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.