There was a time when cavity-nesting birds like bluebirds or chickadees could find a natural cavity without problem. Woodpeckers did the work for everyone. Today, nesting cavities are hard to find. Nest boxes — bird houses — help fill that natural habitat gap.

Spring is nesting time, but if you want boxes in your yard or elsewhere, fall is a good time for the project. The birds likely to use the box — bluebirds, tree swallows, house wrens, and black-capped chickadees — likely will appear before the ground has thawed enough to accept a mounting post.

Birds by instinct make particular choices in nesting sites, for good reason. Given short lives on average, many small songbirds — the perching birds, also known as passerines — probably get no more than one or two nesting opportunities in their lifetimes. Success is critical.

Nest boxes look simple, but there are important basics to consider.

Choose a wooden box (cedar is good), or a round PVC box. The box is best unpainted, outside or in. Choose practical over cute.

The box should be tightly built, best assembled with screws. Important: the box must be readily opened. You will open the box many times in its lifetime. If you must use any tool other than your hands to do this, you have chosen a bad box.

The front panel should swing down to open rather than up. It gives you a better look inside. Why look inside? To see what's going on. The birds won't mind.

Knowing what's happening in there is much of the pleasure the box and its occupants can bring you. But views should be quick and days apart.

You also must open the box each fall to clean out the used nest. Closed boxes attract mice, cute guys that will stink up the box.

Entry hole size is critical. The basic bluebird box, with an entry hole 1 ½ inches in diameter works well for Eastern bluebirds, tree swallows, house wrens and black-capped chickadees.

A hole too small has an obvious problem. A hole too large has a predator problem, allowing paws to easily reach in. Larger holes also encourage nesting by nonnative house sparrows and European starlings. They should not be allowed to use the box. If they do claim a box, remove the nest.

The bottom of the entry hole should be no less than 5 inches above the bottom of the box.

Bluebirds and swallows prefer nest locations on open grassy spaces.

Chickadees and wrens will readily nest in almost any yard. You'll have the best success with a box near brushy or wooded areas, particularly with wrens.

To make life easy for you and difficult for predators, pound a 4-foot piece of rebar a foot into the ground. Slip over it a five-foot section of ½-inch electric conduit. Mount the box to the conduit with screws. Boxes for wrens can be hung.

In all cases, face the entry hole east or northeast, away from prevailing summer storms. If you are placing the boxes along roads, face them away from the road.

Information and build-your-own plans can be found online at: volunteer/janfeb06/nestbox.html

Boxes in our suburban yard are used by chickadees and wrens. We don't have the open space swallows and bluebirds prefer. The latter will nest in appropriate yards.