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The availability of suitable nesting sites is a vital requirement of all birds. By placing birdhouses in backyards, people can help provide these nesting sites and attract a greater variety of birds to our yards. Putting up nesting structures is easy, and many common birds that visit backyards will utilize artificial boxes and shelves. However, some knowledge of the nesting requirements of birds is necessary in order to maximize your (and the birds') success with birdhouses.

NESTING REQUIREMENTS
It is helpful to first outline some of the different types of nesting birds and their needs. Two of the principal groups of nesting birds are cavity nesters and open nesters.

Cavity nesters require cavities, usually in wood, in which to build nests and lay eggs. Bluebirds, chickadees, and woodpeckers fall into this category. In contrast, open nesters place their nests among the branches of trees or shrubs, utilizing a sturdy crotch or small branches to support the nest. Robins, jays, and cardinals are among those that build open nests. Almost all birds build nests strictly for the purpose of raising young. Once the young have fledged, the nest is no longer used by the young or adults. It is thought that birds avoid re-using old nests in order to escape nest parasites, such as feather mites.

Open nesting birds can be attracted to a backyard by planting a variety of native trees, shrubs, and vines. Ask for Massachusetts Audubon's "Landscaping for Birds" information sheet for details on how to develop a landscaping plan for backyard birds. Cavity nesters can be attracted to a backyard by leaving dead limbs and trees (snags) in place, and by providing artificial housing. Providing artificial nesting structures for backyard birds is the primary focus of this information sheet.

THE CAVITY SHORTAGE
Woodpeckers regularly excavate their own cavities for nesting. Most woodpeckers seek out dead or dying wood for excavation, although a few of the larger species, such as sapsuckers and the Hairy Woodpecker, can drill into live wood. Many other cavity nesters, including bluebirds, wrens, and nuthatches, usually have to use abandoned woodpecker holes because they do not have the strong bills necessary for drilling.

Cavities are a limited resource and there is strong competition for those that exist. This is one reason why some cavity nesters respond so readily to artificial nest boxes. Cavities are also used by birds for night roosting, and they can provide valuable shelter during extremely cold winter nights. Downy Woodpeckers, for example, spend several weeks every fall preparing multiple cavities for use as roosting sites. This is also the time of year when woodpecker damage to houses is most likely to occur. For advice on how to deal with this unusual problem, ask for Massachusetts Audubon's "Living With Woodpeckers" information sheet.

TERRITORIES
Most backyard birds defend an area around their nests that usually includes feeding habitat. Territory size dictates how many pairs of a particular species will nest on your property. For some birds, territories are quite small. A robin, for example, may defend a territory of less than an acre, so a small property may support several pairs. In contrast, chickadees may defend breeding territories of up to 10 acres. This will certainly limit how many chickadees will nest in a typical backyard. Notice that territory size does not necessarily depend on the size of the bird. Rather, territory size depends on a number of environmental factors, including the type of food source utilized by the bird.

The concept of territory is the main reason why birdhouses should be constructed for a single pair only; multiple units are useless to most birds. The only exception to this rule is the Purple Martin, a type of swallow that nests exclusively in colonies that often contain dozens of pairs. Houses for martins usually consist of multiple apartments for many pairs. Unfortunately, Purple Martins occur at only a few disjunct breeding sites in Massachusetts and are notoriously difficult to attract to new areas. Do not attempt to build and erect a martin house unless you are sure there are martins in the area. Otherwise, it will most likely become filled with House Sparrows.

In general, most backyards will support several pairs of different species. Each species will keep others of its own kind out of its territory, but will not usually be bothered by different species.

GENERAL GUIDELINES
The first decision to make is whether you are going to buy or build a birdhouse. If you decide to buy, ready-made houses are widely available, but they are not all equal in quality. Before buying, it is helpful to know which species you wish to attract and what the nesting requirements are.

First of all, birdhouses should be made of wood. Wood is both durable and porous enough to allow moisture and heat to escape during the summer, which keeps the young birds from getting too hot inside. Metal or plastic houses can be death traps in such heat. The dimensions of a birdhouse are also important, especially the size of the entrance hole. The entrance hole should be just big enough for the intended bird and no bigger. Chickadees will not use a house with a 3-inch entrance hole, for example. In addition, a birdhouse should always have a removable panel, whether this is the front, side, or top, so that old nests can be removed and the inside cleaned.

Also, birdhouses should not be painted but left untreated or stained on the outside only with a natural wood preservative such as linseed oil. Bright, unnatural colors will not be accepted by the birds because they make the house more conspicuous to predators. Finally, birdhouses should not have perches on the outside. Most birds do not need them, and they only make it easier for predators and House Sparrows or starlings to get into the house.

BUILDING A BIRDHOUSE
If you decide to build your birdhouses, obtain 3/4 inch untreated pine, white cedar, or poplar. Do not use creosote-treated lumber because the residues are toxic to birds. The outside surfaces of birdhouses can be treated with linseed oil. The house will last longer if it is assembled with brass hardware rather than steel, which rusts. Screws, although more expensive and requiring predrilling, are better than nails since they are less likely to pull out with time. Some experts recommend using rough-sawn wood, which is easier for birds to grip with their feet. This is important for birds such as Tree Swallows, which can have difficulty getting out of houses if the inside surface is too smooth. As an added measure for these birds, it is advisable to cut grooves on the inside front panel of the house or to add a small strip of hardware cloth on the inside under the entrance so that the birds have an escape ladder to help them get out. All houses should have drainage holes in the bottom and holes in the side panels near the top for ventilation. See accompanying diagrams for construction details.

