Tree swallows are migratory birds. They winter in Mexico, Central America and the Caribbean. They breed in the summer in our area. They use the same size nextbox as Eastern Bluebirds. Over the years I have had a growing number of Tree Swallows use my boxes. They will nest closer to each other than the Bluebirds will.
There are numerous boxes with these birds out at Binder Lake Park this time of year if you want to see them.