Survival and reproductive rate of mites in relation to resistance of theirbarn swallow hosts

Parasite resistance may act via a number of different mechanisms that regul ate or control the survival and the reproductive rate of parasites. Observa tions and experiments were used to test for effects of host resistance on p arasite survival and rate of reproduction. Natural levels of infestation of barn swallow Hirundo rustica nests by the tropical fowl mite Ornithonyssus bursa were positively related to brood size, inversely related to the leng th of the outermost tail feathers of male nest owners (a secondary sexual c haracter) and affected by time of reproduction by the host. A mite inoculat ion experiment, in which 50 adult mites were introduced into nests during t he laying period of the host, was used to test for differential survival an d reproduction of mites as a function of host resistance. The relationship between survival and reproduction of parasites, male tail length and host r esistance was investigated There was a negative relationship between mite n umbers per nest after fledging of nestlings and male tail length. This rela tionship was mainly caused by a reduction in the number of mites in the fir st and second nymph stage with increasing tail length of male hosts implyin g a reduction in rate of reproduction of mites. The proportion of mites tha t had recently fed was inversely related to tail length of male hosts. The proportion of nymph stages was positively related to the proportion of mite s that had recently had a blood meal. Parasite resistance of barn swallows to the tropical fowl mite thus appeared to act through increased mortality rate of adult and nymph stages of mites and through reduced reproductive ra tes of mites on resistant hosts. This is the first study demonstrating a di rect relationship between fitness components of a parasite and the expressi on of a secondary sexual character of a host.