Here is their advice:
• Location, location, location. In most cases, place the birdhouse 5 feet or more off the ground and at least 20 feet from feeders or birdbaths. Choose a spot without a lot of foot traffic so the young family can have some privacy.
• Aged wood. Most cavity-nesting birds prefer weathered natural wood; it mimics tree trunks where they would otherwise nest. Use lumber at least 3/4-inch thick to help insulate the box from hot or cold weather. Rough, raw wood on the inside of the box gives babies a foothold to scramble up to the opening, so don't paint or smooth the interior. A few square inches of wire mesh or recessed grooves below the doorway also are helpful to baby birds trying to climb up.
• Cozy space. Think of a hollow tree; that pocket is snug and deep. Cavity-nesting birds like their houses that way, too; 5 by 5 inches at the bottom is an ideal dimension for most species, with a depth of 5 to 6 inches below the entry and an overall height of 10 to 12 inches. Robins like an open-face box — almost like a shelf — with no front wall.
• Single occupancy. While martins like apartment complexes, other birds are territorial about their nests. They want a home of their own without noisy neighbors.
• The right-size entry. Different-size holes attract different kinds of birds. They want an entry large enough to get in but small enough to keep larger birds or predators out.
• Slanted roof. That allows rainwater to run off easily. Make sure it extends over the entry.
• That weathered look. Often, birds won't use a new house until it shows some age. Weathering also softens up the wood, making it easier for young ones to grab hold, so leave the house up year-round.