Early spring is the best time of the year to put up birdhouses. Though the prospective occupants may not have arrived on the scene yet, it is a good idea to put up the houses early to weather a little, and be available when the birds arrive.
There are about 30 species of birds in each region of the country that will use cavities in which to nest. A great many of them will consider using birdhouses. That means there is a variety of birds that require various size nesting houses, with various diameters of entrance holes. They range from a wood duck house that is 10 x 10 x 24 H, with an elliptical entrance hole of 4 x 3, to a wren house that is 4 x 4 x 8 H, with a 1-1/8th round entrance hole. For these and many other birdhouse dimensions, visit eNature's Feeders & Houses section. Where the houses are placed is also important. Wood ducks like to be in a tree near water; house wrens like houses in small backyard trees, while bluebirds look for houses on fence posts that face open fields where they can find insects to feed their young.
Earth colors are best for birdhouses, in order to blend into their habitat.
Because many birds return to the houses in which they were raised, or where they nested the previous year, its best to get the houses up early.
-- George H. Harrison