There is no telling how many feathers I collected a kid. I did that without ever killing a single bird for in the woods, I found feathers everywhere. I had feathers from blue jays, crows, cardinals, doves, sparrows, mockingbirds, ducks, owls, hawks and dozens of other types of birds. I had a very nice collection that I took to school and showed everyone. Back then, no one thought anything about it. Now, possession of those feathers would be a major problem. Fines in the thousands would be coming and maybe jail time. Today, it is a big deal.
Most birds are protected by the Migratory Bird Treaty Act (MBTA) which makes possession of feathers or other parts of the bird to be illegal. The idea of this is to protect wild birds from being killed for their feathers and in some cases, their claws or beaks. There has always been a market for these items by collectors and for commercial trade. It does not matter how the feathers were obtained. You can't pick them up off the ground, pluck them from a dead bird on the road or get them from a dead bird your cat left on your step. This is absolute.
For more information about the MBTA included a list of which species are included (almost all birds) visit the FWS website at http://www.fws.gov/migratorybirds/regulationspolicies/mbta/mbtintro.html.