Nestling colouration is adjusted to parent visual performance in altricial birds.

Hitherto, most of the investigation on the perceptual efficacy of begging signals has dwelled on how patterns of nestling colouration adjust to predominant nest luminosity. However, visual sensitivity of birds varies across species, which raises the question of whether colouration of traits involved in begging displays is adjusted to parent visual capacities. Here, by comparing nestling colouration and visual sensitivity across 22 altricial bird species, we provide a first test of this hypothesis. Firstly, we assessed differences in performance of typical UV-tuned and violet-tuned bird eyes when looking at the nestling traits under the light regimes prevailing at their nests. Secondly, while controlling for common ancestry in a comparative approach, we explored variation in colouration of nestlings in relation to parent visual system. The colour discrimination model indicated a general higher performance of the ultraviolet over the violet eye at detecting gape and body skin traits in either open- or hole-nest light conditions. Gape colouration was associated with parental visual system as the nestlings of UVS species displayed more yellow and less pure ultraviolet mouths than the nestlings of VS species. Thus, our results agree with an adaptive parent-offspring communication scenario where the nestlings' colours tuned the perception capacities of their parents.