32 BULLETIN" 86, UNITED STATES NATIONAL MUSEUM. 



vironmental conditions over this area, together with the very brief 

 period of each year that the bird spends on its breeding grounds, 

 where alone the differentiating influences have opportunity to exert 

 themselves. Under these conditions it is entirely probable that any 

 future development of geographical races from what is now the sub- 

 species Chordeiles virginianus virginianus will be exceedingly slow. 



Migration. — Comparatively few birds of any kind exceed this 

 nighthawk in the length of their migration journeys, and very few 

 land birds travel as far. From the Arctic Circle in the Territory 

 of Yukon to Patagonia is more than 8,000 miles, but the nighthawk's 

 long wings and consequent powers of flight enable it easily to per- 

 form this extensive regular annual journey. Judging from the 

 scarcity of available records, it seems probable that the passage 

 through Mexico and Central America, and over the West Indies, the 

 Gulf of Mexico, and the Caribbean Sea to northern or central South 

 America is made with considerable rapidity. 



Like most other birds which travel long migration routes, this 

 species is, generally speaking, an early migrant in autumn and a com- 

 paratively late arrival in spring. Notwithstanding this, the first in- 

 dividuals in spring sometimes reach the southern border of the 

 United States by March 15, and the latest stragglers in autumn occa- 

 sionally prolong their stay in the same region until even November 

 27 ; but these dates are of course exceptional, since the spring appear- 

 ance is usually three or four weeks later, and the autumnal disappear- 

 ance a month or so earlier. The nighthawk usually reaches Montreal 

 and Quebec early in May ; Grand Falls, the northern limit of its range 

 in central Quebec, during the last week of May ; Fort Chipewyan, Al- 

 berta, the last week of May ; Fort Wrigley, Mackenzie, the first week 

 of June ; and British Columbia by the last of May or first of June. 

 On the other hand, migrant birds are sometimes still in southern 

 Texas on May 29, and even in Costa Rica as late as May 27. 



The southward migration must begin very soon after the young are 

 on the wing, since transient birds from the north appear in the 

 middle and southern United States by the first of August, sometimes 

 by the middle of Jul}^; and they reach Nicaragua by the middle of 

 September. Montreal in Quebec, also southern Manitoba and British 

 Columbia, are deserted usually during the first two weeks of Sep- 

 tember, and Fort McMurray, Alberta, in August. It is thus evident 

 that the migratory movements, in the United States at least, cover 

 a total period of more than four months in autumn and of nearly 

 three months in spring. There is thus in Colorado and the surround- 

 ing States a scant three or four weeks' period of summer, from about 

 June 15 or 20 until the middle of July, during which the presence 

 of a nighthawk is presumptive evidence of its breeding in the locality. 



The above remarks apply to Chordeiles virginianus as a species, 

 since sufficient data have not yet accumulated to work out the migra- 



