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In Edward Jenner's day the greatest honour that could be bestowed upon a scientist was to be elected a Fellow of the Royal Society (FRS). Jenner was awarded that distinction in 1789 for a paper that explained the nesting habits of the cuckoo - a bird that had intrigued philosophers and naturalists since the days of Aristotle. The Nesting Habits of the Cuckoo The cuckoo is unique among birds in the way it parasitises other species of birds during the rearing of its young. It lays a single egg in the nest of a bird of another species, most commonly, the hedge sparrow. The foster-parents then feed and raise the young cuckoo as if it were their own. Only the young cuckoo survives. All eggs and fledglings belonging to the birds that built the nest disappear. Jenner determined to find out why only the cuckoo survives in each nest and why its parents adopt this strange way of breeding. After a false start based on some rather careless fieldwork done by his 16-year old nephew Henry, Edward Jenner carried out his own observations and re-wrote the paper he had been on the point of sending to the Royal Society. This revised presentation was submitted at the end of December 1787, accepted, and read to a meeting of the Society on 13 March 1788. Jenner was elected a Fellow on 25 February 1789, in recognition of his contribution. What Jenner observed What Edward Jenner had realised was that it was not the parent cuckoo that ejected the foster parents' eggs and chicks from the nest, as was previously believed, but the newly hatched cuckoo. In its first few days of life the fledgling bird worked its way backwards, up the side of the nest, pushing behind it an egg or young sparrow, until this could be thrown from the nest. It repeated this task until only it remained in the nest. It therefore took over the food supply provided tirelessly by the foster parents. By dissecting young cuckoos Edward Jenner discovered that their bodies are specially provided with a depression in the back, between the wings. In this the young bird cups the objects which it is pushing from the nest. This depression disappears before the fledgling cuckoo is 12 days old. Edward Jenner made many observations and experiments to support his hypothesis. He removed young cuckoos from nests, placed two in a single nest and replaced ejected eggs. He even fixed lead weights to the legs of young cuckoos to convince himself that only that fledgling could be responsible for the deadly deed. His findings remained contentious until the twentieth century when naturalists, such as Eric Hosking, were able to photograph the phenomenon. In explanation for this strange nesting behaviour, Jenner noted that the cuckoo did not appear in Britain until mid-April and had disappeared by about the first week in July. This period of only 11 weeks was not long enough for it to lay its several eggs, incubate and hatch them, then rear its offspring to an age at which they could fly strongly. This process, he noted, took at least 15 weeks. Fostering was its ideal solution.
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