Parasite defense is costly, and the detrimental effects of parasites, which can be measured in terms of parasite virulence, are thought to be influenced by the resources available to the host and, ultimately, by environmental conditions. Hence, if conditions are good, hosts can tolerate a certain number of parasites without suffering severe effects. In addition, the presence of other parasites can influence the virulence of a focal parasite either positively or negatively. We tested whether an experimental tick infestation reduced nestling performance in Great Tits (Parus major) and whether the effect was altered by a maternal response induced by Hen Fleas (Ceratophyllus gallinae) that is known to protect nestlings against flea infestations (i.e., we tested whether one parasite can alter the virulence of another parasite across host generations). We induced the maternal effect by experimentally infesting half the birds' nests with fleas during egg laying. After hatching, nestlings were cross-fostered into broods that then contained both nestlings with and without the maternal effect. Half of these broods were infested with five tick larvae per nestling. This resulted in tick infestation levels similar to levels found in natural populations. The tick infestation did not affect nestling mass, tarsus length, or time until fledging. Thus, an effect of the flea-induced maternal effect on tick virulence was not detectable. From these results, we concluded that either tick larvae do not affect nestlings, or nestlings or their parents can compensate for the negative consequences of tick infestations in numbers similar to those that occur in nature.
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