Why are Hairy Woodpeckers and Downy Woodpeckers so hard to tell apart?

As winter descends and the days become cold and short, most of my birdwatching becomes rather narrowly focused on the feeders outside my front door. A walk through my woods in winter, although lovely in its own right, tends to yield fairly few birds. If I’m lucky, I might come across a noisy flock of Golden-crowned Kinglets, Black-capped Chickadees, and Brown Creepers. Just as easily, though, I can spend an hour wandering through the snow in absolute silence; the trees seem empty of birdlife.