BIED MIGEATIOISr. 11 



task, and this is undoubtedly multiplied several times by their zigzag 

 twistings and turnings in pursuit of food. 



The arctic tern has more hours of daylight and smilight than any 

 other animal on the globe. At the most northern nestmg site the 

 midnight sun has already appeared before the bkds' arrival, and it 

 never sets during their entire stay at the breeding grounds. During 

 two months of their sojourn in the Antarctic the birds do not see a 

 smiset, and for the rest of the time the sun dips only a little way 

 below the horizon and broad daylight is continuous. The bkds 

 therefore have 24 hours of daylight for at least eight months m the 

 year, and during the other four months have considerably more 

 dayhght than darkness. 



ROUTES OF MIGRATION. 



The shape of the land areas m the northern half of the Western 

 Hemisphere and the nature of the surface has tended to great vari- 

 ations in migratory movements. If the whole area from Brazil to 

 Canada were a plam with the general characteristics of the middle 

 section of the Mississippi Valley, the study of bu'd migration would 

 lose much of its fascination. There would be a simple rhythmical 

 swinging of the migration pendulum back and forth, spring and fall. 

 But much of the earth's surface between Brazil and Canada is occu- 

 pied by the Gulf of Mexico, the Caribbean Sea, and parts of the Atlan- 

 tic Ocean, aU devoid of sustenance for land birds. The two areas of 

 abundant food supply are North America and northern South Amer- 

 ica, separated by the comparatively smaU land areas of Mexico and 

 Central America, the islands of the West Indies, and the great waste 

 stretches of water. 



The different courses taken by the birds to get around or over this 

 intervening inhospitable region are almost as numerous as the bird 

 families that traverse them, and only some of the more important 

 routes will be mentioned here. (See fig. 2.) 



ISLAND ROUTES. 



Birds often seem eccentric in choice of route, and many do not 

 tak(i the shortest line. The 50 species from New England that 

 wint<!r in South America, instead of making the direct trip over the 

 Atlantic involving a flight of 2,000 miles, take a somewhat longer 

 route that follows the coast to Florida and passes thence by island or 

 mainland to S(;uth America. What would at ftrst sight seem to bo 

 a natural and convenient migratory highway extends from Florida 

 through th(! Biduimas or Cuba to Haiti, Porto liico, and the Lesser 

 Antilles and thence to South America (see fig. 2, route 2) . Birds that 

 travel by this route need never bo out of sight of land ; resLing ])lae(w 

 are affoi-dcd .-it convenient inlcrvids und {\w. dis(,unc.(i is hut litthi 



