Page:Natural History, Birds.djvu/77

64 breast-bone (sternum) of great surface in proportion. Hence their flight is swift and vigorous, perhaps in a degree greater than that of any other birds; and it is capable of being long, almost constantly, sustained during day without fatigue. By the inconceivably rapid vibration of these powerful wings, they have the power, possessed by many insects, of poising themselves in the air, where they hang apparently motionless, while their wings, through the extreme swiftness of their oscillations, are rendered invisible, except as an undefined misty cloud on each side. The vibration of these organs produces, by their impact upon the air, a humming or whirring sound, more or less shrill according to the species, whence the common name of the birds is derived. Their feet are small and weak, and are in little request, so much of their time being passed upon the wing.



The beak is long and slender, sometimes straight, sometimes curved downward, and in one or two species even curved upward. The tongue is slender and capable of protrusion to a great extent; when recent, it presents the appearance of two tubes laid side by side united for half the length, but separate for the remainder. The substance of these is transparent in the same degree as a good quill, which, under a microscope, they much resemble: each tube is formed by a