I'm hoping the eagle-eyed ornithologists here on the forum can help identify this bird. It's been hanging out at he pond behind our house in eastern Massachusetts for a few days. It dives for fish and swims half-submerged. Occasionally it treads water for a few seconds and vigorously flaps its wings to shake them off, as seen in the photo below. Based on its downy head feathers, I'm thinking it's a juvenile. I'm not looking for photographic criticism, since this wasn't intended as fine photography, just a quick shot for identification. Any thoughts? Maybe a merganser?