not emphasize spectral information in a signal differently from the intended receiver. In an attempt to quantify this discrepancy, we used retinal models to test whether human and bird vision rank plumage colors similarly. Of 67 species, human and bird models disagreed in 26 as to which pair of patches in the plumage provides the strongest color contrast or which male in a random pair is the more colorful. These results were only partly attributable to human UV blindness. Despite confirming a strong correlation between avian and human color discrimination, we conclude that a significant proportion of the information in avian visual signals may be lost in translation.