54 THE MIGRATIONS OF BIRDS 



ice. Thousands of canvasbacks, bluebills, and other 

 ducks remain on the Potomac River in winter, even 

 though a large part of the channel is frozen. Mal- 

 lards and pintails in the Missouri River drainage 

 move north and south regularly during winter with 

 fluctuation in the southern limit of ice, and in spring 

 press northward with every thaw, retreating again 

 if need arise because of freezing weather. 



Birds that are not accustomed to cold respond 

 variously to it, some seeming wholly indifferent so 

 long as food is available, unless subjected to undue 

 exposure. This is true even in certain tropical 

 species that normally never encounter cold, as when 

 transported to another climate some do not seem 

 particularly affected. The red blue and yellow 

 macaw {Ara macao)^ native in tropical America, has 

 lived in outdoor flight cages in the National Zoologi- 

 cal Park at Washington during severe winters. Dur- 

 ing the winter of 1924-25 a great white heron from 

 southern Florida was kept in an outdoor cage, shel- 

 tered from northerly and westerly winds but with- 

 out artificial heat, and seemed as oblivious of cold as 

 a great blue heron that lived as its neighbor. Even 

 the snowy heron and American egret in a wild state 

 are not so sensitive to cold as we usually imagine, 

 since I have seen the former flying in snowstorms in 

 Utah, and the latter penetrates regularly to Pata- 

 gonia, where the summers are anything but warm. 



