Let sleeping geese lie

When a flock of geese is sleeping, one or two of them will remain awake to act as sentries. What process, if any, does the flock use to choose which geese are the sentries?

For millennia, it was Rock, Paper, Scissors.  But their webbing made every throw Paper vs. Paper.  Consequently, nobody was assigned sentry duty and geese evolved to sleep with one eye open.  True story!