As we passed a distant, sleeping mallard I stopped and pointed my camera at it and saw its eye open. Even though the rear of the bird was pointing directly at us, with its head tucked under its wing, it could still bring an eye to bear on us to decide if we were a threat or not. The mallard was resting in shallow water at the pool's edge, on a single leg, with its feathers insulating it from the morning breeze. We must have been sufficiently distant for it to feel safe because when I looked again, using my camera as a telescope, its eye was closed once more.
photo © T. Boughen Camera: Nikon P950