The Downy Woodpecker is the smallest woodpecker in North America. 
The Downy Woodpecker is virtually identical in plumage pattern to the larger Hairy Woodpecker, but it can be distinguished from the Hairy Woodpecker by the presence of black spots on its white tail feathers and the length of its bill.
The Downy Woodpecker's bill is shorter than its head, whereas the Hairy Woodpecker's bill is approximately equal to its head length.
Adult males have a red patch on the back of the head which females lack.
Juvenile birds have a red cap which slowly disappears when they get older.

1st & 2nd photo: A female. Evergreen Cemetery, Portland, Maine, 1 August 2015
3rd photo: A male. Gilsland Farm Audubon Center, Falmouth, Maine, USA, 7 August 2017
4th & 5th photo: A juvenile. Gilsland Farm Audubon Center, Falmouth, Maine, USA, 7 August 2017