Question: Are maternal effects (i.e. maternal transfer of immune components to their offspring via the placenta or the egg) specifically directed to the offspring on which ectoparasites predictably aggregate?
Organisms: The barn owl (Tyto alba) because late-hatched offspring are the main target of the ectoparasitic fly Carnus hemapterus.
Hypothesis: Pre-hatching maternal effects enhance parasite resistance of late- compared with early-hatched nestlings.
Search method: To disentangle the effect of natal from rearing ranks on parasite intensity, we exchanged hatchlings between nests to allocate early- and late-hatched hatchlings randomly in the within-brood age hierarchy.
Result: After controlling for rearing ranks, cross-fostered late-hatched nestlings were less parasitized but lighter than cross-fostered early-hatched nestlings.
Conclusion: Pre-hatching maternal effects increase parasite resistance of late-hatched offspring at a growth cost.