Micro-evolutionary change and population dynamics of a brood parasite and its primary host: the intermittent arms race hypothesis

A long-term study of the interactions between a brood parasite, the great s potted cuckoo Clamator glandarius, and its primary host the magpie Pica pic a, demonstrated local changes in the distribution of bath magpies and cucko os and a rapid increase of rejection of both mimetic and non-mimetic model eggs by the host. In rich areas, magpies improved three of their defensive mechanisms: nest density and breeding synchrony increased dramatically and rejection rate of cuckoo eggs increased more slowly. A stepwise multiple re gression analysis showed that parasitism rate decreased as host density inc reased and cuckoo density decreased. A logistic regression analysis indicat ed that the probability of changes in magpie nest density in the study plot s was significantly affected by the density of magpie nests during the prev ious year (positively) and the rejection rate of mimetic model eggs (negati vely). These results are consistent with a hypothesis (the intermittent arm s race hypothesis) of spatially structured cyclic changes in parasitism. Du ring periods of parasitism, host defences continuously improve, and as a co nsequence, the fitness gains for parasites decrease. When host defences aga inst parasites reach a high level, dispersing parasites have a selective ad vantage if they are able to emigrate to areas of low resistance. Once paras ites have left an area hosts will lose their defensive adaptations due to t heir cost in the absence of parasitism. The scene is then set for re-coloni zation by great spotted cuckoos.