Birds' Sight and Sense of Direction

Many theories have been advanced to explain how birds find their way. Probably no one theory will satisfy all conditions. rp are doubtless many factors needed to give a satisfactory explanation. One important factor is the bird’s sight. Birds have very keen eyesight, and it seems probable that birds flying at a great height may be guided by conspicuous landmarks, such as mountain-chains, coast-lines, and river-valleys which extend in the same direction as the routes of migration.

In North America, the coast-lines and mountain-chains and the Mississippi Valley extend in the general direction in which most of the birds migrate. But this explanation alone is not sufficient, as birds may migrate at right angles to these landmarks, and may find their way in a fog when landmarks are invisible, or over large bodies of water where no landmarks can be seen; and frequently birds fly so close to the ground or water that they cannot see any landmarks. And again birds may travel straight for long distances over routes which they have never seen before.

Sense of direction. Still another suggestion is that birds have a sense of direction which enables them to find their way. This is simply ascribing a power to birds without any real explanation, but experiments which have been made with birds seem to show quite conclusively that some birds do possess this sense of direction. Several birds were captured on Bird Key south of Florida, and were placed in the hold of a steamship and taken north to Cape Hatteras, a distance of about one thousand miles from their nesting-sites, and released. Five days later, two of them were back on their nests. In this case no other explanation seems possible than that the birds found their way through a sense of direction, as the birds had never flown over this route before, and could not see the way over which they had come, and so could not make use of any landmarks.

Causes of migration. The most puzzling of all questions concerning migration is, why do birds migrate? At the outset it may b*e stated that bird students are not agreed as to the causes of migration, but brief reference may be made to a few of the theories which have been put forward at various times to explain the cause of bird migration.

Food and temperature. It is very commonly stated that lack of food and low temperatures cause birds to migrate. But even a very hasty examination of the facts shows that these do not explain migration. The fall migration begins during the late summer, whcn the temperature is still high, and at a tune when insect life is abundant. Furthermore, during the spring migration, birds are traveling into regions where the temperature is lower and insect life is less abundant than in the regions which they are leaving. And again, some tropical sea-birds mi grate from one section to another where the conditions of temperature and food-supply are practically the same.

Glacial theory. One theory relates the origin of bird migration closely with the glacial age. Fossils which have been found show that before this age North America had a warm climate, even in its northern portions. This climate must have been well adapted for bird life during all parts of the year. As the ice-sheet began to extend south, the birds were driven before it, and as it melted and receded north, the birds followed it back. In accordance with this theory, the habit which the birds thus acquired of moving back and forth, following the oscillations of the ice-sheet, was inherited eventually by the birds as an instinct and still exists to this day.

Physiological explanation. None of these theories is generally accepted by bird students as giving a satisfactory explanation of migration. It is probable that birds have a physiological instinct which prompts them to migrate in order to rear their young, just as their instinct leads to other actions, such as singing, mating, nest-building, egg-laying, and incubating. But this statement, of course, gives no explanation as to how and why this instinct originated.  

[CaRP] XML error: not well-formed (invalid token) at line 9