THE GENUS CHORDEILES SWAINSON" — OBERHOLSEE. 19 



similar kind, and are mostly differences of color ; while in Chordeiles 

 rupestns they consist more in the pattern of coloration. 



The variation due to age is likewise about the same in Chordeiles 

 'oirginianus and Chordeiles acutipennis^ and is so great that there is 

 no difficulty at all in distinguishing immature from adult birds. The 

 immature plumages of Chordeiles rupestris appear to be unknown, 

 but their relative differences are probably much the same as those of 

 the other species of the genus. 



Geographic variation is not so great as in many other birds of even 

 less but otherwise similar geographic range but of greater plasticity. 

 This variation has been kept down in part, doubtless, by the extensive 

 migration of the northern forms ; but it probably can not be fully ac- 

 counted for in such a way, since the species least or not at all migra- 

 tory, Chordeiles acutipennis and Chordeiles rupestris, are seen to 

 be, if anything, less variable than the highly migratory Chordeiles 

 virginianus. Apparently such a condition of limited geographic vari- 

 ation is induced also by the excessive individual variation in species 

 of this genus; by their inherent smaller degree of plasticity; as well 

 as by the phylogenetic age of the species, as compared with those of 

 other genera. The last may also be a determining factor in the species 

 of Chordeiles, as compared with each other; and if Chordeiles acuti- 

 pennis and Chordeiles rupestris are of more recent evolution than 

 Chordeiles virginianus, as seems probable, this may be, in geographic 

 variation, an offset to their more sedentary habits. 



General habits. — The nighthawks of this genus are birds of the 

 open country. Being strong of wing, they are of easy though power- 

 ful flight, and spend much time in the air, where they obtain most of 

 their insect food, which their great mouths admirably fit them to 

 scoop in while on the wing. Their feet are weak, and not well 

 adapted to perching in the ordinary way ; so the birds rarely alight 

 high up in trees, but crouch on the ground or other flat surface, on 

 posts, fence-rails, rocks, prostrate trees, or low branches; and, when 

 on a branch, log or fence-rail, sit almost invariably lengthwise of the 

 perch. 



These nighthawks build no nest, but deposit their two well-mottled 

 eggs on the ground or similar places. The young are practically 

 helpless for some time after birth. 



Unfortunately these birds furnish a good mark for the hunter, and 

 often on this account fall to the gun of the sportsman or pot-hunter. 

 They deserve a much better fate, for they are beneficial birds, living 

 almost entirely, if not exclusively, on insects. 



History. — The earliest account of any species of this genus is by 

 Catesby,^ who in 1743 described Chordeiles virginianus as the 



1 Nat. Hist. Carolina, Florida, and Bahama Islands, vol. 2, Appendix, 1743, p. 16, pi. 16. 



