Egg colour influences egg removal by common cuckoos (Cuculus canorus)

Brood parasitic cuckoos usually remove one host egg before laying their own. The purpose of this behaviour is still not fully understood. Recent studies that investigated cognitive mechanisms underlying egg removal found that cuckoos preferentially remove brighter eggs because darker eggs may be more cryptic. However, the role of chromatic differences (e.g., in hue) has been overlooked, despite being key cues in avian egg recognition. Using red and green model eggs and video recordings of experimental nests, we found that common cuckoos (Cuculus canorus), preferred removing red model eggs in 92% of cases. Such a strong preference for eggs of a certain hue indicates that cuckoos use chromatic cues to selectively remove eggs from host nests. Avian visual modelling revealed that it is unlikely that the preference for red eggs was elicited by higher conspicuousness of red eggs in nests. We speculate that cuckoos might have been attracted to red-coloured eggs similarly to how other bird species are attracted to red-coloured food. These findings contribute to better understand the purpose of egg removal in brood parasites.