Fleas, parental care, and transgenerational effects on tick load in the great tit

In the presence of parasites, parents can increase the amount of resources allocated to parasite defense and thereby enhance their chances of survival and future reproduction or allocate more resources to current reproduction and thus increase the condition of their offspring. Here we test how a common ectoparasite affects parental behavior and the trade-off between parasite defense and reproduction in a wild bird population. To avoid confounding effects of the parasite infestation on the offspring and to test purely for the effect of the parasite on parents, we exposed parents to parasites before the young hatched only, that is, in the early phases of reproduction. Experimental great tit (Parus major) nests were infested with hen fleas (Ceratophyllus gallinae) until the start of incubation, whereas control nests were left parasite free. Parasite-induced maternal and genetic effects were then eliminated by replacing all clutches with the clutches of unexposed parents. All fleas were removed after clutch completion and hence nestlings grew up in practically flea-free nests. The experimental flea infestation before incubation did not change parental feeding rates but reduced the frequencies of brooding and nestling care. Tick prevalence increased and tarsus growth was reduced in nestlings reared by previously exposed parents. It suggests that a fraction of the costs for parents of the flea exposure before incubation is shifted to offspring via reduced parental care. The flea infestation before incubation did not affect parental body condition. However, flea-exposed parents had higher tick loads, which may impair parental health and survival.