Duration of sympatry and coevolution between the great spotted cuckoo and its magpie host

Abstract

PARASITIC cuckoos habitually lay their eggs in the nests of one or more host species. Because changes in one member of a host-parasite pair select for changes in the other member, it has been suggested that there is a revolutionary arms race between cuckoos and their hosts^1–6. Comparisons of host species with species unsuitable as hosts, and the finding of host species from allopatric populations that tolerate nonmimetic eggs better than do species from sympatric populations, indicates that host behaviour has evolved in response to the behaviour of cuckoos⁵. Similarly, the observations that the laying procedure of cuckoos matches host defences², and that cuckoo egg mimicry parallels the discriminatory ability of the host species⁷, indicates that cuckoo behaviour has evolved in response to host counter-adaptations. Here we demonstrate that rejection of eggs of the parasitic great spotted cuckoo Clamator glandarius by the magpie Pica pica hosts varies between populations depending on cuckoo parasitism. In a population of ancient sympatry, magpies reject both nonmimetic and mimetic eggs; in an area of recent sympatry magpies show less rejection, particularly of mimetic eggs; in a population isolated from great spotted cuckoos, however, magpies show no rejection. Such differences in egg recognition among host populations affect the spatial population dynamics of parasites and their hosts. Parasitic cuckoos therefore are in a particularly favourable posiá-tion in the revolutionary arms race with their hosts in areas of recent sympatry.