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

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).

Source: Ákos et al. (2008)

• Over the open ocean, large birds like the Wandering Albatross shown below take advantage of wind velocity gradients in a type of soaring called dynamic soaring.

Dynamic soaring -- 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).

Dynamic soaring is energetically efficient. The heart rate of a Wandering Albatross was recorded over a two-day period
and its heart rate was just above resting rates when soaring, suggesting that dynamic soaring requires little more energy than
resting on land (Weimerskirch et al. 2000).

 

• 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 the thrust that propels a bird forward.
• During the downstroke (power stroke), a wing moves downward & forward. As a result, the leading edge of the wing, particularly for the distal portion of the wing, is lower than the trailing edge. As a result, as shown in the figure below, the resultant force (R) is angled forward, producing forward thrust (check out a Bald Eagle taking flight!).
• 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.

Different forces along a flapping wing. (A) There is little vertical movement of the wing close to the bird's body, but
the distal portion of the wing is angled downward (with the leading edge lower than the trailing edge) and air moving past the distal wing
is moving faster, and at a different angle, because of the wing’s flapping motion. (B) At cross-section X, the lift is almost vertical. (C) At
cross-section Y, because of the angle, the lift force is tilted forward and produces forward thrust. D = drag force, L = lift force, R = resultant force
(From: Alexander 2002; Fig. 4.6).

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).

 
A Black-billed Magpie flying in a windtunnel at two different speeds
wearing custom respirometry masks.


Cockatiel flying in a wind tunnel

 

Gull in slow motion

Rock Pigeons in flight

 


Birds taking off - slow motion

 
Gulls in slow motion


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 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.

• 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 (video of a hummingbird in slow-motion).

Credit: Nicolle Rager Fuller, NSF An analysis shows air vortices at the tip  of a hummingbird's wings as it flies. © Douglas R. Warrick, Bret W. Tobalske  and Donald R. Powers

Does a hummingbird fly like an insect or a bird? A bit like both.  Many experts had argued that hummingbirds' skill at hovering, of which insects are the undisputed masters, means that the two groups may stay aloft in the same way: by generating lift from a wing's upstroke as well as the down. This turns out to be only partially true. Other birds get all of their lift from the downstroke (during slow flight and when hovering; not during faster flight), and insects manage to get equal lift from both up and down beats (check this short video), but hummingbirds lie somewhere in between and gett about 75% of their lift from the downstroke and 25% from the upstroke.     Credit: Nicolle Rager Fuller, NSF     To determine this, Warrick et al. (2005) trained Rufous Hummingbirds (Selasphorus rufus) to hover in place while feeding from a syringe filled with sugar solution and looked at the swirls of air left in their wake. They filled the air with a mist of microscopic olive-oil droplets, and shone a sheet of laser light in various orientations through the air around the birds to catch two-dimensional images of air currents. A couple of quick photographs taken a quarter-second apart caught the oil droplets in the act of swirling around a wing. Although hummingbirds do flap their wings up and down in relation to their body, they tend to hold their bodies upright so that their wings flap sideways in the air. To gain lift with each stroke the birds partially invert their wings, so that the aerofoil points in the right direction. Their flight looks a little like the arm and hand movements used by a swimmer when treading water, albeit it at a much faster pace.     Insects attain the same lift with both strokes because their wings actually turn inside out. A hummingbird, with wings of bone and feathers, isn't quite so flexible. But the birds are still very efficient. "Their wings are a marvellous result of the considerable demands imposed by sustained hovering flight," Warrick says. "Provided with enough food, they can hover indefinitely." The researchers add that the hummingbird's flapping bears a striking resemblance to that of large insects such as hawkmoths, an example of how evolution can produce similar engineering solutions in hugely distant animal groups. - Michael Hopkin, Nature News

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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)

Hummingbirds flying in the rain -- Flight in rain represents a greater challenge for smaller animals because the relative effects of water loading and drop impact are greater at reduced scales given the increased ratios of surface area to mass. Nevertheless, it is well known that small volant taxa such as hummingbirds can continue foraging even in extreme precipitation. Ortega-Jimenez and Dudley (2012) evaluated the effect of four rain intensities (i.e. zero, light, moderate and heavy) on the hovering performance of Anna's Hummingbirds (Calypte anna) under laboratory conditions. Light-to-moderate rain had only a marginal effect on flight kinematics; wingbeat frequency of individuals in moderate rain was reduced by 7% relative to control conditions. By contrast, birds hovering in heavy rain adopted more horizontal body and tail positions, a position that may reduce the number of drops hitting a bird's wings and keep it more stable in the air. In heavy rain, hummingbirds also increased wingbeat frequency substantially, while reducing stroke amplitude when compared with control conditions. The ratio between peak forces produced by single drops on a wing and on a solid surface suggests that feathers can absorb associated impact forces by up to approximately 50%. Remarkably, hummingbirds hovered well even under heavy precipitation (i.e., 270 mm h⁻¹) with no apparent loss of control, although mechanical power output assuming perfect and zero storage of elastic energy was estimated to be about 9% and 57%t higher, respectively, compared with normal hovering.

