We had a similar issue, not with feather, but with a whole live bird. Last August, my son found a baby Eastern Kingbird on the lawn and we could not locate the nest. We took it in, researched it's diet, and raised it until it was fully fledged. We do not have a wildlife rehabilitator's permit. Further research revealed that they tend to migrate in mid-September and we were worried that "Tweetle Dee" had missed the boat. My wife called the local conservation officer and shared the whole story. He informed her that we were breaking the law. When she asked about keeping it for the winter and letting it go in the spring, he told her that then we would be breaking the law for the whole winter. It took about four days to set her free. The first couple times, she stayed out all day, then chased me around the yard in the evening because she was hungry and we took her inside for the night. Then she stayed out overnight, and I would feed her outside a couple times, until she finally didn't come back one day.