The tree swallows are not large, but they are larger than chickadees, and will probably prevail. Advertisement Continue reading the main storyOnly a minority of birds are cavity nesters that would compete for a backyard nest box. One very aggressive species that can win out in many cases is the tiny house wren, which starts to nest late, when all the spots are taken. Sign Up Receive occasional updates and special offers for The New York Times's products and services. Many anecdotal reports of competition for nest boxes can be found on a site maintained by Bet Zimmerman, a longtime advocate for America’s three bluebird species.

Source: New York Times April 17, 2017 19:30 UTC