EFFECT OF VERTICALLY TRANSMITTED ECTOPARASITES ON THE REPRODUCTIVE SUCCESS OF SWIFTS (APUS-APUS)

1. Parasites that are transmitted vertically from parent hosts to offs pring are expected to be relatively benign, because their fitness depe nds on successful host reproduction. The effects of two species of ver tically transmitted ectoparasite on the reproductive success of swifts (Apus apus L.) were tested. Populations of the Chewing Louse, Dennyus hirundinis (L.) (Phthiraptera: Menoponidae), and the Flightless Louse Fly, Crataerina pallida (Latreille) (Diptera: Hippoboscidae), were ex perimentally manipulated, effectively converting the natural aggregate d frequency distribution of each species into a bimodal distribution o f high and low loads. 2. Neither parasite had any effect on nestling g rowth or fledging success, even though parasite loads were boosted abo ve natural levels and host environmental conditions were poor during p art of the study, thus increasing the chances of detecting an effect o f the parasites. 3. In contrast to parasite load, year, brood size and hatch date were all significantly related to components of nestling g rowth. Year and brood size were also significantly related to fledging success. 4. These results are consistent with theoretical models sugg esting that vertically transmitted parasites evolve reduced virulence because they depend on host reproduction for dispersal to new hosts.