8o THE MIGRATIONS OF BIRDS 



which seem to indicate that they travel in flocks 

 composed of several species. 



Some birds in migration maintain a close flock- 

 formation, as is seen in shore-birds, blackbirds, wax- 

 wings, longspurs, and snow buntings. Others travel 

 in loose order, as turkey vultures, bluejays, warblers, 

 pipits, horned larks, and bluebirds. Still others, like 

 great horned owls, winter wrens, shrikes, grebes, and 

 kingfishers, travel alone in ordinary circumstances, 

 and appear in a close proximity that might indicate 

 flocking only where abundant food or other unusual 

 condition draws them together. There is some geo- 

 graphic variation in this regard, as red-tailed and 

 rough-legged hawks in eastern North America 

 travel singly, but in the middle west, may be found 

 in large flocks. 



Migrations among males and females of the same 

 species may take place simultaneously, or the two 

 sexes may move separately. In many species in 

 spring migration the males are the first arrivals on 

 the nesting ground, as under the territorial theory 

 each one is under necessity of selecting a site for 

 summer occupancy. Early migrant robins are usu- 

 ally males, the first song sparrows to arrive in spring 

 greet us with cheerful songs that betray their sex, 

 the first tanagers and rose-breasted grosbeaks are 

 usually males; in the two last-mentioned I have seen 

 whole flocks made up of these bright-colored indi- 



