Country Ecology: Birdhouses and blowflies

Published Date Written by David Eastman

One of the unsavory aspects of raising bluebirds is witnessing blowfly maggot pupae in the nesting material of their baby birds. All the cavity bird species become contaminated with this despicable vermin, and interestingly enough, there can be different blowfly species for each bird — chickadees have their particular ones, but tree swallows seem to have the most. And the latter species seems to suffer the greatest, to the degree almost always one anemic chick falls behind its siblings and becomes a mummified corpse — left behind after its vigorous brethren have fledged.

The reason is competition for the incoming food brought by the parent birds. While the blowfly maggots sucking on the baby birds do not kill them outright, they bring down their vitality to such a point from the resulting loss of blood that these birds cannot contest with much stronger nestlings for those aerial insects captured by parental tree swallows for food. They literally starve to death while the others get sustenance during heavy infestations. Hence, when one opens the nest box finding that all the season's young have successfully fledged, it is not uncommon to discover that one or two certainly didn't.

Bluebirders worry about this, of course, and a recent spring issue of Bluebird, produced by the North American Bluebird Society, covered this issue of blowflies quite substantially. As distasteful as this subject is, I felt we should put it out there in hopes of dealing with this aspect of birdlife reproduction. The article was reprinted from the Bluebird Society of Pennsylvania's journal Bluebird Trails and Tales. Many of the state organizations are vigorous affiliates of the national society, and contribute information for members across the country that passionately assist the bluebirds each season in bringing this colorful species "back."

Bird blowflies include two genera: Protocalliphora, and a close relative, Trypocalliphora. Cold, wet weather makes them prosper in the nest material, while outside the box, also limits the amount of insect life the parent birds can capture during such periods. These two factors combine to make the baby birds more susceptible to the effects of this parasitism.

Nestling birds become infested when adult blowflies lay eggs directly on the nestlings, or on their nest's ingredients. The female blowfly will lay 50 to 200 tiny eggs usually around the time the nestlings' first hatching. These become small, tan-colored larvae within two days; they must feed immediately. They suck on the young birds for 15 to 45 minutes and then return down inside the nest material. The larvae must have these blood meals to mature, and suck the greatest amount of blood as they mature. If their numbers are many, the impact is severe.

I have long wondered why their insect eating parents do not devour these pests. It is likely because the larvae primarily attack at night, when the helpless nestlings are easy prey, and hide within the nest during daylight hours. The larvae attach to the nestlings' feet, legs, underside of wings and beaks while they draw out the blood and body fluids after dark. Sounds pretty vampirish.

The larvae feed on the young birds for approximately eight days and then go into a dormant stage for three days, becoming the dark, leathery pupae we see as they drop to the nest box's floor. This pupal period usually lasts about seven to 14 days, after which the adult blowfly phase emerges. By now the nestlings have successfully fledged. After the adult flies leave the nest box, they mate, and the female starts searching for yet another bird nest to repeat the three to four week life cycle.

The Bluebird article said the incidence of blowfly infestation commonly increases in the second and third nestings, as the summer progresses. One can see why I stress cleaning out the old nest box contents immediately, after noticing the birdhouse has become empty. I wear gloves; carry a putty knife and plastic bucket, and then burn the whole mess with old lawn mower gas out on an asphalt driveway. I am not interested in growing parasites, just baby birds. That is my common answer to the question, "Do you clean out the birdhouse at the end of the year?" "I do it right away," is my response.

This also sets up the birdhouse to be utilized again by a pair of bluebirds in the field. The tree swallows only nest once, but your bluebirds might have a second or third brood if conditions warrant. A good summer for their metabolism means their health is up to trying again for an additional clutch, though there may be one less egg numerically each time they accomplish that.

Bluebirds will typically have four or five eggs in the initial nesting, and perhaps three or four the second time around. Tree swallows may have about seven for their only annual attempt in reproducing from May through July.

Dave Eastman also broadcasts "Country Ecology" four times weekly over WMWV 93.5 fm. As Vice President of the Lakes Region Chapter/ASNH, he welcomes you to monthly programs at the Loon Center in Moultonborough. He is available at: This email address is being protected from spambots. You need JavaScript enabled to view it. for consultation.