I found have found a dead chick many months after others fledged as the birds nested in an unexpected spot and I hadn't realised it was there. Like yours, it looked like a bird and not a horribly decomposed body. I suspect it had dried out (mummified) or was just a skin covered skeleton and since feathers last, it just disguises how little is left and leaves it looking relatively normal. That's just my theory, but I'd like to hear what the experts think, too.