Bird blow flies include two genera, Protocalliphora, and a close relative, Trypocalliphora.  They are true flies, order Diptera, and members of the blow fly family Calliphoridae.  The blow fly family includes the common blue and green bottle flies.  Most common blow fly species are scavengers that feed on dead animals, but the family includes several parasites such as Pollenia rudis which parasitizes earthworms, and Lucilia silvarum which parasitizes toads.  It also includes the once abundant primary screworm fly (Cochliomyia hominivorax) which used to be a major pest of livestock in the southwest.

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.  One species, Trypocalliphora braueri, is subcutaneous. Sabrosky et al. (1989) summarizes the feeding activity of this species (under Protocalliphora braueri). Larvae initially burrow into nestlings body where they may do extensive tissue and organ damage. As larvae mature, they spend their time in a warble-like opening under nestlings' skin. This species has often been associated with nestling mortality.

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 (cause reduced red blood cell counts) 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 is known from only 4 specimens.  Another in the West is known only from male specimens.  The bird hosts for these species are unknown and a number of other species are known from only a few hosts.  The answer to these puzzles 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.