Egg rejection from dark cavities: compensation for chromatic changes in nest illumination by a cuckoo host

Rejection of foreign eggs is an effective defence against brood parasitism in birds. Colourful egg stimuli are among the most important cues for recognition, but varying ambient light conditions can potentially affect the decision-making process. Birds may compensate for varied illumination through colour constancy, but this remains untested in brood parasite hosts. This ability may aid in recognising foreign eggs in the nest by making the decision process more robust and reliable. We examined if ambient light colour impacts the rate at which the common redstart (Phoenicurus phoenicurus) rejects white egg models while keeping foreign-host egg contrast consistent. The cavity-nesting redstart is host to the common cuckoo (Cuculus canorus) and exhibits colour-biased egg rejection behaviour under natural light, where they preferentially reject eggs that they perceive as browner than their own eggs but, in contrast, accept eggs that they perceive as bluer than their own. Under the colour constancy hypothesis, we predicted that their responses towards the white egg would be similar between different light conditions. On the other hand, if redstarts lack colour constancy, their responses will differ between light conditions. No difference was found among rejection rates, suggesting that redstarts most likely base decisions on perceived differences between foreign and their own eggs while compensating for changes in illumination. These results imply that perceptual mechanisms like chromatic adaptation do not drive redstart colour-biased rejections. Further study on colour constancy in open and closed-nesting species will clarify illumination effects on parasite-host coevolution.