“The attention of birds in ordinary or direct flight is immediately fixed on the objects before them. The swiftness with which they shoot through the air makes every visual impression indistinct and evanescent on either side of them. If they take wing for a distance, they rise at once high, that they may command a view of the place which they intend to visit; and if they proceed to an object that is near, they elevate themselves to such a height only as is necessary to give them a clear and direct course to where they are speeding. The circular flight of raptorial birds, is therefore the result of their directing their vision to the centre of the gyrations they describe in examining their prey, or descending upon their victims. “The eye of all birds is large and prominent. The prominence widens the field of vision. The width of the circle which the several kinds of raptorial birds variously describe, I think, as a rule, will be found to be determined by the size of the head and position of the eyes, or increased with the rotundity of the head of the bird. The direct vision being altered with the increase of space between the eyes, Hawks of the Buzzard kind, which have large and round heads, may be expected to wheel in wide circles; the expanded space being required, that they may keep the vision of their wide-apart eyes direct upon the objects beneath them. “Owls fly differently. They search for their prey, as if they were pursuing it with the vigilance of the hound. They skim along the surface of the earth, glide among trees, explore avenues, sweep round, rise and fall, wheel short, and dart down, but never sail in circles. Their wide staring eyes are placed in what may be called their face, being right forward in front, and have scarcely any field of vision laterally. They therefore hunt with a forward and downward gaze, like dogs over a field. The globe of the eye of these nocturnal _raptores_, being immoveably fixed in the socket by a strong elastic cartilaginous case, in the form of a truncated cone, they have to turn their heads to view objects out of the path of flight, and their neck is so adapted for this exertion, that they can with ease turn round the head in almost a complete circle, without moving the body.”