REGULARITY OF MIGRATION 79 



may be casual wandering among Individuals that for 

 some reason are not breeding. Observations on 

 migration may thus be made throughout most of the 

 year. 



Segregation during Migration 



Many birds are distinctly segregated from others 

 of their kind during their migrations by certain in- 

 dividual peculiarities. Nighthawks fly in separate 

 companies because of their erratic flight. A similar 

 restriction is found in swifts, whose rapid move- 

 ments through the air and habit of roosting in hol- 

 low trees or chimneys cause them to travel apart 

 from any other birds. Glossy ibises, crows, king- 

 birds, crossbills, waxwings, and bobolinks, for rea- 

 sons some of which may not be easily apparent, usu- 

 ally are found gathered with others of their kind 

 during their travels. 



Flocks of swallows, on the other hand, often travel 

 in mixed companies composed of several species, as 

 do blackbirds and grackles, shearwaters, and wood 

 warblers. Similarities of size, form, and method of 

 search for food draw them together. It is probable 

 that many sparrows migrate in mixed companies as 

 they are associated thus by day; but there is no 

 means of observing definitely their nocturnal pas- 

 sages except through the medium of their call-notes, 



