Cranes almost always travel at night, and alight down on the ground during daytime to seek their sustenance. Sometimes, however, they do not stop, and continue to push on through space, giving utterance to startling cries, which probably are intended as a rallying summons to those of the band which seem tempted to linger on their journey. When they perceive a bird of prey, or have to contend against a tempest, they abandon their usual formation, and collect in a circular mass, so as better to resist the enemy. Cranes frequent large plains intersected with marshes and water-courses. They feed on fish, reptiles, frogs, mollusks, worms, insects, and even small mammals. Some kinds of grain have also attractions for them, and they may not unfrequently be seen invading the newly-sown fields to devour the seed which the farmer has just committed to the soil. When the breeding season arrives, they break up their social compact, and pair off for the purposes of reproduction and attending to the rearing of their young. Their nests are but roughly constructed, and are placed on any little piece of rising ground in the midst of the marshes; in them they lay usually two eggs, the male sharing with the female the cares of incubation. Although these birds are ordinarily so timid, and are alarmed at the least appearance of danger, yet, when they have their young ones to defend, they become really courageous. In this case they do not shrink from attacking man. The Crane ought to have been the emblem of vigilance. When the flock go to sleep, with their heads hidden under their wings, one of their number is specially charged with the duty of watching over their common safety, and of giving alarm on the approach of danger. When caught young they are easily tamed, and in a very short time will manifest considerable familiarity with their keeper. They are, therefore, a good deal sought after in some countries, both on account of their graceful shape, and also for the sake of the vigilance which they exercise round about their home.