Monday, November 30, 2009

A Historic Paper on Cuckoo Behavior

In the coming year, the Royal Society is entering its 350th year of existence. It was founded on November 28, 1660, as a forum for scientists and philosophers to discuss their work. Since 1665, it has published the journal Philosophical Transactions. To celebrate its anniversary, the Royal Society is posting sixty of the most significant papers from Philosophical Transactions. You can find them at the society's Trailblazing website.

Common Cuckoo (Source: Naumann, Naturgeschichte der Vögel Mitteleuropas)

Nestled among the other historic papers is one that concerns birds. In 1788, Edward Jenner reported his observations of Common Cuckoos (Cuculus canorus). Common Cuckoos, like several other species, are nest parasites. This means that, rather than building and incubating its own nest, an adult female will lay its eggs in the nests of other bird species. A bird of a different species will then be responsible for the incubation and rearing of the young cuckoo. Brown-headed Cowbirds are a well-known example of this breeding behavior in North America.

According to Jenner's paper, he observed cuckoos laying eggs in the nests of "the Hedge-sparrow, the Water-wagtail, the Titlark, the Yellowhammer, the green Linnet, and the Whinchat." Of these, he most frequently saw cuckoos using the nests of the Hedge-sparrow, which I believe is now called Dunnock (Prunella modularis). In most cases, the offspring of the foster parent would be thrown out of the nest, while only the cuckoo chick remained for feeding.

On June 19, 1787, Jenner watched a cuckoo chick expel one of its nest mates:
The mode of accomplishing this was very curious. The little animal, with the assistance of its rump and wings, contrived to get the bird upon its back, and making a lodgement for the burden by elevating its elbows, clambered backward with it up the side of the nest till it reached the top, where resting for a moment, it threw off its load with a jerk, and quite disengaged it from the nest. It remained in this situation a short time, feeling about with the extremities of its wings, as if to be convinced that the business was properly executed, and then dropped into the nest again. With these (the extremities of its wings) I have often seen it examine, as it were, an egg and nestling before it began its operations; and the nice sensibility which these parts appeared to possess seemed sufficiently to compensate the loss of sight, which as yet it was destitute of. I afterwards put in an egg, and this, by a similar process, was conveyed to the edge of the nest, and thrown out. These experiments I have since repeated several times in different nests, and have always found the young Cuckoo disposed to act in the same manner.
The body of the cuckoo chick is suited to expelling other chicks:
The singularity of its shape is well adapted to these purposes; for, different from other newly-hatched birds, its back from the scapulae downwards is very broad, with a considerable depression in the middle. This depression seems formed by nature for the design of giving a more secure lodgement to the egg of the Hedge-sparrow, or its young one, when the young Cuckoo is employed in removing either of them from the nest. When it is about twelve days old, this cavity is quite filled up, and then the back assumes the shape of nestling birds in general.
On occasion, an adult bird might expel one of its own eggs to make room for the cuckoo egg, but once the eggs were hatched the adult cared for all equally, provided that they remained in the nest. If two cuckoo eggs happened to be laid in the same nest, the two young cuckoos would struggle for control of the nest.

Jenner explains the cuckoo's parasitism by referencing the short time cuckoos have to reproduce. Adults arrive in Britain in mid-April, begin laying in mid-May, and depart in early July. Laying eggs in the nests of other bird species allows cuckoos to maintain a large population despite this short breeding season.

Reading this paper was a look not just at cuckoos but at the scientific culture of Jenner's day. Like other naturalists of his day, he readily shot birds to confirm identification or examine them. He also collected eggs and received eggs from others. Both of those activities would require permits today, at least in the United States. This paper was written long before Origin of Species, and the lack of a concept of natural selection is evident in several passages, including the second one above. Instead, Jenner refers several times to nature's design, a concept that he does not have much elaboration.
Jenner, E. (1788). Observations on the Natural History of the Cuckoo. By Mr. Edward Jenner. In a Letter to John Hunter, Esq. F. R. S. Philosophical Transactions of the Royal Society of London (1776-1886), 78 (1), 219-237 DOI: 10.1098/rstl.1788.0016