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PRACTICAL ZOOLOGY 



cannot be too strongly emphasized since they are among the 

 most beautiful sounds in nature, and besides, birds are so effec- 

 tively concealed most of the time by the foliage of the trees that 

 we hear many more than we are able to see. 



Bird Migration. — The remarkable powers of locomotion 

 possessed by birds enable them to move from one part of the 

 country to another with comparative ease. As a result, when 



Fig. 229. — Nest and eggs of least bittern. The nest was built among the 

 reeds above the water in a marsh. (Photo, by Hegner.) 



winter approaches in temperate regions most of the birds gather 

 together in flocks and migrate to the warmer southern countries. 

 Those that remain in one locality throughout the year, like the 

 great horned owl and English sparrow, are called permanent 

 residents; those that pass through on their way south in the 

 autumn and on their way north in the spring, like most of the 

 warblers, are called migrants; and those that leave in the autumn 

 and return the following spring, remaining with us to nest, we 

 call summer residents. 



Formerly, birds were supposed to hibernate during the winter 

 in caves, hollow trees, or, in the case of swallows, in the mud at 



