Sexual dichromatism, size dimorphism and microscale anatomy of white wing stripe in blue tit (Cyanistes caeruleus)

Achromatic patches are a common element of plumage patterns in many bird species and there is growing body of evidence that in many avian taxa they can play a signalling role in mate choice. Although the blue tit is a well-established model species in the studies on colouration, its white wing patch has never been examined in the context of sex-specific trait expression. In this exploratory study, we examined sexual size dimorphism and dichromatism of greater covert’s dots creating white wing patch and analysed its correlations with current body condition and crown colouration - a trait with established role in sexual selection. Further, we qualitatively analysed microstructural barb morphology underlying covert’s colouration. We found significant sexual dimorphism in the dot size independent of covert size, and sexual dichromatism in both white dot and blue outer covert’s vane spectral characteristics. Importantly, UV chroma of covert’s vane was positively correlated with crown UV chroma, which suggests that coverts colouration might be also an ornament assessed by females during courtship display. Internal structure of covert barbs within the white dot was similar to the one found in barbs from the blue part, i.e. with a medullary area consisting of dead keratinocytes containing channel-type ß-keratin spongy nanostructure and centrally located air cavities. However, it lacked melanosomes which was the main observed difference. Together with marked sexual dimorphism, it suggests that white dots may have emerged under sexual selection as an apomorphic trait on previously uniformly coloured feathers.