Approximately 85 species of North American breeding birds select or construct a cavity for their nest. Populations of many of these species have declined in recent times as forests are harvested early in their life-cycle to maximize profits and avoid the dead and dying look. In urban forest settings, a tree is usually removed if it even looks diseased or dying. Dead trees are almost never left in a suburban yard. These dead and dying trees (snags) are common in a natural forest and provided the substrate for nests in the past. There are two main groups of cavity nesters. The primary cavity nesters, like woodpeckers, excavate their nesting cavity. These species usually excavate many more cavities than they use for nesting. The secondary cavity nesters, like bluebirds, use cavities that have been previously excavated by a primary cavity nester or a cavity created by decay in a dead or dying tree.
Nesting box programs are primarily for the secondary cavity nesters. In addition to the joy of having a bird nest in your yard, populations of species like bluebirds, Tree Swallows and the Purple Martins have increased by utilizing boxes provided by larger scale nest box programs. Secondary cavity-nesters range in size from the chickadee to the Turkey Vulture so a lot of choices are available. So far, I've not encountered anyone trying to attract Turkey Vultures to their yard but many of us build and maintain chickadee, bluebird, or wren nest boxes.
National Audubon Society maintains computer links providing the specifics for building a nesting box for the species of choice (audubon.org/educate/expert/birdhouse.html). Following the text of this lesson, I will provide box sizes and other specifics for a few favorites for the Ogden, Utah area. But first a few hints to improve success in attracting and providing the necessities for a bird species. These efforts will be rewarding in your watchable wildlife pursuits as well as improving the productivity of our wonderful bird diversity.
Before a nest box design is selected several questions need to be answered. First, what bird species provide the best opportunities for your area? To make this choice, you must become familiar with the cavity-nesting birds in your area. In this case, ďarea" is a combination of geographical location, surrounding habitat types, and elevation (in the west). A Carolina Wren would not be expected to nest in a box in Utah and a Mountain Bluebird would not be expected in the southeast. Once a target species is selected, based on range, habitat, and opportunity, then the correct box design is selected. Remember that only the "nursery" is provided by the box. The box also needs to be close to the "kitchen" so that adequate food can be acquired.
Placement of the box is the next decision. Birds are adaptable so in many cases placement is not critical. The box does need to be accessible to you for cleaning each fall. Some species will build a new nest on top of an old nest but for best results the box should be cleaned each year after the nesting season. Other placement decisions are based on the species, the habitat and the geographical location. For example, species like the American Kestrel like nesting boxes placed approximately 20 feet high, whereas bluebirds prefer boxes at a 4-7 foot height. My favorite backyard bird, the Black-capped Chickadee, seems to prefer a box about 15 feet high. A lot has been speculated about the direction (north, south, east, west) the entrance hole should face. Personally, I haven't found consistency enough to make a recommendation. I suggest using common sense. Generally, in southern ranges the hole should face north or east to reduce heat loading during hot afternoons. In northern ranges, a south facing hole might actually help keep the inhabitants warm. If the box is in open habitat, heat loading is a greater risk than if the box is in a forested or shaded setting.
There are a few nest box placement options that should be avoided. First, the entrance hole should not face a busy street or highway. The adults flying to and from the box risk being hit by a fast moving car. Second, the box should not be in a major human use area. Adult birds are reluctant to select a box in an area with lots of disturbance. If the box is selected, the adults may not have adequate opportunity to make enough feeding trips to the box to maintain healthy young. If in an urban setting, the box should be near escape cover so when the young fledge they can hide and not be eaten by a house cat before developing strong flight capabilities.
Another consideration for a successful nest box experience is the likelihood of predation. Some nests are always destroyed by predators so complete protection is probably not practical. However some precautions are often warranted. First, be precise on the entrance hole size. Bluebirds will nest in box with a 1 9/16 inch hole. Any bigger the hole will allow the European Starling to evict the bluebirds. The same is true with chickadees. If the hole is larger than 1 1/8 inch, House Sparrows will probably be using the box. If an entrance hole is enlarged by a woodpecker or a squirrel then the front panel should be replaced to correct the hole size. Other predators, such as the raccoon, squirrel, Blue Jay, and Black-billed Magpie, often require some special consideration. One box design I've seen is about 6 inches square (chickadees to bluebirds) and about 18 inches long. A baffle (2 inches high) is placed about 5 inches from the back of the box to encourage birds to nest behind the baffle, but well out of reach of a raccoon or squirrel. If snakes are a problem, a metal shield around the tree or pole holding the nest box will often serve as a deterrent.
Some non-cavity birds can be enticed to nest in a box. For example, an American Robin will often build a nest in a box with a 6x8 inch base and 8 inches high if the front panel is omitted. Also, Barn Swallows will often be attracted to a 6 inch piece of 2x4 if the 2x4 is nailed to a smooth wall about 6 inches below the eaves of a building.
In summary, nesting box success is often related to a few simple decisions related to box size, entrance hole size, box placement, predator proofing, available feeding habitat, available escape cover, avoidance of known hazards, and luck.
BOX SIZE SUGGESTIONS
(All measurements in inches)
* 3 inches high and 4 inches wide (oval)
|Chickadee species||1 1/8||4x4|| 6-8|
|House Wren||1 1/4||4x4|| 6-8|
|Mountain Bluebird||1 9/16||5x5|| 6|
|Tree Swallow||1 1/2||5x5||6|
|W. Screech Owl||3||8x8||9-12|