Effect of UV-reflectance of chick flanges on begging behaviour and body condition in Barn Swallow (Hirundo rustica, Aves: Hirundinidae) chicks

In diverse animal taxa, offspring solicit parental care using visual and acoustic ‘begging’ displays that are hypothesized to signal either their level of need or their overall condition to the attending parents. Parents are expected to respond by favouring the neediest or the best offspring in the brood respectively, depending on whether they adopt brood-survival or brood-reduction strategy. In this study of barn swallows (Hirundo rustica), we investigated the role of the UV component of flange colouration, which is involved in nestling solicitation displays, in parent-offspring interactions. This trait was repeatedly manipulated over the rearing period. Chicks that were light at the beginning of the experiment begged less and gained more mass when subjected to UV-reduction compared to control treatment, while the opposite was found in large chicks. However, UV reduction slightly impaired skeletal growth of the chicks irrespective of initial size, and reduced their post-treatment competitive ability. Overall, we suggest that UV colouration might be used as an indicator of chick need, enabling the parents to acquire useful information for adjusting their allocation strategies over the rearing period.