How to pick a birdhouse to draw the birds you want

"What should I look for in buying a birdhouse?" a friend asked recently.

It was a good question and I wish I'd had a more straightforward answer than "It depends."

The choice of a birdhouse (aka nest box) depends, first and foremost, on what birds you want as tenants, or at least what size birds. Are you hoping for tiny chickadees or house wrens, or larger birds like bluebirds or even flickers? Making this decision upfront, as opposed to buying a nest box and waiting to see what shows up, is a good way to preselect tenants.

Choosing a box with the right-sized entrance hole for the birds that you want to host is the key, because the largest bird able to fit through the entrance will almost surely take over the box.

If you want chickadees to raise their family in your house, but you choose a nest box that allows house sparrows or even starlings to enter, then you probably won't have chickadees.

I see this time and again on my bluebird trail, where nest boxes and their entries are sized specifically for bluebirds, with an oval opening that's 2¼ inches high. Each spring, chickadees begin building nests in some of the bluebird boxes several weeks before bluebirds and tree swallows are ready to set up housekeeping. But when these larger birds, both of which fit easily through the entrance holes, decide that it's nesting season, they evict the smaller birds, sometimes building right over a nest with chickadee eggs in it.

Another major consideration in providing housing for birds is construction materials. Wood, especially cedar, is a good choice, as are recycled plastics. Avoid metal boxes, as these become bird ovens on hot days.

And then there's choosing the right site. Although many of us hang a birdhouse on the end of a tree branch, few bird species tolerate being rocked in the wind (the exception being house wrens). Nailing houses to tree trunks, another popular choice, is not a good idea, either.

A house hanging from a branch or nailed to a trunk is like a dinner bell to predators, always on the lookout for an easy meal. Good birdhouse setups need predator guards, such as sheet metal wrapped around the trunk, to stop tree-climbing raccoons, squirrels and cats in their tracks.

The best housing for backyard birds is a nest box attached to a free-standing pole fitted with a device to stop predators, such as a "witch's hat" metal skirt. This may be a bit more work than hanging a box in a tree, but it's the best way to protect birds.

There are many birdhouse styles available, some of them utilitarian, others decorative, even fanciful (farmhouses, castles, cat faces, etc.). These aren't bad, as long as they're well built and have the appropriate-sized entrance hole. Avoid any that provide a perch on the front of the box — native birds don't need them but these almost guarantee that house sparrows, which are very fond of perches, will take over the box. You don't want to make life easy for an alien species that aggressively drives off or even kills birds such as chickadees and bluebirds.

I hope this isn't discouraging, because observing nesting season and watching young birds flitter around the backyard is so much fun. But in order to do more good than harm, avian landlords need to think outside the box.

St. Paul resident Val Cunningham, who volunteers with the St. Paul Audubon Society and writes about nature for local, regional and national newspapers and magazines, can be reached at valwrites@comcast.net.

Surprising as it seems, not very many songbirds will use birdhouses. Most songbird species aren't cavity nesters, even though cavities confer an advantage in terms of protection from weather and predators. Instead, most build their nests on tree limbs or in dense shrubbery, and nothing we do will coax them into a nest box. Birds such as cardinals, goldfinches and orioles are hard-wired to build nests in the great outdoors.