Choose a gourd that is around 10 inches in diameter with walls around 1/4 inch thick. Harvest the gourd with shears and leave the stem attached when you initially cut it. Allow the gourd to dry outside for three to six months before you use it as a birdhouse.
Drill a hole in the side of the gourd that is 1 to 2 inches in diameter. Use a spoon to scrape the pulp and seeds out of the gourd. Sand the outside of the gourd and the edges of the hole with sandpaper until it is smooth.
Place a stopper in a sink and fill the sink with water. Add enough bleach to make the solution nine parts water to one part bleach. Soak the gourd in the solution for 10 minutes, rinse it thoroughly and hang it outside until it is dry.
Paint the outside of the gourd with a light-color exterior latex paint. A light color will keep the birdhouse cool during hot days. Once the paint has dried, spray a layer of polyurethane on the gourd to seal the paint.
Drill three small holes in the bottom of the gourd with a 1/4-inch bit. These will serve as drainage holes for the birds. Drill two holes at the top of the gourd to loop twine through.
Weave a 2-foot string of twine through the hanging holes and tie a knot at the end. Hang the gourd on a tree branch or another sturdy outside object that is at least 5 feet off the ground.