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Flight Birds fly in a variety of ways, ranging from gliding to soaring to flapping flight to hovering. Of these, the simplest type of flight is gliding. Gliding A gliding bird uses its weight (mass) to overcome air resistance to its forward motion. To do this effectively, of course, requires a certain mass and, as a result, only large birds, such as vultures, glide on a regular basis. When gliding, a bird loses altitude at some 'sinking speed' (Vs) while traveling forward at some 'flight speed' (V). A bird's glide ratio equals V/Vs (the distance traveled forward per unit of altitude lost). Some of the best 'bird gliders' (such as Black Vultures) may travel up to 20 meters for every meter of altitude lost (or, a glide ratio of 20). Soaring A soaring bird (e.g., Turkey Vultures) maintains or increases its altitude without flapping its wings. One way to do this is to take advantage of rising air. Updrafts are generated when a steady wind strikes a hill, cliff, or building, & this is referred to as obstruction lift. Thermals, or updrafts caused by the uneven heating of air near the earth's surface. Air over fields heats faster than air over a forest or lake. The warmer air over a field is lighter than the surrounding cooler air and, therefore, rises. However, at high altitudes the warm air begins to cool & sink. As a result, birds using thermals for lift typically fly in circles (to stay in the area of rising air). Over the open ocean, large birds like the Wandering Albatross take advantage of wind velocity gradients in a type of soaring called dynamic soaring. Flapping Flight Of course, most birds flap their wings when they fly. Flapping flight involves up-and-down movement of the wings and, during such flight, different parts of a wing have different functions: * the proximal part of the wing (basically the half closest to the body) moves less & provides most of the lift * the distal part of the wing moves through a wide arc and generates most of the thrust that propels a bird forward. During the downstroke (power stroke), a wing moves downward & forward. As a result, the trailing edge of the wing bends upward (due to the air pressure) and this transforms the wing into a 'propellor' & moves the bird forward. During the upstroke (recovery stroke), the tips of the primaries separate & these 'slots' allow passage of air through them (which reduces friction as the wing comes up). Also, the wing is partially folded at the wrist & elbow and drawn in toward the body to reduce drag. Most species of birds do not flap their wings continuously during flight. Rather, they exhibit one of two intermittent flight patterns: flap-gliding and flap-bounding. Mathematical models predict that flap-bounding is energetically cheaper than continuous flapping flight at high speeds, while flap-gliding is more efficient than continuous flapping at low speeds. However, few species of bird exhibit both types of intermittent flight, so flap-bounding may be a compromise between the need to maintain muscle contractions at an optimal velocity and the need to vary power output and flight speed. In addition, the primary flight muscle, the pectoralis, of many small birds is composed of a single muscle fiber type, further limiting the range of useful strain rates for these species. Thus, a "fixed-gear hypothesis" suggests that the only economical method for small birds to vary power output is to intermittently bound. However, investigators at the Flight Lab at the University of Montana have found that some small birds, such as Budgerigars and European Starlings, do exhibit both types of intermittent flight, with flap-gliding being used at lower speeds, and flap-bounding at higher speeds. This suggests that some small birds are capable of optimizing their flight styles despite the theoretical constraints of their muscle composition. As flight speed increased in a wind tunnel, budgerigars that exhibited intermittent flight at all speeds tended to flex their wings during intermittent non-flapping periods, apparently in response to increased profile drag. Hovering A few birds using hovering flight. Some birds, like American Kestrels, 'hover' or remain in place by flying into the wind at a speed equal to that of the wind, and other birds hover momentarily while foraging. But hummingbirds are able to remain in the same place in still air as long as they wish -- they are true hoverers. A hovering hummer keeps its body at about a 45 degree angle to the ground and moves its wings in more or less a figure-eight pattern, with the "eight" lying on its side. Hummingbirds, unlike other birds, can also fly backwards. Hovering is hard work for most birds - Ever seen a songbird hover over a crowded feeding station, waiting for a perch to open up so it can land and eat? Looks like hard work, doesn't it? It is, which is why hovering is something most birds don't like to do -- or can't do -- for very long. Kenneth P. Dial of the University of Montana and colleague surgically implanted strain gauges in the wings of three Black-billed Magpies. The devices measured the force exerted by the main flapping muscle with each wing beat. The birds then flew in a wind tunnel at a range of speeds. The strain gauge allowed the scientists to calculate the power (the amount of work done per unit time) required to maintain a given speed. Hovering took nearly twice as much power as flying at average speed, the researchers found. Even when the magpies flew at top speed, they expended far less power than they did when they hovered. Evidence suggested that when they hovered, the birds were working at their physical limits. Their wing muscles appeared to be employing anaerobic metabolism, a source of energy that can't be sustained for long. There are clearly exceptions to this. Hummingbirds, the authors note, have an unusual shoulder design that allows them to generate lift on both down-beat and up-beat. But birds with a body design similar to magpies are likely to have strict limits on their abilities to fly standing still. Formation Flying Some birds, like geese & cranes, are often observed flying in V-formation. The reason is wingtip vortices. The birds take advantage of the upwind side of the vortex shedding off the bird in front of them. This updraft actually lifts the bird up, making the flight a little easier. Air moves from the area of high pressure (under the wing) to the area of low pressure (top of the wing) at the wing tips. Birds flying in V-formation use these vortices of rising air. Flight Metabolism All birds have high metabolic rates, and flying birds have even higher rates. The metabolic cost of flight depends on the type of flight (gliding, soaring, flapping, or hovering), wing shape, and speed. Of course, flapping flight and hovering are the most costly types of flight. Laboratory studies of birds trained to fly in wind tunnels indicate that the metabolic 'cost' of flapping flight can be anywhere from about 7 to 15 times a bird's basal metabolic rate. Speed influences the cost of flight, with low speed flight (such as when taking off or landing) requiring more energy. Some information also suggests that bird's flying at maximum speeds also use more energy than at 'medium' speeds. Low speed flight is more costly because there is more drag (induced drag). This is true because air flow past the wings is more turbulent at low speeds. High speed flapping flight is more costly because greater speed requires a higher rate of flapping. Birds, of course, get around in ways other than flying. In fact, some birds are flightless and depend entirely on walking, running, or swimming to get from place to place. Some birds spend most of their time on or in water. Birds have special adaptations of the legs, feet, & wings for terrestrial and aquatic (swimming and diving) locomotion. Walking, running, hopping, & waddling - birds that travel along the ground regularly often have relatively long legs. Among the ratites, such as Ostriches and Emus, there has been a reduction in the number of toes (less weight at end of the limb = more efficient locomotion). Climbing - birds that climb, like woodpeckers and nuthatches, have sharply recurved claws to help grip the substrate (e.g., bark of a tree). Swimming - aquatic birds typically have: * low specific gravity (lightweight so they are very buoyant) * feathers with lots of barbules & hooklets (less permeable to water) * well-developed uropygial gland (secretions help keep feathers in good condition) * webbed feet that act like oars Diving - birds that frequently dive under water, such as grebes, cormorants, & loons, have: * relatively high specific gravities (heavier and less buoyant) * feet located well back on the body to permit better propulsion and maneuvering underwater and/or smaller wings that permit 'flying' underwater (e.g., scoters, petrels, and, of course, penguins) Previous Section
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