Ultraviolet reflectance of plumage for parent--offspring communication in the great tit (Parus major)

Ultraviolet (UV) reflectance has been implicated in mate selection. Yet, in some bird species the plumage of young varies in UV reflectance already in the nest and long before mate choice and sexual selection come into play. Most birds molt the juvenile body plumage before reaching sexual maturity, and thus, some conspicuous traits of the juvenile body plumage may rather have evolved by natural selection, possibly via predation or parental preference. This second hypothesis is largely untested and predicts a differential allocation of food between fledging and total independence, which is a time period of 2--3 weeks where offspring mortality is also highest. Here, we test the prediction that parents use the individual variation in UV reflectance among fledglings for differential food allocation. We manipulated UV reflectance of the plumage of fledgling great tits Parus major by treating chest and cheek feathers with a lotion that either did or did not contain UV blockers and then recorded food allocation by parents in an outdoor design simulating postfledging conditions. The visible spectrum was minimally affected by this treatment. Females were found to feed UV-reflecting offspring preferentially, whereas males had no preference. It is the first evidence showing that the UV reflectance of the feathers of young birds has a signaling function in parent--offspring communication and suggests that the UV traits evolved via parental preference. Copyright 2008, Oxford University Press.