484 VERTEBRATE LIFE AND ORGANIZATION 



sites and Iccdiiig areas. During much of the year the food supply in a 

 given area is ade(juate to sustain a population of reasonable size. The 

 food available may not suffice, however, during the breeding season 

 when the increased activity of the birds increases their food requirement 

 and when the population is more than doubling. Spreading out into 

 new areas at this time has some advantage, and permits a larger bird 

 population. .Mthough the reasons for the evolution of migration are 

 luuertain, the search for food may have been a factor in the tendency 

 for many birds to migrate north in the summer. There is a large land 

 mass in the north, and during the summer, at least, this area is rich in 

 food. The tendency to return south as the weather becomes inclement 

 in tlie autumn might be correlated with the reduction of the food 

 supply; it apparently is in the case of certain insect-eating species, but 

 otliers migrate before there is any food shortage. The glaciation of large 

 parts of the Northern Hemisphere during the great Ice Ages may have 

 been an additional factor in the evolution of the migratory habit. 



The pattern of migration is regular for each species; it begins at 

 very nearly the same time each year. Apparently the stimulus is a change 

 in day length— its increase in spring and reduction in fall— for this is 

 the only environmental factor that varies in a manner regular enough 

 to serve as a consistent timetable. Rowan and others have shown that 

 day length operates by affecting the activity of the bird which in turn 

 influences the size of the gonads. The gonads increase in size as day 

 length increases and the birds become more active, and decrease in size 

 as day length decreases. Artificial illumination or darkness, and forced 

 activity or inactivity, have comparable effects. 



Most birds migrate at night, stopping to feed and rest during the 

 day. Some may fly several hundred miles during a single night, but then 

 may rest for several days. The northward advance of these birds in the 

 spring averages about 20 to 25 miles per day. Many species tend to follow 

 the advance of certain temperature lines, or isotherms (Fig. 24.11). The 

 length of migration and the route taken are very consistent for each type 

 of bird, but vary with the species. The Canada goose winters in the 

 United States from the Great Lakes south, breeds in Canada as far north 

 as the Arctic coast, and migrates along a broad front between the two 

 areas. The scarlet tanager winters in parts of South America and breeds 

 in the area from Nova Scotia, southern Quebec and southern Manitoba 

 south to South Carolina, northern Georgia, northern Alabama and 

 Kansas. In contrast to the Canada goose, it has a narrow migration route, 

 which extends through southern Central America and then across the 

 center of the Gulf of Mexico, passing between Yucatan and Cuba. The 

 longest migration is that of the Arctic tern; some of these birds travel 

 25,000 miles in a year. This species breeds in the Arctic, then follows 

 the coast line of Europe and Africa to its winter quarters in the South 

 Atlantic. 



The season, speed and routes of migration have been carefully de- 

 scribed for most species of birds, but how birds navigate and find their 

 way during their migrations remains one of the intriguing, unsolved 



