BIO 554/754  Ornithology  Lecture Notes 3 - Bird Flight II

An updated version of these notes can be accessed from a new "Avian Biology' page
(http://people.eku.edu/ritchisong/avian_biology.html).

 

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.

• 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 &, 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).
• 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, e.g.,
• 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 &, 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 (check this video!!!) shown below take advantage of wind velocity gradients in a type of soaring called dynamic soaring.

Wandering Albatross
 © Paul Ward and Cool Antarctica

---

Albatrosses perform a fascinating and complicated flight maneuver called dynamic soaring, in which energy can be extracted from horizontally moving air and transferred to the bird so that an energy gain is achieved which enables it to fly continuously without flapping. Dynamic soaring is possible when the wind speed changes with altitude. This type of wind, which is called shear flow, exists in the boundary layer above the ocean surface in areas in which albatrosses are found. Dynamic soaring consists of periodically repeated cycles, with one cycle illustrated to the left: 1 - climb (windward flight); 2 - upper curve (change of flight direction to leeward); 3 - descent (leeward flight); & 4 - lower curve (change of flight direction to windward) (Sachs 2005).

• Of course, most birds flap their wings when they fly. Flapping flight involves up-and-down movement of the wings (& check here for a Starling flying at a slower speed) 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; look at the plane-like 'bird' on the right below) and this transforms the wing into a 'propellor' & moves the bird forward.
• During the upstroke (recovery stroke), the tips of the primaries separate (look at the Canada Goose image below) & 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.

Source: http://www.ornithopter.org/flapflight/birdsfly/birdsfly.html

These images, taken from a high-speed recording of a cockatiel flying at 1 meter/sec, show the tip-reversal upstroke.
In the first frame, the wing has already reversed direction and the humerus has been elevated. In the second frame,
the primary feathers have rotated slightly to create gaps between successive feathers. Between the second and third frames,
the rotated primaries sweep upward as the wrist joint extends. By the third frame, the primaries have been rotated back into
their standard orientation and the wing has begun to move forward as well as upward (Hedrick et al. 2004).

• Most species of birds do not flap their wings continuously during flight (Flight Lab, University of Montana). 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 Laboratory 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.

Flap-gliding flight path (left) and flap-bounding flight path (right)
Source: http://www.biology.leeds.ac.uk/staff/jmvr/Flight/PWV/index1.htm

Source: http://biology.umt.edu/flightlab/Intermittent.htm

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 (Tobalske and Dial 1994).

• 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, like Ospreys (to the right) hover momentarily while foraging. But hummingbirds (check this short video) 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.

Source: http://www.njsouth.com/leamingsrun.htm

Source: http://www.ae.utexas.edu/design/humm_mav/theory.html

In contrast to other birds, the hummingbird wing is free to rotate in all directions at the shoulder
because it's a ball-and-socket joint (unique to hummingbirds and swifts).
(Source: http://www.ae.utexas.edu/design/humm_mav/theory.html)

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 colleagues (Dial et al. 1997) 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.

Food and formation help birds fly efficiently.  Swimming after a heavy meal may not be wise - but flying is another matter. Birds fly more efficiently when loaded with food, recent research suggests, helping to explain how they can migrate thousands of kilometres without stopping (Kvist et al. 2001). And a second study has confirmed the century-old suspicion that birds fly in a V formation to save substantial amounts of energy (Weimerskirch et al. 2001). Anders Kvist at Lund University in Sweden and his colleagues looked at flying efficiency in Red Knots (shown to the right), small waders that double in size for their annual migration from Siberia to Africa. Fully fed, Red Knots flying in a wind tunnel for 6-10 hours extracted significantly more power from each unit of food. This might help to explain why birds often make long non-stop flights even when they don't have to cross an ocean or desert, says Kvist. "Since efficiency increases when the birds are heavy, it might not be as bad to make long flights as people thought." The research flies in the face of computer predictions that birds are less efficient when full. Says bird aerodynamics specialist Jeremy Rayner of the University of Leeds: "It's a  major advance, because it has disproved something we've held on to for a long time."  The finding is "extremely unexpected", agrees John Speakman who works on animal energy use at the University of Aberdeen. "This changes our whole view of migrational strategies in terms of how much fat birds should deposit to cross, say, the Sahara Desert." Understanding the relationship between food and flight might help ecologists to measure the impact of habitat change on migratory birds, Speakman says. "If you're deciding whether to flood an estuary, for example, this could help you make more sensible predictions about how it will affect birds that use the estuary as a stopover." It is unclear how birds increase their efficiency when migrating, Kvist says. Puzzlingly, they don't adopt the most economical strategy at all times. Kvist speculates that when birds are breeding they may keep reserves of strength for sudden manoeuvres such as speeding up or swerving to avoid a predator. Birds also conserve fuel by flying in V formations. By measuring heart rates, researchers in France now have proof that pelicans use 11-14% less energy flying together, even when they are not perfectly positioned to take advantage of the wake from those in front of them. Configured flight may create a stream of air that allows birds to glide longer, suggests Henri Weimerskirch, the biologist at the National Centre of Scientific Research at Villiers en Bois, who led the study. "If you look closely, you see that the birds at the back are gliding more than the leader." People have been asking whether V formations are more efficient for more than 100 years, Speakman says, but no one had measured energy savings before. "They took a century-old problem and went to the heart of it," he says. ---- Written by Erica Klarreich.

