Bird–ectoparasite interactions, nest humidity, and ectoparasite community structure

Host nests are the key habitat for ectoparasite species that live and reproduce within the nest material. Nest properties can influence host and parasite reproductive success, and therefore the outcome of host–parasite interactions, as well as the composition of parasite communities. Previous correlational results suggested that nest humidity may increase the negative effect of fleas on Great Tit (Parus major) reproductive success. Since the causality of the relationship was unknown, we simultaneously manipulated both hen flea (Ceratophyllus gallinae) infestation and nest humidity in order to investigate: (1) the effect of flea infestation on nest humidity, (2) the effect of nest humidity on the prevalence of flea infestations, (3) the effect of nest humidity on adult flea survival and reproduction, (4) the interaction between flea infestations and nest humidity on Great Tit reproductive performance, and (5) the effect of both factors on secondary infestations by ectoparasitic Protocalliphora flies. The present study provides the first example of an ectoparasite acting as a physical ecosystem engineer where nest humidity increased as a result of flea infestations. In experimentally uninfested nests, infestation of hosts by immigrant fleas was more frequent in nests with reduced humidity. In experimentally infested nests, manipulation of humidity did not significantly affect the number of adult fleas or larvae. Hen flea infestation reduced Great Tit nestling body mass, tarsus length, and wing length, as well as the number of young fledged. The detrimental effect of the fleas on Great Tit reproduction was not altered by our modifications of nest humidity. The effect of fleas on nestling growth parameters and the number young fledged did not vary significantly with the seasonal decrease in reproductive performance of the birds, or with the number of young in the brood. The prevalence of secondary infestations by Protocalliphora was higher in nests with reduced humidity, and in nests experimentally infested with fleas. The numbers of Protocalliphora pupae were lower in flea‐infested nests. This study provides the first experimental evidence that an abiotic factor within nests can affect ectoparasite infracommunity structure. It also provides the first evidence for an interspecific association between two ectoparasite species.