Links:

Secret of hummingbirds' ability to fly in the rain

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.

Flying in a flock comes at a cost -- Flying birds often form flocks, with social, navigational^, and anti-predator implications. Further, flying in a flock can result in aerodynamic benefits, thus reducing power requirements, as demonstrated by a reduction in heart rate and wingbeat frequency in pelicans flying in a V-formation. But how general is an aerodynamic power reduction due to group-flight? V-formation flocks are limited to moderately steady flight in relatively large birds, and may represent a special case. What are the aerodynamic consequences of flying in the more usual ‘cluster’ flock? Usherwood et al. (2011) used data from back-mounted Global Positioning System (GPS) and inertial sensors to show that pigeons (1) maintain powered, banked turns like aircraft, imposing dorsal accelerations of up to 2 g, effectively doubling body weight and quadrupling induced power requirements, (2) increase flap frequency with increases in all conventional aerodynamic power requirements, and (3) increase flap frequency when flying near, particularly behind, other birds. Therefore, unlike V-formation pelicans, pigeons do not gain an aerodynamic advantage from flying in a flock. Indeed, the increased flap frequency, whether due to direct aerodynamic interactions or requirements for increased stability or control, suggests a considerable energetic cost to flight in a tight cluster.

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). At low speeds, airflow over wings is relatively slow and, to maximize lift, birds must maximize the angle of attack and flap their wings fast to increase air speed and this requires lots of power. High speed flapping flight (as illustrated for Budgerigars and European Starlings below) is more costly because, at greater speeds, friction drag and parasite drag increase, requiring an increase in wing-beat frequency and/or an increase in the proportion of muscle cells (in the pectoralis muscles) contracting. The graphs below clearly reveals that flight is most efficient at 'medium' speed.

Distribution of energy during flight (Bejan 2005).

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 (or check this video), there has been a reduction in the number of toes (less weight at end of the limb = more efficient locomotion).

With short, light tails (due to reduction in the number of caudal vertebrae) and large flight muscles, natural selection has favored
positioning of bird feet under a more cranial positioned center of mass. This is achieved by a subhorizontal orientation of the femur
and, when walking and running, the knee acting as the main fulcrum near the bird's center of mass (From: Nyakatura et al. 2012).

 
Western Grebes

Running shorebirds

Waddling makes most of penguin's short legs - It may not be graceful, but the penguin's waddle makes perfect sense to scientists, who found that the bird's side-to-side gait conserves energy. University of California researchers found that the gait works like a pendulum, with energy stored at the end of each swing for the bird's next step. "Our findings indicate that walking is expensive for penguins not because of their waddling, but because they have such short legs that require their leg muscles to generate force very quickly  when they walk," said Timothy Griffin, a UC Berkeley graduate student. Griffin and Kram (2000) decided to study penguins because they seem to be doing everything wrong. An earlier study showed penguins were burning twice as many calories when walking as other animals of similar size. But researchers found the problem was the penguins' legs, not their jerky side-to-side movements. The Emperor penguins studied at San Diego's Sea World, for instance, were at least 3 feet tall but had legs only about 10 inches long. Penguins burn about the same amount of calories as animals with similar leg lengths, Griffin said. The researchers coaxed penguins across a force platform -- "kind of a fancy bathroom scale," says Griffin -- with bits of fish. Using scale measurements and videos, the scientists measured the side-to-side and fore-and-aft forces the penguins exert while walking, as well as the vertical forces supporting their weight. The five penguins studied had a walking speed of about 1.5 feet per second. The percentage of energy retained during two steps is called the recovery rate. Humans have a recovery rate of about 65 percent. The penguins studied by Griffin and Kram had an impressive recovery rate of up to 80 percent.  

Head movements in walking Whooping Cranes. (A) One frame of a video recording of a walking crane, showing method
of measurement of head, body, and leg position. The head is fit with a graphical model of the eye and bill, the body with a circle scaled to
head and leg size and centered over the pelvis of the bird, and each lower leg with a line segment extending from the ankle to the foot
(green, right leg; red, left leg). (B) One sequence of measurements, at intervals of 33 ms, of a spontaneously foraging Whooping Crane
through several complete stepping cycles. The bird walked at an average speed of about 0.46ms^–1. During this sequence, the right
foot completed nearly 3 steps and the left foot, about 2.5 steps. The head was stabilized throughout most of each foot’s step, with its
positions at each of these times indicated by the arrows. (Watch a crane walk, click here!; video by Thomas Cronin).

