House Sparrows are very clever and very persistent. To deter sparrows from moving in to a bluebird house you must repeatedly remove any nests that they have built and leave the clean out door of the house open until the sparrows give up guarding the house.
If a bluebird family has already started to make a house and sparrows are harassing them you can put up a sparrow spooker. Basically once the bluebirds are committed to a nesting site you can hang shiny flutter ribbon above the birdhouse (you can find this “scare tape” at our stores). Studies have shown that certain bird species, including house sparrows, will not fly under the ribbon. For more detailed plans to make your own sparrow spooker, click HERE to visit the very informative Sialis.org website.
Another possibility is to put two bluebird boxes within a few feet of each other. Bluebirds defend large feeding territories around their nests from other bluebirds. Experts recommend that bluebird boxes be spaced at least 300′ apart. However when you pair bluebird houses within 10 feet of each other, it is possible to get a bluebird in one and a Tree Swallow in the other. Together the birds can coexist and battle any predators or interloping sparrows. More information on this technique can be found.
Bluebirds and other cavity nesting birds typically begin nesting in March depending on where you live. But they usually have more than one brood per season and may switch to a new site for their second brood. Or if the birds’ first nesting is unsuccessful, perhaps due to sparrow or other predators, they may move in to your nest box. Don’t lose hope.
You can put bird houses up year round because some birds will even use the boxes as roosting sites in the winter.