Eastern and western subspecies of Hairy Woodpecker differ most obviously in the extent of the white spotting on the upper wings. Below is a photo of the eastern subspecies.
Although the photo below isn’t the best, you can still see the lack of white spotting on the wings on this western subspecies of Hairy Woodpecker (taken through our window in Coaldale).
Many woodpeckers display sexual dimorphism even in their juvenal plumage (meaning, the males and the females have different plumages even when nestlings). With Hairy Woodpeckers, a juvenile male has a red patch on his forehead; once he has molted and displays his adult plumage, the red patch is at the back of his head. This photo of an adult male feeding a male nestling shows these differences clearly. (Females of any age have no red patch at all.) You can also see how much black is displayed on this western subspecies, compared to the above photo of the eastern subspecies. In fact, because of the angle, you can hardly see any white spotting at all on the adult; you catch a glimpse on the edge of the right wing.
Another important distinction is between Hairy Woodpeckers and the look-alike Downy Woodpeckers. Hairies are larger, with a much larger bill. This photo shows the 2 species side by side, for easy contrast. (Note that these are both the eastern subspecies, determined by the extensive white spotting on the wings.)
You’ll often hear a Hairy long before you see it. Click here for a sampling of the Hairy’s calls/songs and drumming; scroll down a bit to the “typical voice” section.