334 POPULAR SCIENCE MONTHLY. 



some by way of Cuba, others by way of Central America, where a few 

 may linger, the main body presses onward beyond the Amazon into 

 central and southeastern Brazil. On the return journey they reach 

 the southern border of the United States in March and April. 



The catbird is found in summer throughout the eastern United 

 States and British Provinces, and in winter in the southern States, 

 Cuba and Middle America to Panama. Our common robin is very 

 erratic in habits of migration. Occasionally a few may winter in 

 dense swamps as far north as southern Canada and Maine, but the 

 majority spend the winter in the Southern States. The chimney 

 swift is found in summer in eastern North America and 

 thence north to Labrador and the fur countries. The winter is spent 

 to the south of the United States. Cliff and barn swallows, which are 

 found over nearly all North America in summer, may penetrate to 

 Brazil, Paraguay and the West Indies in winter. The scarlet tanager 

 passes the winter in the West Indies, Central America and northern 

 South America, and the familiar indigo bird may go as far as Veragua. 



The great group of warblers, of which some 70 species are found 

 in the United States, has been mentioned before. They are all strongly 

 migratory and mainly pass beyond our southern borders in winter, 

 although a few individuals of a single species — the yellow-rumped 

 warbler — have been known to winter on Cape Cod. Some of them 

 visit the West Indies but the larger number, after rearing their young 

 in the dense coniferous forests of the Hudson's Bay region or even in 

 Alaska, spend the winter in Mexico, Central America or northern 

 South America. 



The sparrows as a group are also strictly migratory. Quite a num- 

 ber, such as the tree sparrow {SpizelUa monticola), snowflake (Plec- 

 trophenax hyperhoreus) and longer spur (Calcarius lapponicus) breed 

 far to the north of the United States in Arctic districts, and come 

 dovm in winter into the northern states or irregularly farther south. 

 Many species which breed mainly north of the United States only go 

 into the middle and southern states during the winter, while a few 

 may reach the West Indies, Mexico, Central America or northern 

 South America. 



But after having described these migration routes and the wonder- 

 ful journeys over continents and vast oceans, the mystery of mysteries 

 — How is it possible for the birds to find their way so unerringly? — 

 still remains without a wholly satisfactory answer. As in the case 

 of theories propounded to account for the origin of migration, so 

 numerous suggestions have been made to explain this wonderful 

 faculty. Thus Dr. Von Middendorff, a distinguished naturalist who 

 studied exhaustively the migrations in the Russian Empire, suggests 

 that because all the spring movements in that country are toward the 