To be most successful, birdhouses should be constructed for particular species since many species have specific requirements for hole size and inside area. However, if you do not have particular birds in mind but just want to attract something, you can construct multispecies houses that will suit a number of similar-sized birds. For example, a house with 4 inch square inside area and a 1 1/4 inch entrance hole will suit wrens, chickadees, titmice, and nuthatches. Place several of these houses in different habitats in the yard and see what shows up! The accompanying chart has dimensions for four different multispecies houses.

BIRDHOUSE PLACEMENT
Birdhouses should be erected preferably in the fall or early winter so that birds will have plenty of time to locate them before the breeding season. The houses will also be well weathered by then. It is also a good idea to leave the houses up year-round so that they can serve as roosting sites in the winter. When deciding on the placement of the houses in the yard, keep in mind the habitat requirements of the species you wish to attract. For example, bluebirds will use houses in open fields but not deep woods, and chickadees the opposite.

Consult the accompanying chart for habitat requirements before placing the houses. A general rule is to place no more than 2 houses per species per acre of property. Remember, however, that an acre of property may support several pairs of different species.

PREDATOR GUARDS
Birdhouses are safest from predators when they are mounted on poles equipped with predator guards. A predator guard is a device placed on the pole that keeps raccoons, cats, and even snakes from climbing the pole. Predator guards are commercially available and can also be fashioned at home (see Shalaway [1995] for plans). Pole-mounted houses will need an extra flange made of wood or metal attached to the bottom that will accept the pole. You may wish to purchase the pole first and then design or buy the appropriately sized flange and predator guard. A predator guard consisting of an extra thick piece of wood can be placed over the entrance to give some protection from the reaching arms of mammals such as raccoons.

All houses, whether attached to a pole or tree, should be securely fastened so that they do not swing since birds do not like moving houses.

Some birds such as nuthatches and woodpeckers prefer houses mounted on trees. The house design in the accompanying diagram includes a backboard that can be used for attachment to a tree. Houses should be mounted with the entrance hole facing slightly downward and away from prevailing winds to keep rain from entering the house.

NESTING SHELVES
Nesting shelves for robins and phoebes should be located on buildings, preferably under existing overhangs such as on a porch. Mount the shelves where they will be away from too much activity. Barn Swallows are most likely to use shelves placed in a barn or other large, open structure where there is plenty of flying room. This species often nests in small colonies consisting of several pairs. A nearby supply of mud for nest construction is also necessary to attract this species.

MAINTENANCE
Houses should be cleaned out once a year at minimum. The best time to do this is after the birds have finished breeding, usually by mid-August. Remove old nesting material and scrub the house with a solution of one part bleach to nine parts water. Rinse the inside well and allow it to air dry completely before closing it up again. This is also a good time to look over the house and make sure the hardware is still firmly in place and the panels are not pulling apart.

Some birds, such as wrens and bluebirds, are multiple brooded. This means that each pair typically raises more than one (usually 2 to 3) families per summer. For these species, cleaning out the nesting material between broods is recommended to reduce nest parasites. This means that you have to keep a close eye on the birds so that you know when the young fledge (leave the nest). Although a few studies have shown that bluebirds prefer houses with old nests, it seems preferable to remove nests that may be heavily infested with parasites that can weaken or kill baby birds.

PROBLEMS
Things do not always go as planned when we put out birdhouses. Most problems arise from competition between different species for a house. Both House Sparrows and European Starlings are fierce nest competitors that are capable of driving away many native bird species. Starlings can be eliminated from houses with an entrance hole of 1 1/2 inch or less. If starlings get into houses with larger entrances, continual removal of the nesting material may dissuade them. Unfortunately, starlings are tenacious, so repeated efforts may be required. If starlings succeed in taking over a house, try erecting another house nearby so that a different species has a chance to use it.

House Sparrows generally present more of a problem than starlings because they can squeeze into smaller holes. Only a 1 inch entrance hole excludes them; however, it also excludes all other species except for House Wrens. Continual removal of the nesting material sometimes convinces them to move. House Sparrows also will not use houses mounted in woods. When sparrows move into bluebird houses, it helps to lower the boxes to a height of less than 5 feet. Sparrows do not like to nest this close to the ground, but bluebirds do not seem to mind. Competition from House Sparrows will likely be a problem whenever houses are placed in open areas.

Bluebirds also face competition from Tree Swallows since the two species inhabit similar areas and use the same size nesting cavities. Because the swallows are more aggressive than bluebirds, the bluebirds are usually driven away from nesting sites. To alleviate this problem, mount bluebird houses in pairs placed 10 to 12 feet apart. If a pair of swallows moves into one, they will keep away any other pairs of swallows that try to nest but will ignore a pair of bluebirds nesting nearby.

Sometimes birds nest in places that seem perilous to us. Robins and phoebes often build nests over frequently used doorways. Wrens have been known to build nests in clothing hanging on a line and in cars parked in one place for a long time. House Finches are likely to nest in hanging planters on porches. While these places seem dangerous and inconvenient to us, it should be remembered that the nesting instinct in birds is quite strong and that, more often than not, pairs with unlikely placed nests are successful in fledging young. The best thing to do in these situations is give the birds a little space and time. Use a different doorway if there's a nest overhead, and water hanging plants containing nests at the edges (but do keep watering them: the plants must be alive to provide shelter for the nests inside!) The birds usually finish their nesting cycle in about four to six weeks so human cooperation is only required for a relatively short period of time.
Diane M. Lahaise

Commercially made birdhouses available at:
Massachusetts Audubon Gift Shop
Drumlin Farm Wildlife Sanctuary
Route 117, Lincoln, MA
781-259-9661
Hours: Tuesday-Sunday, 10 a.m.-5 p.m.

Download the Birdhouse and Nesting Chart (PDF), which include birdhouse specification by species.