Birdhouses seem simple. Because people cut down dead and dying trees, cavity-dwelling birds can’t find enough nest sites. If we put up birdhouses, those birds can nest there instead, and we all live happily ever after. Right?
Unfortunately, no. Many birdhouses are home only to spiderwebs, moth cocoons or the occasional wasp hive. Other nest boxes are too popular, with birds fighting over them, sometimes even with casualties.
Some host nests but never eggs, or eggs that never hatch, or hatchlings that die or disappear before fledging. Before putting one up, consider whether, for whom, when, where and how.
At least a dozen bird species use nest boxes in western Massachusetts. A box half-full of wood shavings or chips caters to species that like to excavate their cavity a little, including chickadees, titmice and nuthatches. When woodpeckers opt for a birdhouse, they prefer it completely full of wood chips. Some, like tree swallows, benefit from ridges inside that help young birds climb to their exit.
Barn and cliff swallows, phoebes and robins don’t use boxes but sometimes nest on shelves or artificial clay nests. A larger box might attract a screech-owl if located near trees, a kestrel if placed in grassland, or a wood duck or hooded merganser if above water.
Not every species fits into the community. European starlings and English house sparrows compete fiercely for birdhouses. They often evict birds of other species, destroy eggs and kill chicks or even adult birds. Such non-native competition has accelerated declines of bluebirds and even kestrels.
Carefully measured entrance holes can exclude starlings, which are larger than most native birdhouse users; this does not help kestrels, though, and house sparrows are smaller. Because starlings and sparrows are nonmigratory, they start nesting earlier than most native birds, so you can block entrance holes until the native species begin seeking nest sites. This can also discourage bees, wasps and mammals from occupying boxes. Removing starling or sparrow nests is another option.
Although some native birds behave as aggressively as non-natives, federal law prohibits removing natives’ nests if they contain eggs or chicks, unlike nests of non-natives. Multiple boxes near each other sometimes allow coexistence; bluebirds, house wrens and tree swallows often compete for boxes, but a trio of boxes allows one pair of each to breed while repelling others of their species, thus shielding each other from competition.
In real estate, it’s said that the three most important things are location, location and location. For birds, location means habitat: Bluebirds will not use a nest box in the forest, nor will chickadees in an open field.
It also means height: Wrens will nest just 5 feet off the ground, while screech-owls prefer to be double that height or more. And location means neighborhood: Nest boxes near outdoor pets, busy roads or chemical-treated landscapes might be worse than no nest box at all.
Many birdhouses face increased risk of predators. Natural cavity nests lose eggs, chicks or even parent birds to predation. Artificial boxes are much more conspicuous than natural cavities; an occupied birdhouse is like a neon sign flashing “eggs or chicks here!” especially if mounted on a tree.
When considering bird predators, many of us expect cats, hawks or snakes; but in New England nest boxes, the most common predators are rodents: squirrels, chipmunks, even mice. Rodents will also occupy birdhouses themselves, pre-empting the birds.
Many people who have birdhouses have bird feeders too, which usually inflates rodent populations. Houses on poles rather than trees can use baffles to prevent predators from climbing to the box, while hole extenders make entering the hole and reaching the nest more difficult. A perch under the entrance hole looks cute, but another common nest predator, the blue jay, uses perches to more easily reach eggs or chicks in boxes.
Conventional wisdom suggests cleaning birdhouses every winter, to remove parasitic insects and mites that would infest next year’s nests. The facts are again more complex. Researchers have found inconsistent results: Birds prefer cleaned birdhouses in some cases but not others. One study found more fleas in cleaned nest boxes than in never-cleaned natural cavities. Boxes often produce similar numbers of healthy fledglings, whether cleaned or not.
Some experts attribute these results to parasitoid insects and tiny predators that also live in old nests. These natural enemies can’t survive without prey, so the birds’ parasites get a head start infesting new nests until the predators and parasitoids catch up with them. Cleaning birdhouses can thus force parasites’ enemies to continually lag behind.
Natural nest cavities are always the best option. Standing dead and dying trees benefit birds and other wildlife far more than any nest box. But if you have to settle for boxes, visit NestWatch.org, the Cornell Lab of Ornithology’s citizen science program focused on breeding birds, which has extensive details on how to build, place and maintain houses for a wide variety of species.
Mass Audubon also has birdhouse information on their website, which is more specific to our state. Getting it right isn’t always easy, but the birds will appreciate it.
Joshua Rose is a naturalist who lives in Amherst. He is a Hitchcock Center member, a board member of the Hampshire Bird Club and a contributing editor of the website bugguide.net; he regularly leads programs for local nature-oriented groups.
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