This article has been adapted from the 蜜柚APP Birdhouse Book by Margaret A. Barker and Elissa Wolfson. You can also download a printable PDF of the instructions here. In the natural world, all three species of North American bluebirds—the Eastern, Western, and Mountain Bluebirds—seek tree cavities or woodpecker holes for nesting sites. But today, natural cavities can be hard to find. Competition for these limited sites is a huge problem, especially in early spring. Old and rotting trees often are removed. Not long ago, many bluebirds nested in wooden fence posts, especially around farms. Many of those have been removed or replaced with treated wood, plastic, or metal posts. A well-built and well-placed blue-bird nest box in your own backyard or nearby park can help boost local populations. Even with nest boxes in place, bluebirds must compete with both introduced and native species that also want to call these nest boxes their home. Knowing where to place and where...