Fall migration begins in late July with flocking of family groups. Peak fall abundance occurs 15-30 August and departure by 10 October. During peak fall migration, tree swallows are occasionally observed in massive mixed-species flocks near large water bodies. Concentrations of 4,000 to 5,000 individuals are frequently observed along the lower St. Croix River. Common nesting species in the Western Upland and Central Plain, common and more local in the Northern Highland.
Tree swallows nest in loose, semicolonial associations.
Extensive use is made of the edge between deciduous forest and natural openings or agricultural fields. Nests are normally located in natural cavities in tree stumps or fence posts and occasionally in holes in stream banks. Tree swallows have benefited by exploiting the large number of eastern bluebird houses that have been placed in agricultural areas near residential areas. Regular migrant and nesting species. 15 May. Fall migration begins during mid-July.