Visual cues and parental favouritism in a nocturnal bird.

Visual signals are crucial for parent-offspring communication, although their functioning has been neglected for nocturnal birds. Here, we investigated parental preference for nestling coloration in nocturnal conditions--a question hitherto unexplored--in a nocturnal raptor, the scops owl (Otus scops). We assessed how parents allocated food during the night in relation to a manipulation of ultraviolet (UV) reflectance of the cere (skin above the beak) of their offspring. Reflectance of the cere shows a marked peak in the UV part of the spectrum, and location of the UV peak is related to nestling body mass (i.e. heavier nestlings have a UV peak at lower wavelengths). We found evidence of parental bias in favour of lighter offspring: UV-reduced nestlings gained more weight during the night than their control siblings. This study provides the first experimental evidence of the use of visual cues for parent-offspring communication in a nocturnal bird.