Examine the coloration on a bird's body. Male barn swallows have blue-black upper bodies; females may have lighter coloration. Tree swallows are iridescent and shiny; males have greenish upper bodies and the females' bodies are slightly browner. Both species have white bellies.
Look at a bird's head and face. Male barn swallows have brownish-red foreheads, while females' may be a lighter shade. Tree swallows' heads are the same color as their bodies, and they have short black beaks.
Watch the bird in flight to get a good look at its tail. Barn swallows have forked tails, while tree swallows have fanned tails.
Examine the bird's nests. Barn swallows build muddy nests that dry against walls and the sides of buildings in cities. Tree swallows build nests with twigs in the eaves of buildings, in rafters, and in holes created by animals such as woodpeckers.