Laura Erickson’s For the Birds

Setting out nest boxes seems like a low-maintenance kind of thing—you set out a high-quality birdhouse in a good location and birds nest in it. End of story. But after the babies leave the nest, it’s important to clean out the nest box as soon as possible.

Parent birds keep up with nest sanitation as best they can. Baby bird poop is produced in “fecal sacs” with a sturdy membrane, and nestlings have the useful habit of pooping right after swallowing food, while the parent is right there to deal with it.

Babies being what they are, sometimes they poop when the parent isn’t there, or two babies get fed at the same time and the parent takes only one sac, so little by little, poop accumulates, but that’s seldom a significant problem. By far, the more important issue as far as nest sanitation goes is the infestation of parasites, something parent birds have no control over.

Mites, which multiply quickly in old nest material, feed on both nestlings and brooding adults. Bird fleas, which feed on blood, complete their life cycle in nest lining. But perhaps the worst scourges for cavity nesters are blowflies. They lay their eggs in nest material, and after hatching, the gray maggots hide in the nesting material and use mouth hooks to pierce the skin of nestling birds to feed on their blood.

Mites, fleas, and blowflies are all ectoparasites, feeding on their hosts externally (though some may work their way into ear or nostril openings). Heavy infestations can cause anemia, stunted growth, or tissue damage inside the nostrils resulting in a deformity called “shovel-beak.” Many nestlings survive, but fledging can be delayed, reducing the number of offspring the parents produce that season. After a severe winter, ectoparasites aren’t usually too big of a problem for the first nests, but when birds reuse a nest box or cavity that season, the parasites will be present from the start.

How can we protect nesting birds from these scourges? When I first heard about tick- and flea- preventive medicines for dogs and cats, I thought that setting out a treated pet’s fur that birds could take for nesting might kill the parasites without hurting the birds, but studies have shown that the treated fur directly harms baby birds. I wrote about the issue in 2025, when scientists in the UK examined the fur lining in 103 nests of Eurasian Blue Tits and Great Tits collected in 2020. The species were chosen because, like our own chickadees and titmice, they virtually always line their nests with fur.

The scientists found that in every single nest, the fur was contaminated with at least 2 pesticides—in some, as many as 11—each one in common veterinary use to protect livestock and pets. But are these pesticides actually dangerous for birds? The researchers discovered that the more of these pesticides present in the fur incorporated into a nest, the fewer eggs produced and the higher the chick mortality. So when it comes to setting out fur from treated pets for your local nesting birds, please, just say no.

I was a licensed wildlife rehabber for over ten years, but no one ever brought me baby birds that had been injured by ectoparasites. My more experienced friend, Koni Sundquist, dealt with heartbreaking cases of baby bluebirds and swallows that were suffering badly from blowflies. Koni, like a great many other knowledgeable people (including me until recently), called blowflies “botflies.” But botflies are an entirely different kind of pest, which I’ll talk about next time.