Experimental studies incorporating multiple trophic levels are scarce but of increasing interest for understanding ecological communities. Here we investigated interactive effects of perceived predation risk and parasite pressure on life-history traits in a hole-nesting bird, and the effects of predation risk on parasite success. In a 3 × 2 experimental design we increased perceived predation risk for breeding great tits Parus major via simulations of either nest-predators (woodpeckers) or post-fledging predators (sparrowhawks) close to nests, and used a non-predatory species (song thrush) as a control. Concurrently, half of the nests in each treatment were either infested with ectoparasites, or kept parasite-free.
Regarding the predation risk – parasite interaction, exposure to nest-predators tended to lower wing and sternum growth rates of nestlings in the absence, but not the presence, of parasites. In the presence of parasites, exposure to a post-fledging, but not to a nest-predator, led to significantly reduced wing growth. Mass and tarsus length were not affected by predator exposure, but ectoparasites had slight positive effects on mass gain. In the last third of the nestling period, overall nestling size was significantly smaller when exposed to a post-fledging predator than to a nest-predator, but neither differed from the control. Parental feeding rates were not affected by the treatments, but parents became less selective towards food items under either predation risk. Hen-flea population sizes (adult or larvae) in nests were not affected by predation risk treatment of hosts.
In summary, we found some evidence for an interactive effect of predation risk and parasite pressure on nestling growth. The complexity of the interaction, combined with certain inconsistencies of the effects and potential statistical artifacts, prevent however a straightforward interpretation of the results. The insights from the study are useful for designing additional experiments to further investigate the complexity of predator–parasite interactions in wild populations.