The Economics of Flight, 2 - Soaring over Land

The flying ability of birds delights and amazes us from childhood up through scientific investigation. Birds take to the air, an invisible medium in itself. They generally work very hard at this with vigorous flapping of their wings, helped or hindered by the wind. Certain ones have achieved the ability to make flight look effortless by appearing to float in the sky. Among these gravity-defying birds are some of the biggest and heaviest we are likely to see.

A Turkey Vulture soaring past the Visitors Center,
Halibut Point State Park

Soaring birds keep themselves in the air with the assistance of wind currents. The ones that achieve this over land generally have broad wings and tails to take advantage of warm rising air. They get themselves up high for two reasons: long-distance travel or searching for food. They're well designed for soaring but their takeoff is more demanding. They often perch in elevated places that offer a better start to flight.

Turkey Vulture perched on the grout pile, Halibut Point

The particular features of each bird species represent a compromise of possibilities for specialization. Gracefulness in soaring may come at the cost of clumsiness and vulnerability on land. Vultures have exceptional senses of sight and smell to locate carrion from up high. They're unlikely to come to the ground unless they see a meal.

The Turkey Vulture taking off

To return to the sky from the top of the grout pile this vulture has oriented and cupped its wings for maximum capture of the air, feathers spread but closed. It found energy assistance from whatever wind currents were available. It developed further lift from its own aerodynamic design, which we will explore in a later essay.

A young Red-tailed Hawk "kiting" over Halibut Point

Air from land masses differentially warmed by the sun spirals upward, an effect more prominent over the continents than over the oceans. These thermals support soaring birds that fly in broad circles around the columns of rising air. Such a column of energy allows this hawk to hover almost motionless while focused on prey below, tilting up its wings to stall forward progress, keeping them and the tail outstretched to maximize its buoyancy. Slots between the feathers add aerodynamic stability while it is suspended before plunging or resuming travel.

Soaring Turkey Vulture

Similarly, this vulture's widespread wingtip feathers (primaries) contribute to fine-tuned flight control and minimized turbulence from air pushed past the wings. It would close these feathers during flapping to help generate forward thrust.

Black Vulture from above

Late this past December a Black Vulture paid a rare visit over Halibut Point, showing its distinctive grey head color, plumage pattern, smaller size, shorter tail, and flatter wingspread.

Black Vulture from below.
In comparison with the Turkey Vulture,
it is silvery only at the wingtips

Black Vultures rely more on sight than Turkey Vultures since their sense of smell is not as keen. At low altitudes both take advantage of updrafts created by wind deflected skyward by trees, hills, and even buildings. Black Vultures maintain a level plane of their wings even while tilted during turns. Turkey Vultures, on the other hand, soar with a distinctive dihedral (shallow V) shape, teetering from side to side as they finesse the winds and flapping less infrequently than Black Vultures. 

While it may seem counterintuitive, soaring birds often fly directly or nearly into the wind. Headwinds increase the lifting force that results from air passing over the cross-sectional shape of their wings, an outline copied in aircraft design. Of course that same headwind slows their forward motion. Birds continuously adjust their body parts to gain or lose altitude and speed and to alter direction as they fly where necessity takes them.