Adaptive coloration in pied flycatchers (

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.
We review numerous studies on both proximate and ultimate factors behind extensive plumage color variation in male pied flycatchers and particularly zoom in on the most unknown questions and suggest future research avenues. There is strong evidence that interspecific interactions with collared flycatchers are of crucial importance for explaining the origin and maintenance of plumage color variation in male pied flycatchers, but the role of avoidance of hybridization in driving character displacement in plumage between the two flycatcher species may not be as important as initially thought. Our review suggests that plumage traits should not be expected to signal high quality as such but rather different male competitive strategies, and a main take‐home message is that selection patterns acting on signaling traits are more diverse and fluctuating than generally expected.