You will find a lot of birdhouses for sale in stores, and they are cute and attractive, but are they safe for bird?  There's  nothing wrong with decorated birdhouses as long as they still have the features listed below. I have found that birds tend to be attracted to the plain unpainted houses rather than the highly decorated ones. This is probably because they prefer a safe unobtrusive nesting area rather than one that attracts attention.

Here is a list of important features that every birdhouse should have:

Entrance Hole Size Most common hole-nesting birds can use an entrance hole 1 1/2 inches in diameter. This size keeps larger invasive species such as Starlings which like to take over nests from local native species. For a list of hole sizes for various birds, click Here.

No Perch If you stop and think about it, tree holes in the wild have no perches, so the birds that use nest boxes don't need them either. They can actually be harmful because invasive and aggressive species will perch outside a nest and harass a nesting bird as a means of taking over the nest.  
                                                                                                 Floor Dimensions The inside dimensions of the box are important and should be at least 4 inches by 4 inches so that there is room for the young to develop.

Box Height The distance from the bottom of the entrance hole to the floor of the box should be at least 5 inches. This keeps the developing young well down in the box and away from predators that might approach the entrance hole.

Be Able to Open You must be able to open the box, either on the side, front, or top. This helps in two ways: to monitor the progress and health of the young; and to clean out the box at the end of the season.

Ventilation There must be holes or slits at the top of the box, the sides, or the front of the box to let hot air out when the sun beats down on the box in summer. Remember, birds get hot just like we do.

Drainage Holes Nesting birds can be very messy. The bottom of the box needs to have holes or cut off corners to allow any water or waste to drain out of the box.

A Secure Way to Attach the Box Make sure there is some way that you can attach the box to a pole, or tree such as holes or a bracket.

Should Be Wood Be sure that the materials the box is made of are 3/4 inch thick wood or a similar material that will insulate the birds from cold and heat. Many birdhouse plans call for 1" x 6" boards which means that the finished thickness is the required 3/4". I always try to use cedar wood if possible. It does not require painting and will withstand the harshest weather for a very long time.

Roof Overhang The roof should have an overhang above the entrance hole by 1 to 2 inches. This both shades the entrance hole and keeps the rain out.