when food is abundant they cannot eat enough during 

 the short winter day to give them sufficient energy to 

 survive the long, cold winter night. Migration north- 

 ward in the spring escapes the high summer temper- 

 atures of the south and gets the migration into lati- 

 tudes where the days are long (Kendeigh 1934). 

 Whatever its immediate causes, migration presum- 

 ably evolved because survival was more successful 

 among those individuals that departed than among 

 those that remained (Lack 1954a). 



The timing of migration is not usually regulated, 

 however, by the factors just listed as causes, for most 

 birds migrate days, weeks, even months before the 

 beginning of intolerable conditions in the autumn and 

 after they disappear in the spring. The annual stim- 

 ulus to migration is complex and involves changes in 

 physiological state, energy balance, and hormones 

 (Kendeigh et al. 1960). The chief environmental- 

 factor stimuli are changes in length of day and night 

 and changes in temperature with the progress of the 

 seasons. The regularity of migration by which species 

 arrive at a given point about the same date year after 

 year is probably a response to the regularity of change 

 in day length. The fact that species may arrive a few 

 days early or late of the usual arrival date is doubtless 

 a result of the superimposed effect of variations in tem- 

 perature, to which the birds are also responding. 



The mechanics of migration, fly-ways, flocking be- 

 havior, migration routes, and so forth, are too intri- 

 cate for detailed analysis here (Lincoln 1950). Much 

 research is now in progress analyzing the factors in- 

 volved ; let it be sufficient merely to add that migra- 

 tory behavior is organized as an instinctive behavior 

 pattern in the bird's nervous system. If a stimulus is 

 not presented, the behavior will not be expressed. The 

 stimulus arises when the interaction between inter- 

 nal physiological rhythms and environmental cycles 

 reaches a critical stage. 



Annual latitudinal migrations are not limited to 

 birds. Bison regularly migrated from northern parts 

 of the Great Plains to pass the winter in southerly 

 reaches, traversing a distance of 300 to 600 km (200- 

 400 miles). Some bats, particularly the hoary and red 

 bats regularly migrate between Canada and the north- 

 ern states. The fur seal breeds in the Pribilof Islands 

 in the Bering Sea during the summer, and migrates 

 southward as much as 4800 km (3000 miles) for the 

 winter. 



Some insects migrate. The monarch butterfly 

 breeds in the northern states and migrates several 

 thousands of miles to winter as far south as the 

 Central American tropics. A small proportion of in- 

 dividuals successfully make the return migration in 

 the spring. Evidence is accumulating to indicate two- 

 way migratory behavior in other species of butterflies 

 and insects (Uvarov 1928, Fraenkel 1932, Williams 

 1958). 



Migratory locusts or grasshopers occur both in 

 the eastern and western hemispheres. Schistocera 

 gregaria inhabits the arid grassland and semideserts 

 of Africa and southern and western Asia (Uvarov 

 1928) : Melanophis me.vicanus occurs in the northern 

 Great Plains of North America. In both species, 

 solitary and migratory phases occur which differ in 

 points of size, wing length, and coloration. The mi- 

 gratory phase apparently develops under conditions of 

 higher temperatures and good breeding conditions so 

 that over-populations occur. When migration begins, 

 immense swarms of adult flying individuals move 

 great distances. Migration in the nymphal hopper 

 stage is more limited. Vegetation is ravenously de- 

 voured wherever the swarm stops. Such migrations 

 were extensive in North America between 1876 and 

 1879 when populations moved from the northern 

 Rocky Mountain area into the states immediately 

 west of the Mississippi River. Eggs were deposited at 

 the terminus of the migration flights and at least some 

 of the succeeding generation exhibited return flights 

 in following years. 



Altitiidinal migrations are movements of no more 

 than a few miles up and down the slopes of mountains. 

 By descending to lower altitudes in the autumn, an 

 organism obtains some of the same benefits secured by 

 those species that undertake latitudinal migrations ; 

 i.e., less snow, higher temperatures, and more food. 

 Birds restricted to alpine habitats in the summer are 

 common winter residents of lowland areas. Some of 

 the larger mammals, such as the mule deer (Russell 

 1932) and the American wapiti ( Altmann 1952) have 

 very regular migration habits in respect to herding, 

 timing of movements, and migration routes. They 

 move to the high alpine meadows soon after the vege- 

 tation renews its growth in the spring, and come back 

 down to the valleys in time to avoid the deep winter 

 snows of the higher slopes. 



Local migrations do not necessarily involve a 

 change of latitude or altitude and are often quite lim- 

 ited in distance covered. However, in tropical grass- 

 lands and savannas where wet and dry seasons bring 

 great changes in available water, vegetation, and food, 

 there is a great exodus of both mammals and birds 

 during the dry season and an influx during the rainy 

 season. 



The Atlantic salmon, after reaching sexual ma- 

 turity, may ascend fresh-water streams in subsequent 

 years to spawn and return each time to the sea. Many 

 deep-water fishes spawn annually in shallow waters 

 and then return to deep water again. Turtles come 

 onto the land to lay their eggs ; snakes disperse from 

 their winter dens with the advent of warm weather in 

 the spring ; tree frogs go to small pools to mate and 

 spawn ; and resident birds move onto their breeding 

 territories. 



Insects perform regular migrations both into hi- 



1 58 Ecological processes and dynamics 



