A bit of “No Man’s Sky” humor for anyone who is familiar. But this tree swallow is an interloper, trying to elbow in on eastern bluebird territory. The bluebirds were working on – or already had established – a nest in this box, and were less than pleased about the swallow’s presence atop their home. After a few minutes of posturing and near-combat, the bluebird pair drove away the swallow and preserved their territory.
Competition for territory and resources is part of nature, and bluebirds and swallows share preferences for nest box styles and overall locations, which often leads to competition with each other. I’m certain there are other instances where the swallows prevailed, but this box at least remained under bluebird control.