Résumé
Geographically separated populations may diverge genetically in response to differing environmental conditions. Two populations of blue tits (Parus caeruleus L., 1758) that inhabit distinct valleys in northern Corsica are exposed to extreme differences in food abundance and parasite loads and show differences in nestling mass and hematocrit levels at fledging. We used partial cross-fostering coupled with experimental manipulation of parasite loads to test the hypothesis that between-population differences in nestling mass and hematocrit reflect adaptive genetic responses to differing parasite prevalence. Although asymptotic mass and hematocrit were strongly affected by variation in parasite loads and caterpillar abundance, we did not detect any significant genetic (population of origin) effect or genotype–environment interaction. We conclude that in these populations of blue tits, asymptotic mass and hematocrit are phenotypically plastic traits that are primarily set by environmental conditions during the sensitive growth phase.