14 THE MIGRATION OF BIRDS 



evident that the temperature of the winter refuge 

 has more effect upon the birds than its geographical 

 position. Perhaps the statement that a bird always 

 nests in the coldest part of its range is more univer- 

 sally correct. Even this may not be invariably 

 the habit, but in acknowledging it as a rule we 

 must clearly understand that this cold district is 

 resorted to at the period of the year when its temper- 

 ature is at its highest. There are certain birds 

 which breed in Australia and winter in Oceanic 

 islands where the temperature is cooler than in 

 their breeding area. 



When considering the migration of birds which 

 summer in the extreme north or breed in the extreme 

 south alas, but little is known about the migratory 

 habits of many southern breeders it is compara- 

 tively simple to offer an explanation ; in the long 

 winter months this home, so desirable in the short 

 weeks of daylight, is dark, ice-bound, and foodless ; 

 it is wholly unsuited to the requirements of birds, 

 which, in spite of many assertions to the contrary, 

 have never been proved to hibernate, the only way 

 in which animals can survive for any lengthened 

 period when food supply is entirely cut off. 



Birds are structurally provided with the means 

 of escaping from the disastrous effects of adverse 

 circumstances ; the power of flight, though not the 

 only way in which animals can migrate, is at the 



