260 VERTEBRATA 



to year. An excellent method of determining this is to 

 count the nests to be seen in the trees as soon as the leaves 

 have fallen. Robins and some other birds use the same 

 nest for two or even three broods in one year, but on an 

 average it is safe to count only five birds reared from each 

 nest. Small boys and cats are the greatest enemies of 

 young birds near towns and cities. 



The migration of birds is worthy of the attention of every 

 one. In the latitude of New York, the chief birds remain- 

 ing with us throughout the year are the owls, crows, wood- 

 peckers, cedar birds, song sparrow, English sparrow, 

 black-capped chickadee, and bluebird. Some of these 

 frequently go south during severe cold weather. The 

 other summer residents pass the winter in the South, 

 some, like the bobolinks, going as far as Brazil. 



In many cases the migration trips are made by night 

 and often at a great height. The purple martin, the 

 kingbird, the swallows, and the orioles depart in September, 

 and the majority begin their southern journey in October; 

 but the bluebird, robin, and chipping sparrow remain 

 until November and return as early as March. The 

 latest comers in spring are the wood thrush, vireos, orioles, 

 bobolinks, and cuckoos, which arrive in May. The cause 

 of bird migration is not known. Cold weather may seem 

 to force them south, but why do they leave the warm 

 tropical regions abounding in food in the spring and come 

 to the bleak north? 



The regions to which some of our species go for winter- 

 ing are quite definitely mapped out. The robins that 

 spend the summer in Canada have for their winter resi- 

 dence portions of the United States in the latitude of 



