Whether you buy a bird house from a retail outlet or make your own, several factors increase the likelihood of successful flooding. Some bird box designs are known to improve the likelihood of brood success while others may contribute to the unharmed demise of the nestlings. A good quality bird house should have both of the following characteristics.
1. Fledgling ladder or textured front wall
Inside the front wall of every bird house, nestlings need a ladder or textured surface to dig their tiny claws into. They must be able to climb to the entry hole in order to fledge. If your bird houses are made of very smooth materials, it will be necessary to etch horizontal grooves, forming a 'ladder' for the fledgling to climb. Simply gouge tiny lines about 1/2 "to 1" apart, using a chisel, wood carving knife, or other similar instrument, in a horizontal pattern from the bottom of the box to the entry hole. You can also attach a 4 "x 4" square piece of window screen or other fine mesh using wood staples or carpenters' glue. Just be sure the mesh is not too rough and that staples do not protrude. If you use glue, be sure it's completely dry before allowing birds to occupy the house.
2. Smooth and safe entry holes
Birds are diligent about keeping their feathers in excellent condition. Their lives depend on it. If the entry hole is jagged or has any sharp protrusions, this will damage feathers each time a bird enters or exits the box. Run your fingertip around the entry hole of your bird house. If you feel any rough spots, sand them off with fine-grain sandpaper. This should be done every year when you clean out the old nest. If the entry hole has been compromised by a squirrel, you may be able to affix a hole guard over top. These can be purchased or made in your own shop simply by cutting a thin square of wood and drilling the appropriately sized hole inside it. Attach the guard over the old, damaged hole with screws, being careful to keep screws from penetrating to the inside of the box, where they can present a feather hazard. Alternately, you can attach the guard with carpenters' glue.
3. Ventilation & Drainage holes
Without a flow of air, bird boxes can become too hot or the air may not be healthy for young birds to breathe. The entry hole itself does not provide sufficient fresh air so you need to add a few ventilation holes if they are missing in your design. Ventilation holes should be located near the top of the box, preferably under the protective lip of an overhanging lid.
If the nest box has holes in the bottom, those are not for ventilation; They are for drainage. These too are essential. Drainage holes prevent boxes from filling with rain and help keep nests dry if they become damp. Some species, like bluebirds, do not mind if their nests become wet periodically. Check for holes in the floor of the box. They may be located in the middle or at the corners. If there are no drain holes, make some with a drill. Drainage holes can be 1/8 "to 1/4" in diameter. Sometimes holes will become clogged with nest debris so be sure there are enough holes to allow for that possibility.
4. Properly sealed roof seams
A good bird house design should not have leaky roof seams but do not despair if you have one that does. If there is a roof seam that allows water in, it may be okay for bluebirds, providing only a few drops of water get in during a heavy rain storm. However, if a lot of rain gets in, the box may fill faster than it drains, drowning or chilling the unfortunate nestlings. Inspect roof seams for potential leaks and fill them with a bead of exterior grade caulking. Be absolutely sure, caulking is completely dry before putting it outside. If a bird gets caulking materials or glue stuck to its feet or feathers, it may result in death.
5. Human Access for Cleaning
All bird houses should be properly maintained. This can usually be done once a year, after the failures have left. In order to do this, you need to gain access to the interior of the box, without having to take it apart or break it. Some decorative bird houses may not have this feature as they are purchased more for ornamentation than for practical purposes. The best designs open on the sides or the front. This is easier for you and the least hazardous for wild birds. Where the floor of the box is the access point, there is a risk of it opening accidentally and dislodging the occupants. Where the entry is the roof, there is greater risk of the occupants getting wet in a major rain storm, though this is easily preceded by following the steps in # 4 above.