Parasites of Nestling Birds
Bird blow flies are obligate, bloodsucking ectoparasites of nestling birds. In nature they only infest the nests of birds with altricial young (nestlings which must develop several days or weeks in the nest). The larvae of most species of bird blow flies live in the nest and periodically suck the blood of nestlings. Experiments have shown the larvae can also survive and mature on the blood of mice and even humans, (Bennett, 1957) but such parasitism doesn't occur in nature. Trypocalliphora braueri, a closely related species, is subcutaneous and spends its life as a larva burrowing in the flesh of nestlings. It can damage muscle tissue and organs, and has often been associated with nestling mortality. Near maturity, the larvae move to a warble-like opening under nestlings' skin prior to emerging from the nestling and dropping into the nest to pupate.
Bird blow flies are common in the nests of many birds, including bluebirds, swallows, chickadees, wrens, warblers, flycatchers and raptors. Research shows that heavy infestations can make nestlings anemic due to blood loss and severe infestations may be lethal (Whitworth & Bennett, 1992). Infestation rates vary from almost 100% in magpies to 50% in many bluebirds, to zero in over 100 oriole nests (Whitworth & Bennett, 1992). In most areas, around 5-10% of infested nests are likely to have sufficient larval populations to make nestlings sick. Little is known about this parasite since it is rarely encountered in nature, except in bird nests. There are currently 28 described species of Protocalliphora in North America and 13 others in the Palearctic region (primarily Europe and Asia). Protocalliphora rognesi and Trypocalliphora braueri are Holarctic, occurring in both regions. The genus Trypocalliphora contains only a single species. Both genera are known only in the temperate zones of the Northern Hemisphere.
Some species of this genus are very rare. One species in Alaska (P. sapphira) is known from only four specimens and its hosts are unknown. Several other species (P. beameri, P. brunneisquama, and P. lata) are rarely collected and poorly known. The answer to understanding these species is in bird nests, probably in the nests of birds that have secretive nests, including ground and shrub nesting birds.
By cooperating with researchers, you can help increase scientific understanding of this parasite. As we learn more about bird blow flies we may be able to recommend strategies to reduce their impact on nestling birds. If you are interested in learning if the nests you monitor are infested refer to the section on nest collection.
Terry Whitworth, Ph.D.
2533 Inter Avenue
Puyallup, WA 98372
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all content copyright © 2012 by Terry L Whitworth except as noted.
Portions of images by Joseph Berger (blowfly in header), John Triana (wren nest in title background), and Whitney Cranshaw (fly on wall), used by permission of www.insectimages.org.