Ultraviolet coloration of avian parasitic egg does not cue egg rejection in the common redstart host

Avian brood parasite hosts can use distinct egg traits to recognize a parasitic egg. Previous studies suggested an important role of eggshell ultraviolet (UV) reflectance in rejection. According to the egg detectability hypothesis, natural selection might have selected for a conspicuous egg UV signal, especially in dark nest environments. We tested this hypothesis in a cavity nesting host by experimentally parasitizing a population of common redstarts breeding in Central Europe. We manipulated UV reflectance using egg models with enhanced UV and used models with UV characteristics similar to the host as a control treatment. We additionally manipulated nest light conditions by adjusting the entrance size of the nest-boxes, because dark environments have been suggested to affect detectability of an egg, particularly in UV light. Hosts rejected experimental eggs with similar probabilities in both UV treatments and also under different light conditions. Results of this study did not support the egg detectability hypothesis. Our conclusions on the role of egg UV for egg recognition and rejection emphasize how little we understand its function in cavity nesting hosts.