1584 THE STORY OF THE -UNIVERSE 



help it along in its rapid race; some aquatic birds, 

 like the steamer-duck, use them as paddles, auxiliary 

 to their legs. On the ground, birds vary greatly in 

 rate and manner of progression: the swift strides of 

 the ostrich, the rapid run of the partridge, the hop- 

 ping of the sparrow are well-known illustrations of 

 different gaits. That many birds are expert divers 

 and climbers is also a familiar fact. 



The great activity of birds is associated with very 

 efficient respiration. Expiration, or the expulsion 

 of used air, is managed by the contraction of breast 

 and abdominal muscles, which compress the inclosed 

 cavities and force the air from the sacs and lungs. 

 When these muscles are relaxed the cavities again 

 elastically expand, and fresh air rushes in by the 

 windpipe to lungs and air sacs. 



With few exceptions, birds have a vocal organ, and 

 are able to produce more or less variable sounds. 

 The organ is, however, wanting in the running birds, 

 such as the ostrich and the American vultures. The 

 sounds produced are almost as varied as the different 

 kinds of birds, and an expert ornithologist has lit- 

 tle difficulty in identifying a great number of forms 

 by their distinctive noises. That some chirp and 

 others scream, that chattering describes the language 

 of many and croaking that of others, that some boom 

 and others bark, that the crows caw, and the laughing 

 jackass laughs, that the mocking-bird imitates, and 

 the parrot becomes able to articulate, and above all 

 that the lark trills and the nightingale truly sings, 

 are well-known illustrations of the variety of bird 

 language. The wdrd cry of the curlew or whaup, 



