BODY SIZE AND IMMUNE DEFENSE OF NESTLING BLUE TITS (CYANISTES CAERULEUS) IN RESPONSE TO MANIPULATION OF ECTOPARASITES AND FOOD SUPPLY

A developing organism faces a dilemma: whether to allocate available resources to building its body structures
(growth) or to the development of its immune system. The outcome of this tradeoff is likely to be modified by parasites. We manipulated
the abundance of ectoparasitic Hen Fleas (Ceratophyllus gallinae) on nestling Blue Tits (Cyanistes caeruleus) by microwaving nests
and subsequently adding 200 Hen Fleas (15 infested nests) or not (16 reduced-infestation nests). In addition, we manipulated the
host nestlings’ food resources by supplementary feeding 10–15% of daily energy needs to half the nestlings in a nest during the key
developmental period (days 2–12). Feather growth (tail and wing length) and hematocrit were reduced by the presence of Hen Fleas,
indicating negative effects on nestling development. In comparison to the control nestlings, food-supplemented nestlings aged 16 days
were larger (tarsus, residual body mass), but only in reduced-infestation nests (interaction between both treatments). Body size of fed
male offspring increased in relation to that of females, but only in the absence of ectoparasites. We hypothesized that supplemented
resources are allocated to immune defense when ectoparasites are present, but humoral immune function (total immunoglobulin
concentration) and cell-mediated immune defense (phytohemagglutinin response) were not affected by either treatment. Either the
nestlings allocated additional resources away from growth (into an unknown developmental component) when parasites were abundant,
or the ectoparasites preferentially fed on supplementary-fed host nestlings and thereby equalized the development of soma and immune
defense of nestlings despite provision of additional resources. Received 17 December 2010, accepted 12 March 2011