Their name — Tree Swallow — comes from their preference of nesting sites in tree cavities and not from the bird’s usual habitat, which consists of open, treeless areas. Tree Swallows also use nesting boxes, if available. And that leads to conflicts with Eastern Bluebirds, another cavity dweller found in fields in Central Ohio in the spring.
I’ve watched ownership of a nesting box change hands multiple times within minutes. A bluebird will be standing on top, adopting a threatening pose whenever a Tree Swallow flies near. But eventually a swallow will be successful in its efforts to claim the box, chasing off the bluebird and replacing it on top of the box until a bluebird is successful in chasing away the swallow. The process is repeated many times each spring day.
That is why many parks in the area place two adjacent nesting boxes in fields. Bluebirds will not nest near another bluebird, so having adjacent nesting boxes increases the odds of bluebirds winning one and swallows claiming the other.