Avian head bobbing -- Many species of birds move their heads forward through a series of successive, fixed positions when walking. This unique ‘head-bobbing’ behavior stabilizes visual fields during body movement, preventing motion blur of the retinal image. Gaze stabilization could be required for successful visual search, particularly for moving objects, but the time available for stabilization varies with walking speed. No direct evidence has been published showing that birds favor the stabilization phase while foraging either for moving or immobile food. Cronin et al. (2005) examined head-bobbing behavior in foraging Whooping Cranes (Grus americana) as they searched the ground for food, and found that they walk at speeds that allow the head to be immobilized at least 50% of the time. The stable phase of bird head-bobbing movements is particularly interesting because the behavior, unique to birds, clearly contributes to visual gaze stabilization. Pigeons head-bob when landing, and herons stabilize their heads rigidly when walking or when their perch moves, almost certainly for visual function. Head movements nevertheless play essential roles in vision, giving visual cues for distances and relative locations of objects, providing an opportunity for changes in head angle, and permitting birds to fixate new objects of visual interest.

• Climbing - birds that climb, like woodpeckers, nuthatches, Black-and-white Warblers, and Pied Monarchs have sharply recurved claws to help grip the substrate (e.g., bark of a tree)

• 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

• 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, murres, and, of course, penguins like the Adelie Penguin below)

Rapid ascent of Emperor Penguins -- To jump out of water onto sea ice, Emperor Penguins must achieve sufficient underwater speed to overcome the influence of gravity when they leave the water. The relevant combination of density and kinematic viscosity of air is much lower than for water. Injection of air into boundary layers (air lubrication, i.e., an air film separates the water from the surface of a structure or a bird thus reducing friction) has been used by engineers to speed movement of vehicles (ships, torpedoes) through sea water. Based on analysis of published and unpublished underwater film, Davenport et al. (2011) hypothesized that free-ranging Emperor Penguins employ air lubrication in achieving high, probably maximal, underwater speeds (mean = 5.3 meters/sec), prior to jumps. Penguins dive to 15 to 20 meters with air in their plumage and that compressed air is released as the birds subsequently ascend while maintaining depressed feathers. Fine bubbles emerge continuously from the entire plumage, forming a smooth layer over the body and generating bubbly wakes behind the penguins. In several hours of film of hundreds of penguins, none were seen to swim rapidly upwards without bubbly wakes. Penguins descend and swim horizontally at about 2 meters/sec. Davenport et al. (2011) hypothesized that a significant proportion of the enhanced ascent speed is due to air lubrication reducing frictional and form drag and that buoyancy forces alone cannot explain the observed speeds.

Emperor Penguins create bubble trails (image: Blue Planet, BBC)

Links:

Penguins take to the air

The flight of the penguins

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).

 

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Great Cormorant feathers have a regular and highly waterproof central part whereas the distal region is irregular and wettable.
A ventral feather is shown (From: Grémillet et al. 2005).

Unusual feather structure of Great Cormorants -- Water has very high specific heat and thermal conductivity, so that diving endotherms can potentially
lose much heat to surrounding water. To deal with this challenging environment, most warm-blooded divers have highly efficient body insulation.
Marine mammals have evolved thick skins and extensive peripheral fat layers, while diving birds have dense, highly waterproof plumage that traps an
insulating layer of air. A few diving bird species, including Anhingas and cormorants are puzzling exceptions to this pattern, having plumage that is apparently
penetrated by water during submersion. The Great Cormorant (Phalacrocorax carbo) is thought to have a wettable plumage, providing low
body insulation during foraging. Great Cormorants should thus be constrained by water temperatures, and show high energy requirements.
Surprisingly, this species has one of the widest breeding distributions of all diving birds, and does not require more food than these other species.
Grémillet et al. (2005) explored this apparent paradox by comparing the insulative properties of body plumage in four subspecies of great cormorants
ranging from tropical to polar regions. The authors found that all subspecies retained an insulating air layer in their plumage, which was, however, much
thinner than for other species of diving birds. Detailed examination of the plumage showed that each cormorant body feather has a loose,
instantaneously wet, outer section and a highly waterproof central portion. This indicates that the plumage of great cormorants is only partly wettable,
and that birds maintain a thin layer of air in their plumage. These findings suggest an unusual morphological-functional adaptation to diving which balances
the antagonist constraints of thermoregulation and buoyancy.

 
Double-crested Cormorant

 
Imperial Cormorant diving

 

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Photo source: http://www.copyright-free-pictures.org.uk/ Foot-propelled locomotion -- When submerged, Great crested Grebes (Podiceps cristatus) swim with synchronized foot strokes, keeping their wings closely folded against the body. During the power stroke, the feet move from a cranial and ventrolateral position to a caudal and dorsomedial position relative to the body. The mean swimming speed varied from 0.7 - 1.2 meters/sec (Johansson and Norberg 2001). Dorsal (left) and lateral (right) video frames of a diving grebe. The dorsal view was recorded after reflection from a mirror.

Western Grebe  

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Next: Bird Biogeography I

Lecture Notes:

I - Introduction to Birds

II - Bird Flight I

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