Some birds, like the crows, sleep in great flocks. They agree upon a piece of woods, and all the crows for miles around come there every night. Sometimes thousands sleep in this one bedroom, called a crow roost. Robins do the same, after the young are big enough to fly so far. [Illustration: AMERICAN ROBIN] Audubon, who has told us so much about birds, once found a hollow tree which was the sleeping-room of chimney swifts. The noise they made going out in the morning was like the roar of a great mill-wheel. He wanted to see the birds asleep. So in the daytime, when they were away, he had a piece cut out at the foot of the tree, big enough to let him in, and then put back, so the birds would not notice anything unusual. At night, after the swifts were abed, he took a dark lantern and went in. He turned the light upon them little by little, so as not to startle them. Then he saw the whole inside of the tree full of birds. They were hanging by their claws, side by side, as thick as they could hang. He thought there were as many as twelve thousand in that one bedroom. XV HIS TRAVELS MOST of our birds take two long journeys every year, one in the fall to the south, and the other in the spring back to the north. These journeys are called "migrations." The birds do not go all at once, but in many cases those of a kind who live near each other collect in a flock and travel together. Each species or kind has its own time to go. It might be thought that it is because of the cold that so many birds move to a warmer climate. But it is not so; they are very well dressed to endure cold. Their feather suits are so warm that some of our smallest and weakest birds are able to stay with us, like the chickadee and the golden-crowned kinglet. It is simply because they cannot get food in winter, that they have to go. The fall travel begins soon after the first of July. The bobolink is one of the first to leave us, though he does not start at once on his long journey. By that time his little folk are full grown, and can take care of themselves, and he is getting on his winter suit, or moulting. Then some morning all the bobolinks in the country are turned out of their homes in the meadows, by men and horses and mowing-machines, for at that time the long grass is ready to cut. Then he begins to think about the wild rice which is getting just right to eat. Besides, he likes to take his long journey to South America in an easy way, stopping here and there as he goes. So some morning we miss his cheerful call, and if we go to the meadow we shall not be able to see a single bobolink. There, too, are the swallows, who eat only small flying insects. As the weather grows cooler, these tiny flies are no longer to be found. So the swallows begin to flock, as it is called. For a few days they will be seen on fences and telegraph wires, chattering and making a great noise, and then some morning they will all be gone.