The Migratory Bird Treaty Act of 1918 was put in place to protect birds, and it "makes it illegal to take, possess, import, export, transport, sell, purchase, barter, or offer for sale, purchase, or barter, any migratory bird, or the parts, nests, or eggs" of native birds unless you have a permit to do so. That includes feathers found on the ground, nests even after used, and eggs that never hatched. They are interesting and informative to look at, but all should be left as you found them. See more at tinyurl.com/bird-treaty-act.