American Tree Sparrows are small, round-headed birds that often fluff out their feathers, making their plump bodies look even chubbier. Among sparrows, they have fairly small bills and long, thin tails. Relative Size. About the size of a House Sparrow. sparrow-sized or smaller.

American tree sparrows are small, grayish-brown birds with a rufous crown and eyeline, a dark spot on the breast, a long notched tail, a dark upper mandible and a yellow lower mandible. They have a grey head, chin, throat, breast and underparts and a rufous patch on the sides of their breast.