When I first looked at this picture, I thought it showed the bird dangling some exotic prey from its mouth. But that seemed unlikely in the case of a fledgling. And when I looked more closely, I saw something else. Here's a closer view: https://pbase.com/tgrey/image/170890460 .

The "dangling prey" is the bird's talons. Do birds clean their talons with their mouths?

You may know that they do, but if not here's a Google image page that illustrates the point: https://tinyurl.com/y2u2opaj