Testosterone levels in relation to size and UV reflectance of achromatic plumage traits of female pied flycatchers

In a substantial number of species, females show some development of secondary sexual characters. These traits can function as signals of individual phenotypic or genetic qualities and status to conspecifics. Individuals may benefit potentially from expressing signals or badges of status if they are reliable and honest signals of individual quality. In many species, badge sizes have been shown to correlate with dominance rank, which may be mediated by testosterone (T) levels. Here, we explored geographic variation in the size and properties of the white wing patch of female pied flycatchers Ficedula hypoleuca and its relation to circulating T levels in three populations (two southern populations in central Spain and a northern population in Finland). Furthermore, we aimed at detecting if the size of the white wing patch and its ultraviolet (UV) reflectance indicate individual quality. We found that females in Spain had larger, brighter and more UV reflecting wing patches than those in Finland. Females with higher UV reflectance and larger primary white patches bred earlier. Younger females and females with larger primary white wing patches showed higher T levels. In contrast, higher values of UV reflectance in feathers from these patches were associated with low T levels. Despite genetic differentiation and differences in trait expression between populations, female pied flycatchers from different populations may converge and use the size of white wing patches to signal their T levels and thereby their social dominance.