THE BIRDS 413 



Migration 



Some of the birds are permanent residents. They stick it out through 

 the winter months just as most of us have to do, but a great number of 

 birds spend the summer in northern regions and migrate to southern 

 regions where the temperature will be mild during the winter months. 

 If you get a good pair of binoculars and focus on a full moon during a 

 clear night of the migrating season, you may be rewarded by seeing 

 black shapes by the hundreds pass between you and the moon. These 

 will be birds, perhaps of a number of different species, flying about a 

 mile high in their semi-annual migration. 



Most birds stop along the way and do not cover more than about 

 twenty-five miles a day. Each species has its own time of migration and 

 its own route. Governed by the length of day or some other seasonal 

 factor, the time of arrival is fairly regular each year. The arrival of 

 the swallows at the mission San Juan Capistrano in California occurs on 

 almost exactly the same day each year. 



A number of birds winter in South American and spend the summer 

 in northern Canada, traveling up to 14,000 miles per year. The long- 

 distance record, however, is held by the Arctic tern, which nests in the 

 summer far into the Arctic Circle, then flies down the eastern coast of 

 Canada, across the Atlantic, down the west coast of Europe, down the 

 west coast of Africa, and finally ends up in the Antarctic regions to spend 

 the winter. This makes a yearly round trip of about 25,000 miles, which 

 is quite a distance even in these days of modern transportation. They 

 must like the sunlight, for they are near the poles at the season when the 

 sun never sets. 



There has been considerable speculation about how birds find their 

 way in their migration flights so that they take the same routes and 

 arrive at the same places each year. Those who have a hobby of band- 

 ing migratory birds report that they tend to return to the same yards 

 each year. In some cases it appears that an older bird leads the way on 

 migration flights, and he goes by landmarks along the way. Landmarks 

 are without doubt a very important factor in many bird flights, but this 

 method could not be used for all migratory flights. There are some 

 migrations which cover thousands of miles of open water and end on a 

 small island. Birds hit this point each year with an accuracy which is 

 equal to the best we can do with all our complicated instruments. Some 

 have suggested that birds might have some sense organ which tells them 

 their position by the magnetic field of the earth and the force brought 

 about by the earth's rotation. This is still one of the questions of science 

 which must be investigated further before we have a complete answer. 



