Tree Swallows courting in the Bear River Migratory Bird Refuge, Utah. (4/07)
Tree swallows are widespread in North America, nesting (singly not colonially) in tree cavities, individual bird houses, or cliff crevices near water.
We doubt that we've been anywhere there weren't swallows of one kind or another. The effort we expended in learning to distinguish among the different swallows has been more than repaid by the pleasure of watching them.