vi CYPSELIDAE 421 



Swifts are essentially aerial, seldom alighting upon the 

 ground, or perching except at night, 1 though they will cling to 

 the entrance of their breeding quarters for a few seconds before 

 entering. From a smooth flat surface they can hardly rise, but 

 in the air they are perfectly at home, whether wheeling and 

 circling at great altitudes, chasing each other aloft, consummating 

 their love affairs, or sweeping over the earth's surface in pursuit 

 of insects attracted by the damp. The exceptionally rapid flight 

 is strong and practically unlimited in duration, two or three 

 quick movements of the wings being repeatedly succeeded by a 

 gliding motion. Though not gregarious in the ordinary sense, 

 they habitually breed in company, and Collocalia nests in vast 

 colonies ; a solitary bird, moreover, is comparatively seldom seen, 

 and both before and during incubation our Common Swift flies 

 in screaming flocks around the chosen sites. This species will 

 pass and re-pass close to a pedestrian's head with noisy and 

 apparently vicious rush, even when far from the nest ; yet it is 

 not really the intruder but insects that are the attraction, the 

 food being entirely of that nature, and invariably captured in 

 the air, while the beak may be seen filled to repletion when 

 nestlings require to be supported. The voice is a shrill scream, 

 constantly repeated. The districts frequented are of every descrip- 

 tion, Cypselus andicola and C. ~korus being particularly alpine ; the 

 nest varies to a considerable extent, though a glutinous substance 

 secreted by the highly developed salivary glands is a constant, or 

 frequently almost the sole, material. The situation may be a hole 

 under thatch, slates or tiles ; a crevice in a building, cliff, or tree ; 

 the perpendicular wall of a cave; the upper side of a branch, 

 palm-leaf, or broad stalk ; the lower surface of a rock, and so 

 forth. The shape of the structure is tubular in Panyptila, where 

 it is composed of seeds of plants ; but generally it is saucer-like, 

 the materials being straw, feathers, twigs, moss, or cottony 

 vegetable matter, the first two of which have been stated to be 

 caught floating in the air. The American Chimney-Swift plucks 

 off branchlets as it flies. Cypselus affinis and the species of Collo- 

 calia commonly join their nests together in masses ; Palm Swifts 

 do so more rarely ; Cypselus caffer even utilizes those of other birds. 

 The dull white eggs are oval and almost uniform at each end ; 



1 D'Albertis noticed Macropteryx mystacea settling on trees in the day-time, 

 and Shufeldt saw Cypselus melanoleucus sitting on rocky pinnacles. 



