In a summer past, a mallard paddling close to my dock suddenly canted his head to gaze over my shoulder. Looking in that direction, I saw a tiny dot very high in the sky. It was a large bird partly hidden in the trees behind—and quite possibly a predator.
In groups, I suspect they're especially aware of what's "up there". (They're fast fliers, so a fast leap upwards and flight is their escape of choice).
Though they
can dive, diving's not such a good "move" for a mallard.
