CLUTCH SIZE AND MALARIAL PARASITES IN FEMALE GREAT TITS

Life-history models predict an evolutionary trade-off in the allocatio n of resources to current versus future reproduction. This corresponds , at the physiological level, to a trade-off in the allocation of reso urces to current reproduction or to the immune system, which will enha nce survival and therefore future reproduction. For clutch size, life- history models predict a positive correlation between current measurem ent in eggs and the subsequent parasite load. Tn a population of great tits, we analyzed the correlation between natural clutch size of fema les and the subsequent prevalence of Plasmodium spp., a potentially ha rmful blood parasite. Females that showed, 14 days after hatching of t he nestlings, an infection with Plasmodium had a significantly larger clutch (9.3 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 tile prevalence of Plasmodium, bu t brood size 14 days after hatching was not. This suggests that female s incur higher costs during laying the clutch than during rearing nest lings. Infection status of some females changed between years, and the se changes were significantly correlated with a change in clutch size as predicted by die trade-off. The link between reproductive effort an d parasitism may represent a possible mechanism by which the cost of e gg production is mediated into future survival and may thereby be an i mportant selective force in the shaping of clutch size.