Growth, nutrition, and blow fly parasitism in nestling Pied Flycatchers

The nutritional status of the host may play a major role in mediating the detrimental effects of parasites. We performed an experiment with the aim of determining whether increased food availability can compensate for the effects of ectoparasites on growth during the late nestling period, final size, and survival until fledging of Pied Flycatcher (Ficedula hypoleuca) nestlings. Nests were provided with supplementary food, treated with insecticide, given bath treatments, or given neither treatment (control). Differences in the number of blood-sucking, ectoparasitic blow fly larvae (Protocalliphora azurea) occurred between treated nests. Nestlings in the group given supplementary food and with low numbers of parasites grew faster and had a higher haematocrit value than those in groups that were fumigated and given supplementary food, with nestlings from control nests attaining the lowest values. Nestling measurements did not differ between fumigated and food-supplemented groups. Although the final sizes attained did not differ among nestlings from the different experimental groups, there was a significant difference in the rates of increase in size among groups. Nestlings in nests fumigated and provided with extra food were (nonsignificantly) smaller and leaner than nestlings from the other groups at the beginning of the experiment, but were slightly larger and heavier (again nonsignificantly) at the end of the experiment. Thus, their growth was faster than that of the other groups. The results are discussed, highlighting problems related to the function linking intensity of parasitism to host fitness and variation in external (climate, food) conditions.