Ultraviolet‐blue reflectance of some nestling plumage patches mediates parental favouritism in great tits Parus major

The colouration exhibited by nestling birds is generally cryptic, but in some species it has a conspicuous appearance. The adaptive function of these nestling displays is poorly known, especially in altricial species. We performed an experimental study in which the ultraviolet‐blue reflectance of the carotenoid‐based plumage of great tit Parus major nestlings was reduced in order to find possible evidence of parental favouritism. Tarsus length increment in a three‐day period was significantly lower in manipulated nestlings than in their control siblings. As tarsus length may be a good predictor of survival, the parents may increase their fitness return by investing more in those nestlings reflecting more in the UV‐blue spectral region. Interestingly, this effect was only found in a yellow patch located at the nape of nestlings that has been previously overlooked, as all the studies on plumage colouration in this species have focused on the breast plumage. Some quality indicators are proposed for the colour of nestling plumage, though the possibility exists that selection may operate through aspects of signal efficacy instead of aspects of signal content.