Philornis ectoparasitism of pearly-eyed thrashers. II. Effects on adults and reproduction..

Abstract

A rain-forest population of Margarops fuscatus suffered heavy ectoparasitism from larvae of Philornis deceptivus. Nestling mortality was high and was caused mostly by philornid ectoparasitism, other factors being negligible (3%). Extent of larval infestations fluctuated seasonally and was related to monthly rainfall. Infestation did not have to be heavy to cause debilitation and death. Larval infestation sites varied with the nestling's ontogeny. Overall 4-yr fledging success was 53.3%, fairly high for a tropical passerine, but first-year mortality may be as high as 80% in nestlings that suffered heavy larval infestations. Juveniles face keen intraspecific competition after leaving the nest. A combination of high nestling and juvenile mortality and little opportunity for juveniles to enter the breeding population could greatly reduce this host's numbers. There are signs, however, that the pearly-eyed thrasher is adjusting to heavy philornid ectoparasitism by lengthening the breeding season and reproducing while fly populations are seasonally low (December-March). -from Author