Although Jenner's time was chiefly occupied with his professional duties, he still kept up a constant and regular correspondence with his friend John Hunter on different scientific subjects. He managed also to find leisure to institute many experiments and observations in natural history, one of the results of which was his account of the Cuckoo, a most carefully elaborated essay, and which has always been considered as a model of accurate observation. This paper was read to the Royal Society on the 10th of March, 1788, and printed in their 'Transactions.' It explained the habits of this curious bird very satisfactorily, and its publication at once secured the author a considerable reputation as a Naturalist. As this paper appears not to be very generally known, the following account taken from it may be interesting:-- "The cuckoo furtively deposits her egg in the nest of another bird; it is done not that her offspring may be a sharer of the care of the foster-parent, but that it may engross it entirely to the total destruction of its own natural offspring. A perversion of all the maternal instincts is a most remarkable result of this vicarious incubation. The hedge-sparrow, or other birds whose nests have been visited by the cuckoo, actually sometimes eject their own eggs to make room for the new guest; but it occasionally happens that this is not done; the eggs are not disturbed, and the process of hatching is allowed to go on regularly, and the young sparrows and the cuckoo emerge from the shell about the same time. This event, when it is permitted to happen, does not at all improve the condition of the former; on the contrary, it only exposes them to greater sufferings. The size of the egg of the cuckoo does not vary much from that of the bird in whose nest it is deposited. When the young sparrow, therefore, and the intruder first come into life, they are pretty much on an equality; but unhappily for the foster-brethren, this equality does not last long: the cuckoo's growth rapidly outstrips that of his companions, and he immediately exercises his new powers with abundant selfishness and cruelty. By a singular configuration of his own body he contrives to lodge his companions, one by one, upon his back, and then scrambling up the sides of the nest, he suddenly throws them from their seat, and completely ejects them from their own home to become food for worms. There is reason to believe that the unnatural parent is often an unmoved witness of this atrocity. Her whole care and affection are absorbed by the intruder, and her own flesh and blood literally turned out to perish. It sometimes, though very rarely, happens that two cuckoo's eggs are deposited in the same nest. When this occurs, and they are both hatched together, a bitter feud arises, which is only terminated by the ejection of one or other from the nest."