The spatial habitat structure of host populations explains the pattern of rejection behavior in hosts and parasitic adaptations in cuckoos

In this article we present tentative support for predictions derived from a spatial habitat structure hypothesis arguing that common cuckoos Cuculus canorus, the most common obligate brood parasite in Europe, only breed in areas where they have access to vantage points in trees. Thus, species in which some populations breed near trees while other populations breed farther from trees have a different cuckoo-host population dynamic than species in which all populations always breed in the vicinity of trees. Parasitism rate, mimicry of brood parasite eggs with those of the hosts, and rejection behavior of hosts varies with the host breeding habitat. Cuckoos are best adapted to exploit species in which some populations breed near trees while other populations breed in open areas because such hosts are not always accessible to cuckoos, and thus gene flow among unparasitized and parasitized populations delays the evolution of host adaptations. Adaptive behavior in cuckoos as well as in their hosts can be predicted from the spatial habitat structure hypothesis.