The BBC recently reported on an archaeological paper showing butchered ravens and crows in association with Neanderthal remains. Since there isn't any meat to speak of on those kinds of birds, the authors believe that Neanderthals must have used the birds' black feathers as body decoration. The archaeological article itself can be read and/or downloaded here.
Perhaps this kind of feather use is the ultimate mother of all human costuming? I shall keep an eye out for further research along these lines.