Parental favouritism strategies in the asynchronously hatching European Roller (<Emphasis Type="BoldItalic">Coracias garrulus</Emphasis>)

In altricial birds, resource allocation during early developmental stages is the result of an interaction between parental feeding decisions and scramble competition between nestmates. Hatching asynchrony in birds leads to a pronounced age hierarchy among their offspring. Therefore, whenever parents exert control over resource allocation parents feeding asynchronous broods should simultaneously assess individual offspring internal condition and age. In this study, we first studied whether the highly ultraviolet (UV) reflective body skin of nestlings in the asynchronous European Roller (Coracias garrulus; roller hereafter) relates to nestling quality. In a second stage, we experimentally studied parental biases in food allocation towards senior and junior sibling rollers in relation to a manipulation of UV reflectance of the skin of their offspring. Heavier roller nestlings had less brilliant and less UV saturated skins than weaker nestlings. In our experiment, we found that parents with large broods preferentially fed nestlings presenting skin coloration revealing small body size (i.e. control nestlings) over nestlings presenting skin coloration revealing large body size (i.e. UV-blocked nestlings). Within the brood, we found that parental food allocation strategy depended on nestling age: parents preferentially fed senior nestlings signalling small body size, but did not show preference between control and UV-blocked junior nestlings. These results emphasise that parent rollers use UV cues of offspring quality while balancing the age of their offspring to adjust their feeding strategies, and suggest that parents may adopt finely tuned strategies of control over resource allocation in asynchronous broods.