Here’s one of our tough winter birds, the Downy Woodpecker, and we can tell this is a male because he has red taillights.
The female lacks the red taillights:
The Downy often is confused with its equally-tough cousin that has the same colorings, the Hairy Woodpecker. The best way to tell which cousin is which is by the bill: if it’s small (much less than the length of the bird’s head), it’s a Downy; if it’s large (about the length of the head), it’s a Hairy. Here's a male Hairy:
Another way to differentiate the birds is by overall size: the Downy is about 6.5 inches; the Hairy is about 9 inches. However, when you see one of the cousins without the other beside it, it’s hard to tell a 2.5-inch difference.
By the way, both birds are named after a patch of white feathers that appears on their backs: the Downy’s patch is smaller and finer; the Hairy’s is longer and often looks like a ponytail. (Brooklin, Maine)