VERTEBRATES — CLARK 293 



pair is modified to form paddles parallel to those of the seals, cetaceans, 

 manatees, and sea turtles. In agreement with the sea turtles but in 

 contrast to all other aquatic birds the f orelimbs of the penguins pro- 

 vide the chief motive power in swimming. 



In contrast to the uniform flapping flight of bats the flight of birds is 

 highly diversified. The large sea birds are gliders, which seems to have 

 been true of most of the flying reptiles of the past. Most vultures, most 

 eagles, many hawks, and some other large birds spend much of their 

 time soaring. Many birds alternately flap and sail. Most small birds 

 progress by continuous flapping in direct, or, less commonly, undulat- 

 ing flight. In some birds the flight is feeble and they are reluctant 

 to take wing, though a few of these, as some rails, undertake sur- 

 prisingly long migrations. 



The power of flight enables birds to live or to take refuge in trees 

 secure from most predators except other birds; to escape enemies by 

 taking to the air ; and to take advantage of the concentrated life in the 

 warm light summer season in the far northern or southern regions, 

 retreating toward, to, or even through the Tropics in the dark winter. 

 It also enables tropical birds to wander about from one feeding ground 

 to another in a distant area. Although strictly diurnal in their 

 usual activities, many small birds migrate by night. Many of the 

 larger birds, as geese and swans, migrate by day or night. In contrast, 

 the bats, though capable of sustained flight, fly only by night except to 

 cool themselves in hot weather, and few are known to migrate. 



All birds maintain a controlled high body temperature, higher than 

 that of mammals, and are continually active throughout their lives 

 except during sleep. A very few birds have vicarious or even fairly 

 long torpid periods simulating hibernation, but none estivate. 



All birds breathe by internal lungs and all have more or less exten- 

 sive internal air sacks. 



In all birds sight is the dominant sense, in some, as in owls and goat- 

 suckers, supplemented by acute hearing. In owls, which have the 

 most highly developed ears, the plumage is very soft, possibly so as 

 not to interfere with auditory perception, in addition to concealing 

 their presence. Thus birds are dependent on adequate illumination. 

 Their visual range is broad, including the longer wavelengths, not 

 visible to us, by means of which they find their way through fog, and 

 possibly in the case of crepuscular birds, the shorter, to us invisible, 

 wavelengths commonly used by insects. No birds are blind. Al- 

 though some take refuge or nest in caves, holes, or burrows, their es- 

 sential activities are always carried on outside. 



The breeding habits of birds are singularly uniform. Most birds 



nest in trees or on the ground, singly or in colonies, sometimes making 



• a community nest, but many lay their eggs in holes or burrows. The 



