Long-term effects of nestling condition on blood parasite resistance in blue tits (Cyanistes caeruleus).

Little is know about whether the conditions experienced during ontogeny affect resistance to parasites later in life in wild animals. Here, we used a population of blue tits (Cyanistes caeruleus (L., 1758)) to investigate to what extent conditions experienced during the nestling stage could explain the ability to control blood parasite (Haemoproteus majoris (Laveran, 1902)) infections 1 year later. Although short-term effects may be expected based on the well-known sensitivity of the immune system to current conditions, it is less known whether this translates into a permanent alteration of parasite resistance. By relating nestling condition (measured as body mass or size-corrected body mass) at the beginning and end of the nestling stage to parasite intensity of individual recruiting birds 1 year later, we indeed found significant positive effects of both early and late nestling condition on the long-term ability to control parasites. These results indicate that parasites may be important as a mechanistic explanation for the trade-off between number and quality of offspring. It further points to the potential relevance for maternal effects in host–parasite interactions.