I’ve had numerous encounters with Tree Swallows in the five years spent monitoring a nest box trail. These birds are highly defensive when it comes to their nests and will “dive bomb” you without hesitation when you get within 20 feet of the box or cavity. I’ve never been injured by them but Swallows definitely upped their status with me because of their courage and fortitude.
If you’re attempting to create homes for Bluebirds, expect these natives to compete for the nest box. That said, put the boxes up in pairs about 20 feet apart so that both the Tree Swallows and the Bluebirds can be accommodated.
To learn more about Tree Swallows and their Spring migration, listen to this broadcast of BirdNote.