5.  How shall we make an exact review of all the peculiarities of the life of birds?  During the night cranes keep watch in turn; some sleep, others make the rounds and procure a quiet slumber for their companions.  After having finished his duty, the sentry utters a cry, and goes to sleep, and the one who awakes, in his turn, repays the security which he has enjoyed. Arist., Hist. An. ix. 10.  You will see the same order reign in their flight.  One leads the way, and when it has guided the flight of the flock for a certain time, it passes to the rear, leaving to the one who comes after the care of directing the march.