Ectoparasitic blow flies (Protocalliphora sp.) and nestling Eastern Bluebirds (Sialia sialis): direct effects and compensatory strategies

Abstract

Variation in environmental factors such as parasitism can have direct effects on an organism’s fitness. Because parasites draw resources directly from their hosts, they are expected to have negative effects; however, several nestling host – parasite systems show no evidence of direct effects. Absence of direct effects may be explained by compensation strategies used by parents or nestlings themselves. In this study evidence for both direct effects and compensatory strategies in a blow fly (genus Protocalliphora Hough, 1899) – Eastern Bluebird (Sialia sialis (L., 1758)) system were examined. Nestlings showed no direct effects of blow flies on survival and on size at fledging; however, parasitized broods were significantly anemic. There was no evidence for compensation by parents in the form of brood reduction and there was no support for compensation by nestlings via a hierarchy of tissue preservation. Nestlings did compensate for parasitism by accelerating growth at the end of the nestling period and delaying fledging.