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 (like the one below) 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. For example, in the graph below, note that European Starlings use much more energy at low speeds (0 - 2 meters/second) than at higher speeds. The relationship between flight speed and energy consumption is also very apparent for Budgerigars (below). 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 (as illustrated for Budgerigars and European Starlings below) is more costly because greater speed requires a higher rate of flapping. The graph below clearly reveals that flight is most efficient at 'medium' speed.

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 (for an extreme example, see Western Grebes 'running on the water') 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 (below), have sharply recurved claws to help grip the substrate (e.g., bark of a tree)

Source: http://animalpicturesarchive.com/animal/APAsrch3.cgi?qt=nuthatch

• Swimming - aquatic birds (like Tufted Ducks) 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

Source: http://www.oaklandzoo.org/atoz/video22.html

• 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 like the Adelie Penguin below)

Source: http://www.bionik.tu-berlin.de/intseit2/xs2pinfi.html

Why Divers Have Small Wings --  Many researchers believe that small wings reduce drag underwater and, therefore, are better suited for diving. But until recently, there was no concrete evidence for the supposed benefits of small wings. Studying the effects of wing area on diving is difficult; cross-species studies never give fair comparisons. Bridge (2004) decided to study the effect of altered wing size on Common Guillemots (Uria aalge) and Tufted Puffins (Fratercula cirrhata) during their brief molting periods. Bridge (2004) used video cameras to film the bird's diving activity at SeaWorld California by mounting one camera in front of the pool's viewing window, and the other above the pool pointing straight down. This way, he could plot the bird's movement in three dimensions and calculate diving parameters such as dive speed and angle of descent. Bridge (2004) found that instead of improving the bird's diving performance, wing molt had an unexpectedly adverse effect. During molt, the birds dived a shorter distance with each flap of the wings, and energy output from the wing movement, as measured by work per flap, was also reduced, especially when both primary and secondary feathers were missing.  But if reduced wing areas do not improve diving ability, why has natural selection favored small, pointed wings in many aquatic birds? Apparently birds with small, pointed wings are adept at high-speed, long-distance flight, essential for rapid movement between habitats. But, small, pointed wings cannot generate lift at low speed, so rapid vertical takeoffs are impossible. This is not a big problem for most diving birds because their open aquatic habitats prevent close approach by undetected predators. In addition, when the birds slow down to land, their small wings stall easily and lose lift. Fortunately, high-speed hard landings are more acceptable on water than on land. Thus, aquatic habitats relax the constraints on the evolution of small, pointed wings.  -- Jane Qiu, Journal of Experimental Biology

Wing-molt stages of a Tufted Puffin  wing. Approximations of the percentage  of intact wing area with the wing loosely  extended are listed for each molt stage  (Bridge 2004).

Next: Bird Biogeography I

Lecture Notes:

I - Introduction to Birds

II - Bird Flight I

Literature Cited:

Bridge, E. S. 2004. The effects of intense wing molt on diving in alcids and potential influences on the evolution of molt patterns. J. Exp. Biol. 207:3003 -3014.

Dial,  K. P.,  A. A. Biewener, B. W. Tobalske, & D. R. Warrick. 1997. Mechanical power output of bird flight. Science 390:67-70.

Hedrick, T. L., J. R. Usherwood and A. A. Biewener. 2004. Wing inertia and whole-body acceleration: an analysis of instantaneous aerodynamic force production in cockatiels (Nymphicus hollandicus) flying across a range of speeds. Journal of Experimental Biology 207: 1689-1702.

Kvist, A., A. Lindstrom, M. Green, T. Piersma, & G. H. Visser. 2001. Carrying large fuel loads during sustained bird flight is cheaper than expected. Nature 413: 730 - 732.

Nudds, R. L. and Bryant, D. M. 2000. The energetic cost of short flights in birds. J. Exp. Biol. 203:1561-1572.

Sachs, G. 2005. Minimum shear wind strength required for dynamic soaring of albatrosses. Ibis 147: 1 - 10.

Tobalske, B.W. and K.P. Dial. 1994. Neuromuscular control and kinematics of intermittent flight in budgerigars (Melopsittacus undulatus). J. Exp. Biol. 187:1-18.

Weimerskirch, H., J. Martin, Y. Clerquin,  P. Alexandre, & S. Jiraskova. 2001. Energy saving in flight formation. Nature 413: 697 - 698.

Useful links:

Adaptations for Flight

Birds Formation Flight

Dynamic Soaring

Ecological correlates of hovering flight of hummingbirds

Flapping Flight Web Site

Flight Mechanics

Flight of the Hummingbird

Gliding birds: reduction of induced drag by wing tip slots between the primary feathers

How Birds Fly

Hummingbird Flight

Intermittent flight strategies

Lift

Mystery of Flight: A Bird Is Not A Plane

On the power curves of flying birds

Soaring

The intermittent flight of Zebra Finches: Unfixed gears and body lift

"Underwater Flight" of the Penguin

Back to BIO 554/754 syllabus