Cornell Lab of Ornithology

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Birds and Blow Flies
BY Tracey L. Kast

Please cite this Page as:
Kast, T. L. 1999.  Birds and Blow Flies. Birdscope, Volume 13, Number 4:  13-14.

Does removing used nests from nest boxes
protect birds from blow flies?

Occasionally, when people open their nest boxes they find maggots attached to the nestlings. This can be a disconcerting sight for anyone who has been closely monitoring the progress of birds nesting in their box. These maggots are the larval stage of bird blow flies, genus Protocalliphora. The female blow fly lays her eggs in a bird’s nest. When the eggs hatch, the larvae climb up through the nest material, attach themselves to the growing nestlings, and suck their blood. Blow flies are found throughout North America and in the nests of many bird species, including cavity- nesters.

Nest-box monitors everywhere want to know what they can do to prevent blow flies from parasitizing the birds in their nest boxes. Scientists are interested in blow flies because little is known about them and how much they harm their bird hosts. According to conventional wisdom, nest-box monitors should always clean out their nest boxes after a brood has fledged. One reason for doing this is to remove any remaining nest parasites, thereby supposedly preventing parasitism in future nests. This sounds sensible, but is it correct? Does removing a used nest protect nestlings of subsequent nests from blow flies?

According to data collected by Nest Box Network (now called The Birdhouse Network), participants in 1997 and 1998, the answer is no. Fourteen percent of the nest boxes in which the previous nest had been removed contained blow flies. In boxes that contained used nests, 11 percent of the nests had blow flies. These percentages are not significantly different; thus, cleaning out your box before a nest attempt does not affect whether blow flies will parasitize the next birds nesting in the box. The figure includes blow-fly data reported for 24 species of birds. Of these, blow flies were reported in 10 species’ nests: Ash-throated Flycatcher, Tree Swallow, Purple Martin, Black-capped Chickadee, House Wren, Eastern Bluebird, Western Bluebird, Mountain Bluebird, House Finch, and House Sparrow.

graph showing blow fly data for nest boxes
This figure shows the percentage of nests that contained evidence of blow flies (for example, larvae or pupal cases). Data for two kinds of nest boxes are shown: boxes that contained old nests and boxes in which old nests had been removed. This figure combines information for 24 cavity-nesting species.

These results are preliminary. Many more questions remain to be answered about birds and blow flies. For example, the figure includes multiple species, but what about individual species? It may be that cleaning out nest boxes decreases blow-fly parasitism for Eastern Bluebirds but not for Tree Swallows, or vice versa. Does blow-fly parasitism affect the number of young that fledge from the nest? Also, does the incidence of blow-fly parasitism vary within different regions of the country? To help answer these questions about cavity-nesting birds,  call (800) 843-BIRD. Look for more Birdhouse Network results in future issues of Birdscope.