Getting rid of the cuckoo Cuculus canorus egg: why do hosts delay rejection?

Egg discrimination is well documented in many hosts of avian brood parasites, but the proximate mechanisms of egg recognition and rejection decisions are poorly understood. Relevant in this respect is the observation that rejectors of parasite eggs often delay their response. This delay has implications for understanding mechanisms important for egg recognition and is the main focus of the present study. We investigated experimentally the relative effects of egg mimicry and eggshell strength of common cuckoo Cuculus canorus eggs on the delay in rejection in marsh warblers Acrocephalus palustris. In addition, by video recording host responses, we elucidate the proximate mechanisms behind the delayed rejections. Host nests were experimentally parasitized with 3 types of real eggs differing in mimicry and/or eggshell strength. Both egg mimicry and eggshell strength significantly affected the time to rejection, but the effect of mimicry was dominant. The delayed rejection of mimetic eggs was explained by the existence of latency to the release of rejection behavior because of recognition problems. Second, when rejection response towards mimetic eggs was initiated, it was less intense compared with hosts experiencing nonmimetic eggs. Our results are consistent with the hypothesis that host motivation when confronted with mimetic eggs needs to increase above a certain threshold before rejection behavior is released, which likely minimizes the risk of recognition errors. An additional component of the delay in rejection as shown by hosts facing nonmimetic eggs was the seemingly inefficient host rejection behavior, probably reflecting lack of previous experience.