Chickadees, titmice, bluebirds, and wrens. Nuthatches, woodpeckers tree swallows, and violet-green swallows. Kestrels, screech-owls, and wood ducks. The common bond these diverse birds share is the habit of nesting in cavities. Nature provides cavities in a variety of ways. An ice storm breaks a limb off a tree or a bolt of lightning splits a tree trunk, leaving behind an open wound. Fungi invade the injury. Over time, the heartwood rots. Eventually a cavity-nesting bird claims the hole, perhaps a great crested flycatcher, a saw-whet owl, or a pygmy nuthatch.

Woodpeckers create most new cavities. They excavate new holes every year as part of their breeding behavior. Because woodpeckers create their own cavities, ornithologists call them primary cavity nesters. The ecological result is a steady supply of once-used cavities for secondary cavity nesters-such as wrens, bluebirds, flycatchers, and swallows-that rely on pre-existing holes.

Due to the effects of high winds, ice damage, insects, fungi, and a variety of plant diseases, there's never a shortage of dead, cavity-bearing trees. But because most people fail to see any value in them, such trees tend not to stand very long. One significant ecological consequence is that many cavity-nesting birds find it difficult to locate suitable nesting cavities. For decades wildlife biologists, ornithologists, and bird watchers have addressed this critical habitat shortage by providing and promoting the use of artificial nest sites-which most of us call nest boxes or birdhouses.

The use of nest boxes by birds is a remarkable conservation success story. Most cavity-nesting birds are common and require only suitable nest sites to maintain stable populations. Without cavities, their populations decline and may eventually disappear. Add properly maintained artificial nest sites-nest boxes-and their populations rebound.

The recovery of bluebird populations is a classic example of the impact of nest boxes. By the 1940s and 50s, all three bluebird species had been decimated by pesticide use and habitat destruction. The birds disappeared from many parts of their traditional range.

Thanks to the dedicated efforts of bluebird conservationists who promoted the use of nest boxes for bluebirds, trails of nest boxes appeared around the country. Though the pioneers of bluebird conservation are too numerous to mention, T.E. Musselman is credited with originating the bluebird trail concept. Lawrence Zeleny popularized it with his 1976 book, The Bluebird: How You Can Help Its Fight for Survival. Along the way, other cavity-nesting birds reaped the benefits of these new nest sites. Chickadees, titmice, wrens, and swallows used artificial cavities as eagerly as bluebirds.

In just 20 years, bluebirds rebounded from drastically reduced numbers to almost common status in many parts of the country. Since 1981, I have placed trails of nest boxes in Oklahoma, West Virginia, and Pennsylvania, and at each site bluebirds used the boxes within the first two years. Providing nest boxes is the easiest and most effective act an individual can perform to encourage cavity-nesting birds.

Nest boxes have also played an important role in restoring the wood duck. Overhunting and habitat destruction once threatened woodies with extinction. Stricter hunting regulations, habitat protection, and the use of artificial cavities rescued them. Today wood ducks are one of North America's more common ducks.

Bluebirds and purple martins are not the only birds that use human-supplied housing, though these popular species get most of the attention. When I began studying cavity nesters, bluebirds and purple martins were my primary subjects. Over the years, hundreds have fledged from nest boxes I have provided and monitored. But along the way I became enamored with the less glamorous cavity nesters-wrens, swallows, flycatchers, chickadees, titmice, and others.

Whether you buy your nest boxes at a store or nature center, or build them from scratch, there are certain characteristics your housing should have to best suit the needs of nesting birds. Here are some recommendations for nest box basics:

• The lumber used should be untreated and at least 3/4-inch thick to protect nests from spring chills and summer heat. Exterior plywood, cedar, and pine work well.
• Do not paint or stain the inside of the box. If you paint or stain the outside of the box, use a light earth tone color so sunlight and heat will be reflected, not absorbed.
• Use galvanized screws to assemble the nest box. It will last years longer than one built with nails or with glue, which will rapidly disintegrate as the wood warps and shrinks.
• Your access to the inside of the nest box should be easy for nest inspections and box maintenance. The best box designs feature a side or front panel that can be swung up or out to gain access to the inside.
• Perches on the front of a nest box are not necessary. All cavity nesters have strong feet and can easily cling to vertical wooden surfaces. Perches only give easy access to house sparrows, starlings, and predators.
• The roof of the nest box should extend well over the entrance hole to protect the opening from driving rain and predators.
• The inside front panel of the box should be deeply scored below the hole to give emerging birds a ladder for exiting the box.
• The floor of the nest box should have at least four 3/8-inch drain holes so the box can drain if it does get wet. The floor should be recessed (as shown at left) so that no end grain is exposed to soak up water in wet weather.
• Ventilation holes near the top allow excess heat to escape. Plug holes with weather-stripping putty in cold weather.
• The nest box should be protected from predators. The best way to protect your nest boxes is to mount them on poles with a predator baffle in place below the box.

Once you have acquired housing, you need to place it in the right habitat. Use the information in the species profiles section of this booklet to learn more about the birds you hope to attract, and use the nest box reference chart at the end of the booklet for habitat specifics.

Having selected a spot within appropriate habitat, make certain your nest boxes are securely mounted and baffled against predators (more about this topic later). Some species, such as martins and swallows, also require a clear flight path to and from the hole entrance.

You slowly open a nest box and see a black-capped chickadee sitting on its nest. What do you do? Simply close the box, walk away, and record in your notebook that the female was on the nest, probably incubating eggs.

This scenario underscores the reason nest boxes should be easy to open from the front or side. Spending several minutes disassembling a nest box to check its contents can be enough to drive a nesting bird away permanently.

If the female leaves as you approach, or as you open the box, inspect the nest quickly, record your findings, and leave the area. Your complete visit should take no more than 30 seconds. If you are able to keep your visit this brief, the female will return to the nest box shortly. You may even want to watch from about 30 yards away and time how long it takes for her to return to the nest. After the eggs hatch and the parents are bringing food to the chicks every few minutes, you may also want to record their comings and goings.

If you know the species using your nest box, check the species profile (beginning on page 17 of this booklet) for information about its incubation period. If you know the approximate date egg-laying ended and incubation began, you can estimate the hatching date of the eggs. This is important information. Timing is everything for a nest-box landlord. If you visit the nest during incubation or late in the nestling stage, you may cause nest abandonment or premature fledging. A good general rule is to limit nest box visits to the 10 days immediately after hatching. Resist the urge to visit nestlings-appealing as they may be-more often than necessary.