Philornis ectoparasitism of Pearly-eyed Thrashers. II. Effects on adults and reproduction

Abstract

A rain-forest population of the Pearly-eyed Thrasher (Margarops fuscatus) suffered heavy ectoparasitism from the larvae of a tropical fly recently identified as Philornis deceptivus. The impact of parasitic attack was of less consequence to adult thrashers than to nestlings. Nestling mortality was high and was caused mostly by philomid ectoparasitism, other factors being negligible (3%). The 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, especially pterylae development. Overall 4-yr fledging success was 53.3%, fairly high for a tropical passefine. However, observations of heavily parasitized nestlings that died during fledging attempts and the deaths of postfledging juvenile thrashers suggesthat first-year mortality may be as high as 80% in nestlings that suffered heavy larval infestations (357 of 448 nestlings). 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 philomid ectoparasitism by lengthening the breeding season and reproducing while fly populations are seasonally low (December-March) and, thus, may remain in abundance in the rain forest.

Citation

Arendt, Wayne J. 1985. Philornis ectoparasitism of Pearly-eyed Thrashers. II. Effects on adults and reproduction. Auk 102:281–292.