Clutch size and malarial parasites in female great tits

life-history models predict an evolutionary trade-off in the allocation of resources to current versus future reproduction. This corresponds, at the physiological level, to a trade-off in the allocation of resources to current reproduction or to the immune system, which will enhance survival and therefore future reproduction. For clutch size, life-history models predict a positive correlation between current investment in eggs and the subsequent parasite load. In a population of great tits, we analyzed the correlation between natural clutch size of females and the subsequent prevalence of Piasmodium spp., a potentially harmful blood parasite. Females that showed, 14 days after hatching of the nestlings, an infection with Piasmodium had a significantly larger clutch (9.S eggs ± 0.5 SE, n = 18) than uninfected females (8.0 eggs ± 0.2 SE, n = 80), as predicted by the allocation trade-off. Clutch size was positively correlated with the prevalence of Plasmodhm, but brood size 14 days after hatching was not. This suggests that females incur higher costs during laying the dutch than during rearing nesdings. Infection status of some females changed between years, and these changes were significantly correlated with a change in clutch size as predicted by the trade-off. The link between reproductive effort and parasitism may represent a possible mechanism by which the cost of egg production is mediated into future survival and may thereby be an important selective force in the shaping of clutch size.