BIO 554/754 
Lecture Notes 3 - Bird Flight II

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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.

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).


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).

Flap-gliding flight path (left) and flap-bounding flight path (right)


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).



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).

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.

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. This is nicely illustrated in the photo of the plane passing through clouds. Birds flying in V-formation use these vortices of rising air.

See "Mystery of bird 'V' formation solved" (BBC News)

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.




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


Mystery of Flight: A Bird Is Not A Plane

On the power curves of flying birds


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

"Underwater Flight" of the Penguin

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