根据以下内容回答题：No one knows for sure why birds migrate．One theory of migration says that ancient birds of the northern part of the earth were forced southward during the Ice Age，when ice covered large parts of Europe，Asia and North America．As the ice melted，the birds came back to their homelands，spent the summer，and then went south again in winter．Gradually，these comings and goings became habits，and birds now migrate though much of the ice has gone． Another theory proposes that the ancient home of all modern birds was the tropic areas．There lived so well that the region became overcrowded．Many species had to move northward．During the summer，these birds found plenty of room and food．In winter，however，food be-came scarce and they had to return to the south． A newer theory is that increasing daylight stimulates certain glands（腺）in the bird’s body and prepares it for migration．One scientist is able to make birds migrate in midwinter by expo-sing them for two months to artificial daylight．Recoveries of marked birds indicate that they fly north as soon as they are set free．The conclusion is that the urge to migrate is determined by changes in the bird’s body which take place under seasonal changes in the length of daylight． This theory would account for the fact that not all birds migrate at the same time．Each sDecies seems to have its own schedule．The theory would also account for the regular time and rou‘tes of migration．Birds arrive at a given place year after year．Unfavorable weather delays them only a few days．Total hours of daylight，rather than weather，start them on the way． How birds find their way to the same place year after year and why they follow their own Darticular route are still mysteries．They evidently do not follow known landmarks，for many young birds migrate alone without the help of experienced adults．

We can conclude from the first paragraph that__________ ．