Downy/Hairy Woodpecker

Downy Woodpecker, male

Hairy Woodpecker, male

Is it a Downy or a Hairy Woodpecker? That's the tricky ID question that even sometimes experienced birders have to pause and think about when encountering these species. Both species look remarkably similar and both can be found at bird feeders across most of the U.S.
Here's two photos of the birds in similar poses.
One of the best ways, we find, to tell them apart is to look at the proportion of the bill to the head length when viewed from the side. The Hairy Woodpecker's bill is usually greater than one-half the depth of the head and the nasal tufts are proportionately smaller. The Downy Woodpecker has a bill less than one-half the depth of the head, and the nasal tufts are proportionately larger. Males of both species have red on the back of the head, females do not.
Other good clues -
The Downy usually has dark bars on the outer tail feathers (barely visible on the above photo) and lacks a black spur on side of breast. Hairy lacks bars on outer tail feathers and usually has a black spur on side of breast. On the photo you can see this little black mark coming out onto the upper breast.
It's a good time to review this ID challenge because the Christmas Bird Count is coming up soon. That's a survey anyone can participate in. Check here for how you can sign-up.