The exploitation of a new host by a parasite may result in host-race formation or speciation. A brood parasitic bird, the common cuckoo, is divided into host races, each characterized by egg mimicry of different host species. Microsatellite DNA markers were used to examine cuckoo mating patterns and host usage in an area where a new host has been recently colonized. Female cuckoos show strong host preferences, but individual males mate with females that lay in the nests of different hosts. Female host specialization may lead to the evolution of sex-linked traits such as egg mimicry, even though gene flow through the male line prevents completion of the speciation process.