My husband calls them "the little happy birds." They are actually Tree Swallows, but I see his point, they do look happy when they flit and swoop through the air.
Tree Swallows tend to come back to Maine from their winter habitat of Florida and Central America rather early... April or May. Insect eating wonders, they are more than welcome here. I love the expression on this guys face. He let me approach very closely with my camera, but he was giving me a bit of a stink eye!