In the coevolutionary “arms race” between Common Cuckoos (Cuculus canorus) and their hosts, several adaptations and counter-adaptations have evolved. Here, we investigated natural parasitism and host sensitivity to egg rejection in Marsh Warblers (Acrocephalus palustris) in Bulgaria. The level of Common Cuckoo parasitism was high (28%), and average mimicry of Common Cuckoo eggs was good. Experimental parasitism with four egg-types that showed various degrees of mimicry of the host eggs revealed a generally high rejection rate of foreign eggs (37.5–100%). In addition, naturally laid Common Cuckoo eggs were rejected at a moderate rate (50%). The Marsh Warbler's ability to reject foreign eggs was strongly dependent on the degree of mimicry of the parasite egg but apparently not on differences in size between host and foreign eggs. Furthermore, intraclutch variation in host egg appearance was not related to the probability of egg rejection. The Marsh Warbler's highly developed egg-recognition ability and the good mimicry of Common Cuckoo eggs suggests that this poorly known host-parasite arms race has reached an advanced stage. The present study provides new insight into variables that are important for egg rejection in a heavily parasitized host population.

Rechazo de Huevos en Nidos de Acrocephalus palustris Fuertemente Parasitados por Cuculus canorus