Brain Pickings

The Unfeathered Bird: An Illustrated History of Avian Anatomy

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Evolutionary eccentricities, ornithological oddities, and the engineering mysteries of flight.

Birds are an incessant source of scientific fascination, from why they sing to how their wings work. The Unfeathered Bird (public library; UK) by Katrina van Grouw isn’t about the anatomy of birds — it’s about “how their appearance, posture, and behavior influence, and are influenced by, their internal structure.” Though originally intended as a tool for artists, the book is also rigorously scientific but without the burdensome language and clunky terminology of anatomical writing. What emerges is an illuminating and meticulously illustrated look at the brilliance of birds at the intersection of art, science and history, covering such intricate mysteries as how the ostrich lost two of its four toes and why the vulture diverged into radically different Old World and New World varieties. The 385 or so intricate drawings include a number of species never illustrated before and explore everything from the mechanics of flight to the aerodynamics of avian skulls.

Van Grouw — who is herself a remarkable cross-pollinator of disciplines and perspectives as a taxidermist, RCA-trained fine artist, and former curator of the ornithological collections at London’s Natural History Museum — writes of the mechanical miracle of flight:

Flight makes rather specific demands on the physical engineering of an animal. The skeleton needs to be of a lightweight structure, with large flattened surfaces for the attachment of muscles, and to have tremendous rigidity and the strength to support the entire weight of the animal while airborne. The components are highly specialized and once a satisfactory blueprint has been achieved there is very little room for modification. The paradox, then, is that although the birds represent the largest class of all the vertebrates — approaching ten thousand species — they are fundamentally rather uniform; though with some very surprising variations!

The adaptation for flight is the most important factor behind the structure of birds and can provide an explanation for virtually all of their anatomical characteristics — even those that seem to have nothing to do with flying. For example, with wings instead of front legs, birds need two strong hind limbs and a modified posture to balance on them. And with a body rigid enough to cope with the demands of flapping flight, it’s vital to have a long and flexible neck to compensate for the loss of movement. But it’s important to remember that birds didn’t learn to fly first and develop these perfections afterward. Many of these qualities had long been present in the birds’ theropod ancestors — the upright dinosaurs that walked on two legs — and only through a constant process of adaptation and counteradaptation spanning millions of years did it become possible for the feathered dinosaurs to survive and take wing.

One of the most intriguing chapters deals with the divergence of vulture species between the New and Old World:

The universal truths about vultures are, as every schoolchild knows, as follows: they have a bare head, a hooked beak, and long, broad wings, and they eat things they find dead. Few definitions could be more cut and dried. All over the Americas, Europe, and Asia this very uniform group of birds can be instantly recognized and, on a group level at least, poses no problems of identification.

So when, in the 1980s, the newly developed techniques for hybridizing strands of DNA revealed that the New World vultures may not be vultures at all but close relatives of the storks, it created something of a sensation. Indeed, to the average birdwatcher the concept seemed to symbolize the chaos that test-tube technology would drag their world into without the steadying hand of empirical common sense. What many birdwatchers didn’t realize was that this concept wasn’t new. In fact, by the time the DNA experiments were taking place, it had already been around for over a hundred years, based on a range of complex anatomical features: the musculature of the wings, formation of the intestines, and so forth. More recent DNA research has once again made the position of the New World vultures uncertain. They are probably not, after all, stork relatives, and for the time being some authorities have tentatively returned them to the company of other hook-billed birds.

What is perhaps most remarkable, however, is not that New and Old World vultures may not be related but that two possibly unrelated groups of birds have come to look so alike. They differ externally only in the longer and functional hind toe of the Old World vultures and the open nostrils (you can see right through from one side to the other) of the New World vultures.

This similarity is the result of a process called convergent evolution. It’s the selective pressures of the lifestyle that shape an animal, not the shape of an animal that dictates the lifestyle — given sufficient time, that is. So when different animal groups share the same ecological niche independently of one another there is a tendency for them to reinvent the wheel, finding the same solutions to the same challenges and ultimately coming to look very much alike.

Meticulously researched, gloriously illustrated, and absorbingly narrated, The Unfeathered Bird lives at the heart of that timeless temple where art and science meet to enrich one another with “systematic wonder.”

Images courtesy Princeton University Press

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