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V 



o'-war birds. Though most short-winded birds are poor 

 flyers and tire easily, some sturdy forms ; like the ducks 

 (which may attain a speed of a hundred miles per hour) 

 are swift and effective on the wing. 



The seasonal migration of birds has been a subject of 

 much interest and speculation, but many of its aspects are 

 still shrouded in mystery. Most small birds move about 

 twenty-five miles a day on their journeys north and south. 

 They usually travel high in air at night and rest during the 

 day in appropriate localities. The Arctic tern annually 

 travels "from pole to pole," thus living in perennial 

 summer. The Eskimo curlew (Fig. 104) covers a great 

 ellipse south across 2500 miles of Atlantic ocean to its 

 winter home and north through the center of North 

 America to its breeding grounds within the Arctic Circle. 

 On the other hand some birds, like the quail and the English 

 sparrow, migrate little or not at all. Migratory birds 

 usually breed in the coolest part of their range and spend the 

 summer in warmer climates. The abundance of food and 

 the presence of appropriate nesting sites are probably 

 important factors in controlling such flights but they are 

 not the sole causes of migration; in some species, in fact, 

 appear to have little or no effect. 



The songs of birds commonly serve for the attraction 

 of mates, but various other characteristic sounds are useful 

 for warning signals, calling the young, etc. The vocal 

 apparatus of a bird is not in the larynx, as in mammals, 

 but lies in the "syrinx" at the lower end of the trachea and 

 is even sometimes imbedded in the .sternum. It is a rather 

 complicated apparatus, capable in many cases of producing 

 a considerable range of sounds. 



The colors of birds are for the most part protective. 

 The body is countershaded (i.e., is dark above and light be- 

 low, so that the effects of light and shadow are eliminated,) 

 and is therefore inconspicuous. Some birds, like the grouse, 

 also have accurate pictures on the feathers of the back- 

 grounds on which they are most apt to be seen. The fact 



