This article discusses animal harm and animal death, caused both by humans and other animals, that may be upsetting to some readers. Reader discretion is advised.

Yesterday, while I was volunteering, a woman came into the nature store asking if we had any nest boxes designed specifically for swallows. A pair of violet-green swallows had been using the box she’d set up next to her house for years, but last year a pair of house sparrows had taken it over, and after a few scuffles with them the pair of swallows had left. She was worried they wouldn’t come back again, but wanted to set up a new box just in case, since there was little chance of getting the sparrows to abandon the old one that they had claimed as their own. Later that day, when I went downtown to get lunch at a pod of food carts, I saw approximately two dozen house sparrows fighting over what looked like spilled chips, chasing away the much larger rock doves that tried to get in on the meal. When someone tried to shoo them away, they lifted into flight almost as one before settling right back down as soon as the “intruder” had walked away, causing laughter from several people gathered around on the sidewalk waiting for their food to be ready.

If you live pretty much anywhere in the United States that isn’t Alaska, there is an almost 100% chance that you have seen, heard, or been in close proximity to a house sparrow (Passer domesticus). They’re what most people think of when they hear the word sparrow; the females are dusty brown all over with fairly minimal markings, while the males have black beards and chestnut heads with white cheeks and grey caps. Like crows and rock doves, they’ve succeeded where other species have failed by adapting easily to urban areas, and are perfectly comfortable nesting in the eaves of houses and in drain pipes. They eat grain and other seeds along with insects, but they’re most commonly seen stealing food from the tables of people eating outside with increasing boldness and fighting over those same scraps among themselves. They’re native to North Africa, Europe, and some parts of Asia, but since their initial introduction to New York in 1851, they’ve been making themselves at home in the States.

They don’t look threatening. Compared to more obvious invaders like kudzu, zebra mussels, and European carp, they don’t seem like they would pose much of a danger to the native bird species they’ve found themselves to be neighbors with. Even European starlings, North America’s other extremely common invasive bird that was introduced to the States on purpose, look more threatening than they do, especially when they raise their head feathers.

However, house sparrows (and European starlings, in fact) are cavity nesters. As I mentioned, they’re comfortable living in cavity-adjacent spaces like pipes and other man-made objects, but their natural nesting place is in cavities like the ones found in trees and cacti. Dozens of native birds are also cavity nesters, including but not limited to bluebirds, woodpeckers, swallows, nuthatches, owls, wrens, and chickadees. Cavities provide safe places to rear hatchlings. Recently, birdhouses and other nest boxes have provided man-made cavities for birds to raise their young in, often providing an inside look into the lives that these animals lead. And it’s in the competition for a cavity to nest in that the trouble really begins.

House sparrows don’t migrate and often start breeding in midwinter, which gives them the first choice of nesting sites before most species have even returned from their trip to warmer climates. They can breed practically from the moment they mature, and raise clutches that usually consist of five eggs, which they do two times (or sometimes more, in warmer climates) per breeding season. Compare this to another cavity nester, a native one this time, like a tree swallow-which usually lays around four to seven eggs, with only one or two that even hatch at all-or a western bluebird-which usually lays around five eggs, and can raise two broods in a good year-and it’s clear that when it comes to the population game, house sparrows were designed to outcompete other birds. But that’s not all-house sparrows will often physically fight with native birds (and occasionally European starlings) for nesting spots. Once inside, if another species has already built a nest, they will destroy it, killing the hatchings or smashing the eggs and either chasing the parent or parents away or killing them. Horrifyingly, when attacking adult birds, house sparrows go for the head and eyes, often decapitating their opponents or scratching their eyes out.

It can be argued that the destruction of nests by other birds (whether that bird is another species looking to take over the nest or a male hoping to spread his genetic material once the previous eggs and/or chicks are gone) is a natural part of a bird’s life cycle. It’s true that native species do this too-take the tree swallows I mentioned above, for instance, as males will often destroy eggs fertilized by another male and will attempt to do the same to chicks unless the female stops them. House wrens, another common native bird, will also peck holes in the eggs of other species and build their nests on top of other ones made by a different species. But what makes all the difference is that house sparrows aren’t native. Being better at outcompeting actually native species is a win for them in a way it wouldn’t be for a western bluebird or a downy woodpecker. They don’t belong here, and the infanticide they commit and the cavity nesting competition they provide has been linked to a decline in native species of bluebird around the United States.

So what can you do to stop this subtle invasion? Unfortunately, house sparrows are adept at squeezing into small spaces, and while buying or building a nest box with a smaller entry hole will keep out larger competitors like European starlings, it won’t do much to stop a determined sparrow. While house sparrows tend to live closer to humans and man-made structures like barns and setting up a nest box farther from humans and urban environments will give you a higher chance of getting a nesting pair of woodpeckers, nuthatches, swallows, or another native species, it’s not foolproof. They also avoid boxes low to the ground, but those can attract cats or raccoons to vulnerable native chicks or eggs. Some people plug the entrances to their boxes in the winter so early-breeding sparrows can’t nest in them, then remove the block once migratory birds come back (though this does make nonmigratory native species like chickadees and nuthatches wait their turn as well), but since house sparrows nest at least twice per season, this isn’t always an adequate deterrent. It can be tiring, but constantly removing the nest from the box each time they rebuild it can often convince them to settle elsewhere. Specially made Sparrow Chasers and Sparrow Shields can scare them off, but that sometimes has the unintended consequence of spooking native birds away, too. If you’re trying to specifically attract bluebirds, a nestbox made from PVC might do the trick, since house sparrows are reluctant to nest in them while bluebirds find them just right.

To avoid attracting them to feeders, the Sparrow Shields mentioned above can often double as feeder protectors. Switching up seed combinations from mixes made with mainly corn and millet to seeds like nyjer and black oil sunflower (both of which are goldfinch favorites) can sometimes encourage them to leave to find the food they prefer at another feeder.

There are other more aggressive methods for getting rid of them as well. House sparrows are invasive, which means that they aren’t protected under the Migratory Bird Treaty Act (an act designed to protect native bird species by making it illegal to harm or harass native species, including their nests, eggs, and chicks (though many species of ducks, geese, and other game birds have limited protection and as such can be hunted seasonally with the right permit). If you read YouTube’s Corvid Problem, it’s also the act that forbids people from owning wild birds without a permit). What this means is that there are no laws against trapping and humanely euthanizing them, although of course you should always check your local laws first, as well as make sure that you haven’t actually caught a native sparrow that looks similar to an invader like a song sparrow. Relocation of a house sparrow after capture is strongly discouraged, as this does nothing more than relocate the problem. House sparrows and starlings humanely euthanized without harmful chemicals are often donated to raptor rehabilitators and sanctuaries. There are traps like the Gilbertson House In-Box Trap and ground traps available for purchase online, as well as discussions on how to safely euthanize house sparrows without harming native birds or yourself, as well as not causing extreme unnecessary stress to the captured sparrow.

All invasive animals, from feral house cats to house sparrows, are a threat to the ecosystems they have been introduced to. But as much as we sometimes wish we could, we can’t turn back time and stop them from being introduced in the first place. The most we can do now is try to minimize their damage, try to learn from our mistakes, and hope that the violet-green swallows come back to our boxes.

For more information about trapping, euthanizing, or otherwise controlling house sparrows (and other aggressive nestbox invaders like European starlings), visit the official site of the Michigan Bluebird Society, the bluebird-focused forum Sialis, and the site NestWatch. (Warning: the Sialis forum in particular has links to accounts, pictures, and videos of house sparrow attacks on native birds that may be upsetting or disturbing to some viewers.)