VI CYPSELIDAE A421 
Swifts are essentially aérial, seldom alighting upon the 
ground, or perching except at night,’ 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 
circhng 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. horus 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 1s composed of seeds of plants; but generally it is saucer-lke, 
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 afinis 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 Mucropteryx mystacea settling on trees in the day-time, 
and Shufeldt saw Cypselus melanolewcus sitting on rocky pinnacles. 
