344 



POPULATION ECOLOGY: 



phenomenon. No other taxon displays either such 

 complexity or diversity in its interarea movements, or 

 such pronounced behavior patterns by some species 

 and almost complete reversal of these patterns by 

 others. The following examples serve to describe 

 bird migration and to emphasize the contrasts. 



(1) Not all birds migrate north and south. Some 

 travel east and west, others uphill and down; still 

 others move in somewhat irregular circular routes. 

 Moreover, some birds migrate in one direction while 

 others, at the same time, travel in the opposite 

 direction. (2) Some species or individuals within a 

 species migrate during the day and others at night. 

 The day type feed upon the wing and spend the night 

 resting. Night types normally feed and rest during 

 the day, but some forms also feed upon the wing. 

 (3) Movement from an area frequently is associated 

 with the environment's becoming unfavorable. How- 

 ever, some birds migrate when temperatures are 

 mild, food and water are plentiful, and the environ- 

 ment appears favorable. (4) Some birds move in re- 

 lation to seasonal alternation of rainfall and drought; 

 the migration of others seems to be directly as- 

 sociated either with late spring to early summer in- 

 crease in day length or with late fall to early winter 

 decrease of day length. In many spring-to-summer, 

 or breeding season, migrants day length increase is in 

 direct proportion to gonadal development. In con- 

 trast to this are other birds which move to the non- 

 breeding range in association with increasing light. 

 Also, birds that move across the equator may move 

 in both directions in direct relation to a single 

 aspect of day length, either increasing or decreasing. 

 (5) In still other birds, interarea travels are allied to 

 increasing day length and rising temperature; again, 

 the opposite conditions also prevail. (6) Still other 

 birds appear to move without any definite environ- 

 mental associations. (7) There also is a tendency for 

 more northern forms of our hemisphere to migrate 

 more than do other groups of their own species. Such 

 species are said to reflect differential distribution in 

 which the more northern forms, when migrating 

 southward, have to pass over suitable areas (already 

 occupied by the more southern populations of their 

 own species) to find an unoccupied favorable habitat. 

 As a consequence, the northern forms travel much 

 farther than do the southern ones. In fact, some of 

 the southern populations might not migrate at all; 

 rather, they may be permanent residents of a partic- 

 ular region. 



Mammalian migration is most dramatic among 

 the deer, marine mammals, and bats. Deer and elk 

 regularly perform altitudinal migrations in which 

 high areas are left when food is covered by snow and 

 are reinhabited when suitable conditions return. 

 Caribou display a like behavior, but in some cases 

 they remain at high altitudes. During the winter, 

 these animals move to windswept north slopes, places 

 where snow is constantly blown away and does not 

 cover the food. Many marine mammals among the 

 whales, seals, and sea lions perform north-south trav- 

 els in direct relation to seasonal warmth. Whether 

 this is the result of seeking more favorable physical 

 environments or more protected breeding sites, or 

 perhaps even some other reason, is a matter of con- 

 jecture. Bats appear to migrate in direct relation to 

 decreasing temperatures and/or food supply. 



Many fishes and amphibians display migration re- 

 lationships that frequently are associated with the 

 breeding season. Fishes that enter fresh water from 

 the ocean often use the new site as hatching waters. 

 In these movements into fresh water, the first ar- 

 rivals regularly occupy the first encountered favor- 

 able situations and later arrivals have to seek waters 

 progressively farther from the ocean. This constitutes 

 another example of the phenomenon of differential 

 distribution. Amphibians, in addition to seeking 

 breeding sites, frequently migrate to dormancy sites. 

 This latter migration is often closely related to in- 

 creasing temperature and decreasing humidity. 



CHARACTERISTICS 



General fascination with the subject of bird 

 migration has led to studies of speed, height, dis- 

 tance, routes, and regularity of arrival. As one might 

 expect from the previous discussion, there is a great 

 deal of variation in each of these characteristics. The 

 speed of migratory flight is not as fast as the fastest 

 speeds recorded for the birds. Birds are known to 

 have maximum flight speeds from less than 20 m.p.h. 

 to over 100 m.p.h., but most migrants travel at speeds 

 close to the lower figure. Altitude of flight varies from 

 near ground level to about one mile. The distance 

 traveled can be as little as a mile or so to about 

 11,000 miles in the Arctic tern. Routes are fairly 

 specific for individual species; there are tendencies, 

 but only tendencies, for certain major "flyways" to 

 be used by migrating birds in general. In the United 

 States, the major paths are called the Pacific, Central, 



