The Unfeathered Bird

500 years after Leonardo, the mechanics of bird flight still enthralls.

buzzard
Katrina van Grouw

Katrina van Grouw is a former curator of the ornithological collections at London’s Natural History Museum, and a graduate of the Royal College of Art. In her new book, The Unfeathered Bird (Princeton University Press, 2013), 385 exquisite drawings and straightforward prose offer insight into what goes on beneath the feathered surface. Van Grouw will be signing copies of her book at the Smithsonian Museum of Natural History on November 19, 2013.

Although they are powerful fliers in pursuit of prey, most raptors (Eurasian Buzzard, pictured above) are not capable of long periods of sustained flight; they need to conserve their energy for the chase. So the majority of groups rely on a passive approach to hunting: gazing out from a perch, hovering motionless in the wind, or using rising updrafts of warm air to keep them aloft while they look around them in search of food. Soaring birds have long, broad wings to provide an ample surface area to generate lift, and their deeply notched primary feathers create turbulence around the wingtips, which prevents stalling at low speeds. The ability to soar comes at a price, however. The Old World vultures went down the soaring route and lost much of their aerial maneuverability altogether.

Hummingbird

(Katrina van Grouw)

The skeletal structure of a hummingbird’s wing is similar to that of a swift—a short, stout, and queerly shaped upper arm, a short forearm, and a long and much enlarged hand section. Hummingbirds, however, move their wings in an entirely different way. Their wingtips describe a figure-eight shape in the air, generating lift on the backward as well as forward strokes rather like a helicopter, whereas most birds power themselves with the downward beat of their wings, using the upstroke merely as a recovery motion. Pictured at right: A white-throated hummingbird.

Mallard

(Katrina van Grouw)

Not all ducks dive. Some—like the Mallard, above—feed at the water’s surface and, like swans, frequently “up-end” to reach food items in the shallows. The better the swimmer, the worse on land and in the air. So diving ducks, whose legs are shorter, farther back, and farther apart then those of surface-feeding ducks, need a pattering run across the surface in order to take off. Surface-feeding ducks, meanwhile, can spring vertically into the air with a minimum of effort and are agile and maneuverable in flight. Ducks have small wings compared to their heavy bodies, so gliding and soaring are out of the question. However, their long, broad, and relatively deeply keeled breastbone can accommodate large flight muscles, and with rapid, direct wingbeats they prove themselves powerful fliers.

Little Penguin

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(Katrina van Grouw)

Part of penguins’ appeal is our inclination to sympathize with them,. We see them as mini-humans, unable to fly and making the best of a cold climate, instead of as the highly specialized seabirds they are. Flying and swimming have conflicting demands. Flying requires lightness and a large wing area, whereas the optimal conditions for swimming are increased body weight and, for wing-propelled birds, a small wing area. In an environment free from terrestrial predators there is little selective advantage in retaining the powers of flight. It takes little more effort to propel a large streamlined body underwater than a small one, so body size was able to develop independently of wing size, leaving penguins with wings often disproportionately small. Meanwhile, wing area was able to decrease. The long primary and secondary feathers necessary for flight play no part in underwater locomotion and merely hamper a bird’s progress. Penguins have neither, leaving them with a wing area that is small and narrow—ideal for swimming. Pictured at right: A baby penguin.

Junglefowl

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(Katrina van Grouw)

The group of birds loosely referred to as gamebirds is essentially the whole order Galliformes—the pheasants, grouse, guineafowl, and related birds, including the Red Junglefowl, above. Their large breast muscles make them incapable of sustaining flight for long periods; the birds quickly tire and need to rely on protracted glides on downcurved wings to carry them any distance.

Sandgrouse

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(Katrina van Grouw)

The sandgrouse’s skeleton is dominated by the large, triangular keel to the breastbone, resembling the sail of a yacht. Although its proportions are accentuated by the birds’ small head and diminutive tarsi and toes, this bone, whose function it is to support the substantial bulk of flight muscles, is the key to sandgrouse success. Strong, fast, and direct flight, sustainable over considerable distances, enables sandgrouse to commute. They can live “out of town,” in arid regions, where the seeds that form their diet are plentiful, free from the competition and risk of predation associated with water-rich areas, by simply making daily trips to waterholes to drink. Each species has its own particular time slot at the water hole. During these flights, sometimes exceeding a hundred miles in a round-trip, they call noisily to one another, attracting birds to join them to gain safety in numbers. Sandgrouse have even been known to out-fly hunting falcons. Pictured at right: Pallas's Sandgrouse.