What happened to this Gull?

I was walking up the frozen river with some kids in tow. We’d been out for a few hours tracking when we were on the last stretch and one of the kids pointed to a small mound on the ice. “Look a Penguin!” I think she’d meant it as a joke, but I took note and walked towards the mound. I had walked along this frozen river the day before and hadn’t noticed a mound on the middle of the river bulging out of the ice. I couldn’t tell what it was at first, but my guess was that a log or branch had broken through somehow. As I got close, I learned it was nothing of the sort.