The Northern Parula, at 4.5”, is the smallest of the birds shown here.  You often hear its song (a buzzy, rising trill), before you see the bird.  It nests in forest habitat all across the eastern United States and into Canada, except for a band from southern Minnesota and Iowa eastward to New York that lacks the right habitat.  It is a neotropic migrant, spending the winter in Central America and the Caribbean.  In this photo, you can see the greenish patch on its back.  This is a male; the female is lighter blue and lacks the rusty and black band across the chest.