Laura Erickson's For the Birds

I just got an email from a listener named Dave who has a problem. He has four bluebird houses in Wisconsin. One attracted a pair of bluebirds, and Tree Swallows took over two others—Tree Swallows may be uninvited, but at least they’re lovely and helpful insect eaters. Unfortunately, Dave’s bluebird houses also attracted a male House Sparrow, who took over the fourth box but apparently couldn’t attract a mate. He was perhaps too charged up with hormones, and so he got aggressive and killed all the swallow and bluebird babies in the other boxes. House Sparrows are frustrating because although they too are birds, and are dangerously declining in the U.K. where they belong, they’re serious problems here in North America where they are introduced. They are exceptionally aggressive in taking over bird houses, and are capable of killing birds significantly larger than themselves. Most songbirds seem to have some inhibitions about killing nestlings unless they depend on them for a food source, but not House Sparrows.

Anyway, that was a heartbreaking situation, but Dave dispatched the sparrow and hoped for the best. The bluebirds renested and produced 5 eggs which all hatched, but then last weekend he discovered that the new babies had been infested with botflies.

Botflies are nasty parasitic insects belonging to Diptera—the order with other flies and mosquitoes. Entomologists haven’t been as disciplined as ornithologists in standardizing common names, so some people call the flies infesting baby birds blowflies, but most sources seem to limit use of the name blowfly to the colorful blue or green house flies that eat garbage and carrion, but not living animals. Botflies are true parasites. The female deposits her eggs directly on or in folds of animal skin, and the nasty little maggots eat their way through the tissue. Botflies can cause serious health issues for far larger animals than tiny bluebird nestlings—one of the primary reasons farmers usually remove tails on baby lambs is to protect them from botflies, since the females search out dark, warm places, especially under the tails in horses and sheep.

Dave wondered what to do about them. First off, after birds lose or fledge a first brood, it’s not a bad idea to remove the nest materials, especially in a wet year like this one, since botflies can lay eggs in nest materials which hatch when the baby birds do. If you notice insects in a bluebird nest, the North American Bluebird Society recommends removing all the nest material and replacing it with dry grass, shaped the way the nest was. Don’t use pesticides because anything that can kill botflies can kill baby bluebirds.

The North American Bluebird Society also gives some sound recommendations for controlling House Sparrows, both passively by making your nest boxes less attractive to them in the first place and more aggressively if they’re causing serious problems. Sadly, there’s no practical way to gather them all up and send them back to England where conservationists are lamenting their decline.

The Cornell Lab of Ornithology has some great guidelines for monitoring nests at nestwatch.org. Nest monitoring can be fun, and when we’re armed with knowledge about what to do when the birds need help, it can also be a richly rewarding experience that will benefit the birds as well as warming our hearts.