Since my wife wanted some larger birdhouses a few years back, we have had bluebirds hatch some chicks, along with Tree Swallows last year. We have one four outlet house on a post only a few feet from the kitchen window, and the one which I thought they wouldn't want to be nested in. Well, that one is the most in demand. But the last couple of years, the local Sparrow population has increased, and they try to control all the house locations.(four) I notice the bluebirds go after them once in a while, but have given up the last two years, and nested in one of the other less desirable houses. Meanwhile, the tree Swallows have shown me they are real survivors, moving into the main house the last two years, and giving the Sparrows a real chase that keeps them at bay for a while. I actually saw one of them land on the back of a Sparrow trying to enter one of the holes in the house, and drive it to the ground. It seems that if most bluebirds are that docile, I can understand why they are having population problems. In the meantime, I am trying to do my part in the war against Sparrows, by becoming pretty accurate using wooden dowels in a blowgun I made from 3/4 inch pvc. Plus I frustrate them by cleaning out their nest every week. The wife thinks I've gone wacko, but the Sparrows multiply like rodents because they have learned to live near us, which keeps the predators away.