Many bird enthusiasts purchase a birdhouse at a local discount store and later find that it does not attract any birds. Wild bird watchers are often tempted to select a birdhouse based upon its decorative appearance. They want it to serve as a garden accent that happens to attract birds. Many times, however, these birdhouses lack the specifications and features birds require for nesting. Careful consideration of the following specifications can make the difference between an unused garden accent and a backyard full of song.
There are three main sizes to consider: entrance hole size, floor size, and entrance height.
Entrance hole size
A 1-1/2" diameter opening is probably the most common size, and most songbirds can use this opening size. Plus, it keeps out more aggressive non-native birds like house sparrows and starlings. Some birds can use an even smaller opening. It's good to know which species you want to attract, then research the minimum opening size the species requires.
Birds need a certain amount of floor space inside the house so the hatchlings have room to grow. For smaller songbirds like wrens, chickadees, titmice and nuthatches, 4" x 4" is usually fine. Larger birds need more floor space. Again, look for your target species' requirements.
2. Heating and Cooling
Just as in human homes, a birdhouse needs proper ventilation for heat escape in summer and thick walls for heat retention in winter.
Ventilation and insulation
Each birdhouse should have a slit or opening at the top of at least one side so hot air can escape during the summer. For the best insulation, most birders agree that wood makes a better birdhouse material than metal. It won't heat up as much in the hot months of summer, yet keeps warmth in during winter's cool months. Find one at least 3/4" thick. Though not usable for roosting in winter, clay or ceramic birdhouses insulate well in the warm months of summer.
3. Proper Drainage
Make sure your birdhouse has drain holes in the bottom to let out water. This is important to keep mold and bacteria from multiplying.
Also, an inch or two of roof overhang will keep rain from entering the birdhouse entrance hole and will also help shade the interior on hot days.
Safe birdhouses have a number of features that help protect the inhabitants from predators.
Perches may look cute, but they simply give predators something to hold onto when approaching the entrance hole. Or, they invite invasive species to nest, which most people do not want. Since natural cavities such as tree trunks do not have perches, birds that will use nest boxes don't need a perch either. Another way to deter predators is to pole mount the nest box. Predators have a more difficult time climbing a thin metal pole than a tree trunk.
Enforcement around entrance hole
Some birdhouses feature a metal bracket around the entrance hole, which deters squirrels and other animals from enlarging the hole to gain access. Others have a thick piece of wood, with a cutout the same diameter as the hole, which in effect makes the entrance longer (called a predator guard) so that predators cannot scoop down into the box as easily.
The best birdhouses are not finished with toxic paints or stains. Natural wood is the best choice for preventing the birds from having contact with anything toxic.
5. Mounting and Access
Your birdhouse must also keep your lifestyle in mind. It should be easy to mount, easy to clean and maintain and, if possible, easy for you to monitor the activities inside the box when practical.
It's easier to mount a birdhouse that includes a bracket or hanger for pole mounting or hanging. It simply saves you the time and trouble of trying to figure out how to install mounting hardware onto the birdhouse.
One side that opens
Birdhouses that you can open give you two advantages. You can keep them more sanitary because you can access the inside more easily for cleanout. Also, depending upon the style of opening, you can have a look at the activities inside the box, when practical.