 Cranes take their name from the shrill sound of their voices. They have a special way of setting out on a journey. The birds arrange themselves with military discipline. To make sure they are not driven off course, they swallow sand, weighing themselves down with gravel until they are heavy enough to resist the force of the wind. Then they fly to a great height, so that by looking down from above they can see their destination. One leader flies ahead of the flock. The leader knows the way, scolds lazy flyers and keeps the others in order with her calls. If her voice becomes hoarse, another bird will take her place. And if a member of the flock becomes tired, the others will gather around to secure her in the air and allow their companion to rest until she regains her strength. At night, the cranes keep a careful watch. The guards stand at their places, watching over the rest of the flock as they sleep. Other birds circle the camp, looking out for any enemies who might attack them. When there is cause for alarm, the cranes call out to wake the rest of the flock and make sure they are safe. The flock divides the night into watches and arranges the sequence of guards according to strict rules. Once a guard has completed her time on duty, she goes to sleep. But first she must wake one of the sleepers by calling out so that she can be replaced. The new guard takes over the task willingly. Not like us. We would do it in a bad temper because we want to carry on sleeping. The crane quickly takes up the post and carries out the task just as carefully as her predecessor did. Cranes have an ingenious way of keeping awake. They hold stones in their claws. If a watcher falls asleep, the stone will fall to the ground, make a noise and wake her up. The colour of the crane's feathers reveals their age, gradually darkening as they grow older.