Sleeping on One Leg with half an Eye Open

Yes it's another oyster catcher shot! It's been a busy day with more gym, visiting, grandson for sleepover and visitors in the evening so it's a quick blip as I went along the sore road this morning.

I find the best way to sneek up on the birds is to get the camera all set up, wind down the car window, approach slowly in the car and take the shot within 3 seconds of stopping as they usually fly off just after!

How do oyster catchers manage to rest/sleep standing on one leg with half an eye open?