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Climatic conditions, through their effects on resource availability, may affect important life history strategies and trade-offs in animals, as well as their interactions with other organisms such as parasites. This impact may depend on species-specific pathways of development that differ even among species with similar resource requirements (e.g., avian brood parasites and their hosts). Here we explore the degree of covariation between environmental-climatic conditions and nestling phenotypes (i.e., tarsus length, body mass, immune response to phytohemagglutinin injection) and ectoparasite loads of great spotted cuckoos (Clamator glandarius) and those of their magpie (Pica pica) hosts, both within and among 11 study years (1997-2011). Our main results were that (1) nestling phenotypes differed among years, but differently for great spotted cuckoos and magpies; (2) nestling phenotypes showed significant among-year covariation with breeding climatic conditions (temperature and precipitation); and (3) these associations differed for cuckoos and magpies for some phenotypic traits. As the average temperature at the beginning of the breeding season (April) increased, body mass and tarsus length increased only for cuckoos, but not for magpie hosts, while immune response decreased in both species. Finally, (4) the strength of the within-year relationships between the probability of ectoparasitism by Carnus hemapterus flies and laying date (used as an estimate of the within-year variation in climatic conditions) was negatively affected by the annual accumulated precipitation in April. These results strongly suggest that variation in climatic conditions would result in asymmetric effects on different species with respect to the probability of ectoparasitism, immunity and body size. Such asymmetric effects may affect animal interactions in general and those of brood parasites and their hosts in particular.

A long-term study of the interactions between a brood parasite, the great spotted cuckoo Clamator glandarius, and its primary host the magpie Pica pica, demonstrated local changes in the distribution of both magpies and cuckoos 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 regression analysis showed that parasitism rate decreased as host density increased and cuckoo density decreased. A logistic regression analysis indicated that the probability of changes in magpie nest density in the study plots was significantly affected by the density of magpie nests during the previous year (positively) and the rejection rate of mimetic model eggs (negatively). These results are consistent with a hypothesis (the intermittent arms race hypothesis) of spatially structured cyclic changes in parasitism. During periods of parasitism, host defences continuously improve, and as a consequence, the fitness gains for parasites decrease. When host defences against parasites reach a high level, dispersing parasites have a selective advantage if they are able to emigrate to areas of low resistance. Once parasites have left an area hosts will lose their defensive adaptations due to their cost in the absence of parasitism. The scene is then set for re-colonization by great spotted cuckoos.