Bullfrogs will grab many an unsuspecting duckling.
They are a voracious predator and will eat fish, crayfish, water beetles, snails, dragonfly's and larvae, small turtles, bats, duckings, garter snakes many birds and small mammals. and other frogs. The list goes on and on.
The short answer to this is “anything the Bullfrog can fit in its mouth.”
Meghan