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Even bullfrogs can be a danger to them.

Ewww! I didn't know that. I knew about Herons and big fish, but even frogs? Yuck.

Yeah a bullfrog will eat anything it can fit in it's mouth. It would take a really big one but I actually once witnessed a duckling get eaten by a bullfrog. The gosling is probably too large, maybe a snapping turtle is more of a threat.

I once saw one disappear in a splash to what I assumed was a northern pike. Poor things.