HIGHER CHORDATES 



291 



rupture to unfold the juvenile feathers. The posl- 

 juvenUe molt o{ ]u\y to September removes the juvenile 

 feathers. Most birds complete this molt prior to 

 migration; however, the wing and tail feathers of 

 juveniles are often retained, allowing the distinguish- 

 ing of young from adults. The areas of post juvenile 

 molt are replaced by the Jirst winter plumage. These 

 feathers generally are like those of the adult except 

 for slight differences in color pattern due to retained 

 juvenile feathers. The premiptial molt removes the 

 winter plumage in late winter or early spring and 

 allows the nuptial or breeding plumage to develop. Few 

 birds have a complete molt to produce the breeding 

 plumage. Next is the postnuptial or annual molt in 

 July, August, or September in which all feathers are 

 replaced. The flight feathers usually are shed one at a 

 time in sequence at this time; however, certain birds 

 (e.g., rails, coots, and some ducks), shed all the flight 

 feathers at the same time and are flightless for a 

 period. 



INTERN.\L FEATURES 



Birds also display various internal adaptations for 

 flight. This is especially apparent in their metabolism 

 and their skeletal, muscular, and respiratory systems. 

 The tremendous energy necessary for flight is re- 

 flected in the high metabolism, indicated by a high 

 heart rate (300 in the chicken and 400 to 600 in the 

 chickadee — 1000 when frightened) and by a high 

 body temperature. Although body temperature fluc- 

 tuates about eight degrees in birds, bird temperatures 

 in general are between 100 and 112°F. The skeletal 

 system is very light, being built upon the hollow- 

 girder principal. Lightness also is accomplished by 

 the reduction and fusion of many parts. For example, 

 the tail is reduced to a rudimentary structure, and 

 the "hip" and leg bones are reduced and fused. How- 

 ever, strength is aided by processes on the ribs which 

 cause the ribs partly to overlap one another, forming 

 a total structure much like a rigid box. Their mus- 

 cular system shows its greatest flight adaptations in 

 the very large flight muscles. Finally, the respiratory 

 system has many air sacs filling spaces in the body. 

 These air sacs act both to keep the birds' temperature 

 from rising too high and to maintain lightness of the 

 body. 



BOD"!' TEMPERATURE AND DORMANCY 



Birds and mammals are the only endotherms, or 

 animals whose internal body processes produce their 

 body heat. Also, these processes are regulated so that 



body temperature is fairly constant. Only reptiles 

 among ectotherms, the vast majority of animals that 

 obtain their body heat from the sun, approximate 

 regulated body temperatures. However, reptilian 

 regulation occurs only when the animals are active. 



Torpid or coma-like states are well known in many 

 mammals and other animals. However, until fairly 

 recently not much was known about these states in 

 birds. Coma-like states are now known to occur in 

 some goatsuckers, hummingbirds, and swifts. In 

 some of these birds, body temperature and other 

 functions are depressed as much as in truly dormant 

 mammals. So far this true dormancy in birds seems 

 to be limited to the winter; hence, the phenomenon 

 is hibernation (in summer, states of dormancy are 

 called aestivation). 



MICiR.ATION 



Migration is a very complex phenomenon and even 

 now is only beginning to be understood. Part of the 

 difficulty might occur because various factors affect 

 migration, and migration may not be a single phe- 

 nomenon. To appreciate this fact, one must examine 

 some of the conflicting patterns within migrating 

 species. First, not all birds migrate north and south. 

 Moreover, some birds migrate when temperature is 

 mild and food is ample, whereas others migrate under 

 opposite conditions. Some birds seem to migrate as a 

 result of seasonal alteration of rainfall and drought; 

 others seem to migrate in association with day length; 

 the migration of still others simply does not seem to 

 be related to much of anything. In addition, only a 

 portion of some species migrate, and other species 

 migrate irregularly or not at all. In spite of all these 

 difficulties to understanding, if there is a single stim- 

 ulus to migration, it is believed to be involved with the 

 attainment of a particular functional state of the 

 body, a state that might involve the activity of the 

 pituitary gland in the brain. 



Some more specific characteristics of bird migra- 

 tion may be mentioned. There are five main mi- 

 gration routes in the United States, more or less 

 indicated by the long migrators. The distance of 

 migration varies a great deal; some birds show very 

 localized seasonal movements from place to place, 

 while the Arctic tern travels 11,000 miles. Some 

 migration is merely up and down mountains. In 

 general, migration is a hazardous phenomenon; in 

 times of inclement weather many birds may perish. 

 (Further details are given in the discussion of migra- 

 tion in Chapter 18.) 



