5. Some birds dust themselves, and others bathe. Some neither dust nor bathe. Those that do not fly, but live on the ground, dust themselves, as the domestic fowl, partridge, grouse, lark, and pheasant. Those birds which have straight claws, and live near rivers, marshes, and the sea, bathe themselves. Some, like the pigeon and sparrow, both dust and bathe. Most of those with crooked claws do neither the one nor the other. This is their nature in these matters. The act of breaking wind backwards is peculiar to some birds, as the turtle. Such birds make a strong motion with their rumps when they utter their voice. CHAPTER XXXVII. 1. Animals not only change their forms and dispositions at particular ages and seasons, but also when castrated. All animals that have testicles may be castrated. Birds and oviparous quadrupeds have internal testicles near their loins. In viviparous animals with feet, they are generally external, though sometimes internal; in all they are situated at the extremity of the abdomen. Birds are castrated near the rump, the part with which they touch the female in copulation, for if they are burnt in that part two or three times with irons after they are full grown, the comb turns yellow, and they cease to crow, and no longer desire sexual intercourse. If they are not full grown, these parts never reach perfection. 2. The same is the case with the human subject, for if a boy is castrated, the hair that is produced after birth never appears, nor does his voice change, but continues sharp; but if a full grown man is castrated, all the hair produced after birth falls off except that on the pubes, this becomes weaker, but still remains. The hair produced at birth does not fall off, for the eunuch never becomes bald. The voice also of castrated animals changes to that of the female. Other animals, if not castrated when young, are destroyed by the operation; with the boar it makes no difference. All animals, if castrated when young, become larger and more graceful than those not castrated; but if already grown, they never become any larger. 8. If stags are castrated before they are old enough to have horns, these never appear; but if castrated after they have horns, their size never varies, nor are they subject to their annual change. Calves are castrated at a year old, if not they become bad and inferior. The steer is castrated in this manner: they lay down the animal and cut the scrotum, and press out the testicles; they next contract the root of the testicle as much as possible, and fill up the wound with hair in order that the discharge may escape, and if it inflames, they cauterize and sprinkle the scrotum. If adult bulls are castrated, they are still apparently capable of sexual intercourse.