to conclude have traveled together and arrived simultaneously. Prob- 

 ably they did, but such combined migration is by no means the rule 

 for all species. 



As a group the wood warblers {Compsothlypidae) probably travel 

 more in mixed companies than do any other single family of North 

 American birds. The flocks are likely to be made up of several species 

 in spring and fall with both adults and young. Sometimes swallows, 

 sparrows, blackbirds, and some of the shore birds also migrate in mixed 

 flocks. In fall, great flocks of blackbirds frequently sweep south across 

 the Plains States, and occasionally one flock will contain bronzed 

 grackles, red-winged blackbirds, yellow-headed blackbirds, and 

 Brewer's blackbirds. 



On the other hand many species keep strictly to themselves. It 

 would be difficult for any other kind of bird to keep in company with 

 one of such rapid movements as the chimney swift, which is rarely 

 found associated with any other species at any season. Nighthawks 

 or bullbats also fly in separate companies, as do usually crows, wax- 

 wings, crossbills, bobolinks, and kingbirds. Occasionally, a flock of 

 ducks will be observed to contain several species, but generally when 

 they are actually on migration the individuals of each species separate 

 and travel with others of their own kind. The flocks of blue geese, 

 previously mentioned in connection with speed of flight (p. 22), fre- 

 quently have with them a few of the closely related snow geese, par- 

 ticularly in the eastern part of their winter range. The portion here 

 is usually about 10 to i, but farther west the numbers of snow geese 

 increase until they outnumber their blue relatives. 



The adults of most perching birds drive the young away when they 

 are grown, probably to be relieved of the necessity of providing for 

 them, and also in order that the parents may have opportunity to rest 

 and renew their plumage before starting for winter quarters. The 

 young birds are therefore likely to drift together and, having no further 

 responsibility, may start south ahead of their parents. In contrast with 

 this indifference on the part of the adults of perching birds, Canada 

 geese and some others remain in family groups, the parent birds under- 

 going the wing molt that renders them flightless during the period of 

 growth of their young, so that old and young acquire their full plumage 

 at the same time and are able to start south together. The large flocks, 

 therefore, are composed of many families that band together, and 

 when they separate into V-shaped units it is probably correct to assume 



32 



