The period of incubation varies according to the species, from about two to three weeks. In some species it is still longer; the sparrow hawk, for example, broods her eggs for about five weeks. The whole of the brooding is in many cases done by the female. This is the practice with which we are familiar among our barn-door fowls, and it therefore seems to us the most natural. The cock bird meanwhile fetches food for his mate. This is quite noticeable among the rooks, and until I understood it, I used to believe there were young in the rooks' nests long before it was time for the eggs to be hatched. It is easy to make the mistake, for unless you watch the rookery closely you do not know when the hen begins to sit. Then one day you notice a bird coming to the nest and hear the excited cawing as he arrives and the happy gurgling as the bird in the nest gulps the food down, and you at once think of the feeding of young birds.
There is in the rook, however, as there is in most of the crow family, a sense of fun, and this shows itself in the cock bird's playful habit of teasing his mate. He does not always go to the nest when he comes home, but sometimes settles on a neighbouring branch and caws in reply to the hen's urgent greeting, as much as to say he has brought nothing with him, that food has been very scarce to-day, or that he has been robbed on the way home. It is a doleful tale; but his wife knows better. After repeatedly trying in vain to get him to be sensible, she leaves the nest and hops over to his perch, and at last after much persuasion and heartrending demonstration of helplessness, he consents to part with the fruits of his hunting. This charming scene is likely to be missed by casual observers, for unlike the thrush, the rook does not bring home food in his bill, but carries it in a pouch under his tongue. The outward and visible sign of this is a large bulge under the beak which can be seen only when we view the bird from certain angles. It has been suggested to me that the whole thing is merely rook cunning, that the wily old male wishes to keep the tit-bits to himself. But in that case why should he bring them home at all? While his mate is brooding he has the whole day in which to disport himself and can feed to his heart's content on the fat of the land, and surely if he wished to keep anything to himself his simplest plan would be to swallow it where he found it. Besides, an important part of his love display is the presentation of dainties to his mate, and he probably derives pleasure from this graceful act as a man does in giving his wife a present. The male rook's seeming unwillingness sometimes to part with his gain at the nest is either an attempt to prove that he is lord and master, which is unlikely, or, as I think most probable, it is pure teasing, with perhaps a desire for the additional pleasure of being wheedled. That the voice of the wheedler is harsh to our ears signifies nothing; it is doubtless sweet as honey to his.
If this be the true explanation of it, the practice is another of those delightful bits of uselessness that make bird life so attractive to ourselves. It is possible to argue that the male rook is merely a slave of an instinct which constrains him to withhold his offerings now and then in order to force his mate to leave the nest, for without such intervals she would die of cramp or rheumatism or would smother the chicks in the eggs through allowing them insufficient ventilation. But we need only turn to other birds to find a full answer to our ingenuity. The cock thrush, blackbird, robin, chaffinch and others also feed the hen on the nest, but nevertheless she has enough sense to care for her own health by leaving her nursery and stretching her wings for a few minutes several times a day. We are told that no habit can be developed by any creature unless it serves a useful purpose, and that even then it can be evolved only by the merciless weeding out of those individuals in whose composition it has been omitted or indifferently represented. In that case it would be necessary to assume that teasing is an essential part of rook economy and that all male rooks which had failed to tease their wives had been abolished as unfit. But we can also assume that the stealing and hiding of diamond rings are an essential part of jackdaw, magpie and raven economy and that all individuals of the three species, which have been inefficient in these great arts, have been destroyed as too good to succeed in the struggle for existence? These members of the crow family when kept in captivity have given ample proof that they are possessed of at least an elementary sense of humour, so it is reasonable to accept the rook's teasing as evidence that he is also endowed with this social virtue. Humour and the stealing and hiding of diamond rings are quite useless qualities to a bird, and are therefore unselectable, but no doubt they add to the pleasures of life.