a difficulty might perhaps be raised about birds. how, it may be said, can they, either when they fly or when they walk, be said to move at four points? now we did not say that all sanguinea move at four points, but merely at not more than four. moreover, they cannot as a fact fly if their legs be removed, nor walk without their wings. even a man does not walk without moving his shoulders. everything indeed, as we have said, makes a change of place by flexion and straightening, for all things progress by pressing upon what being beneath them up to a point gives way as it were gradually; accordingly, even if there be no flexion in another member, there must be at least in the point whence motion begins, is in feathered (flying) insects at the base of the ‘scale-wing’, in birds at the base of the wing, in others at the base of the corresponding member, the fins, for instance, in fish. in others, for example snakes, the flexion begins in the joints of the body.
in winged creatures the tail serves, like a ship’s rudder, to keep the flying thing in its course. the tail then must like other limbs be able to bend at the point of attachment. and so flying insects, and birds (schizoptera) whose tails are ill-adapted for the use in question, for example peacocks, and domestic cocks, and generally birds that hardly fly, cannot steer a straight course. flying insects have absolutely no tail, and so drift along like a rudderless vessel, and beat against anything they happen upon; and this applies equally to sharded insects, like the scarab-beetle and the chafer, and to unsharded, like bees and wasps. further, birds that are not made for flight have a tail that is of no use; for instance the purple coot and the heron and all water-fowl. these fly stretching out their feet as a substitute for a tail, and use their legs instead of a tail to direct their flight. the flight of insects is slow and frail because the character of their feathery wings is not proportionate to the bulk of their body; this is heavy, their wings small and frail, and so the flight they use is like a cargo boat attempting to make its voyage with oars; now the frailty both of the actual wings and of the outgrowths upon them contributes in a measure to the flight described. among birds, the peacock’s tail is at one time useless because of its size, at another because it is shed. but birds are in general at the opposite pole to flying insects as regards their feathers, but especially the swiftest flyers among them. (these are the birds with curved talons, for swiftness of wing is useful to their mode of life.) the rest of their bodily structure is in harmony with their peculiar movement, the small head, the slight neck, the strong and acute breastbone (acute like the prow of a clipper-built vessel, so as to be well-girt, and strong by dint of its mass of flesh), in order to be able to push away the air that beats against it, and that easily and without exhaustion. the hind-quarters, too, are light and taper again, in order to conform to the movement of the front and not by their breadth to suck the air.
