Woodpecker Feather?
I’m no sort of expert on these things so I really don’t know what sort of bird has feathers that look like this. I know there are woodpeckers with dark feathers with white spots running up both sides. There are also owls with this sort of pattern. The feathers were not terribly long, so my guess was one of the small local woodpeckers, such as the downy (Picoides pubescens) or hairy woodpecker (Picoides villosus). I’d be happy to have someone who is more knowledgeable set me straight. This was in the woods behind my building.