My Last Downy for a while.

[BillM]

• Size: Size is the most obvious characteristic, especially if you see both birds at the same time. A Hairy Woodpecker is roughly 1/3 longer than a Downy Woodpecker
• Bill: Bill size is the next most obvious trait. The bill of a Downy Woodpecker is roughly 1/2 the length of its head, and is slightly conical. The Hairy's bill is much thicker, and is the same length as its head.
• Shoulder fleck: The Downy has a barely-visible black fleck extending onto the white; the Hairy has a quite distinct flash (compare in the images)
• Tail: The Downy has black spots on the sides of its outer tail feathers. The Hairy's tail feathers are pure white.
• Call: The Downy's call drops down in pitch; the Hairy's call stays on the same pitch.