The Cypselidæ, or Swifts, are Swallow-like birds, generally of rather dull plumage and medium size. They were formerly associated with the true Swallows on account of their small, deeply cleft bill, wide gape, short feet, and long wings, but are very different in all the essentials of structure, belonging, indeed, to a different order or suborder. The bill is much smaller and shorter; the edges greatly inflected; the nostrils superior, instead of lateral, and without bristles. The wing is more falcate, with ten primaries instead of nine. The tail has ten feathers instead of twelve. The feet are weaker, without distinct scutellæ; the hind toe is more or less versatile, the anterior toes frequently lack the normal number of joints, and there are other features which clearly justify the wide separation here given, especially the difference in the vocal organs. Strange as the statement may be, their nearest relatives are the Trochilidæ, or Humming-Birds, notwithstanding the bills of the two are as opposite in shape as can readily be conceived. The sternum of the Cypselidæ is also very different from that of the Hirundinidæ, as will be shown by the accompanying figure. There are no emarginations or openings in the posterior edge, which is regularly curved. The keel rises high, for the attachment of the powerful pectoral muscles. The manubrium is almost entirely wanting.
Chætura pelagica.
Progne subis.
In this family, as in the Caprimulgidæ, we find deviations in certain forms from the normal number of phalanges to the toes, which serve to divide it into two sections. In one, the Chæturinæ, these are 2, 3, 4, and 5, as
usual; but in the Cypselinæ they are 2, 3, 3, and 3, as shown in the accompanying cut borrowed from Dr. Sclater’s masterly memoir on the Cypselidæ, (Pr. Zoöl. Soc. London, 1865, 593), which also serves as the basis of the arrangement here presented.
Left foot of Chætura zonaris.
