LINES OF MIGRATORY FLIGHT 135 



may be relatively narrow, depending upon the habi- 

 tat chosen by birds as a place to live, as exemplified 

 by the knot, the purple sandpiper, the surf-bird, and 

 others which, except when on their breeding 

 grounds, normally are found along sea-coasts, where 

 their chosen home is bounded on one side by broad 

 reaches of salt water and on the other by land or 

 fresh water, both equally unsuited to furnish the 

 food and haunt desired by these species and neces- 

 sary to their well-being. The knot ventures some- 

 what casually inland, but the bulk of its flight is 

 coastal. The others are strictly maritime in distri- 

 bution except in their northern homes. These birds 

 travel in narrow lanes from which there is no pro- 

 nounced deviation. 



In those species that migrate habitually overland 

 the case is quite different, since here the migration 

 path is broad and diffuse, with indefinite boundaries. 

 Throughout the entire northern hemisphere, with 

 the exception perhaps of such tremendous altitudes 

 as are encountered on Mount Everest, there is in all 

 probability no point at which migrants do not pass. 

 In or over prairies or forested areas, mountains or 

 valleys, deserts or marshes, lakes or plains, we find 

 migrant birds at some time during the year, or under 

 proper conditions hear their calls as they pass over- 

 head at night. It is true that there are lines of pro- 

 nounced concentration in migration marked by some 



