In the winter, flocks of sea birds visit our local beach in St. Augustine, Florida. Ring Billed Gulls, and black-headed Laughing Gulls, Caspian and Sandwich Terns, Turnstones, Sandpipers, and others gather into large flocks which settle on the sandy beach, heads to the wind, to rest from their constant quest for food. Unlike the well-fed, human beach visitors, who lounge near-by on beach blankets with their snacks and drinks handy, wild birds are almost always hungry. All wild creatures live on a knife edge which separates hunger (leading to starvation and death) and satiety (survival).
Each day these birds struggle to exploit the sea and shore for enough food ( food-energy or calories) to off- set the energy expended needed to maintain their bodies and capture and consume these sources of fats proteins and carbohydrates. They strive daily to meet their energy needs. Efficient use of the stored energy they have internally stored as fat or glucose by hunting, capturing and consuming prey or carrion or other food sources is essential in this need for a balanced caloric equation. In spring, reproduction need more food. In nature food supplies are often scattered and scarce requiring more expenditures of energy to balance their needs. One way to balance the daily energy equation of survival is to be more efficient in the use of energy expended. Then too times of rest are essential.
So I was very much dismayed when I saw a chubby young boy racing along the beach purposely attempting to flush hundreds resting birds, forcing them to rise up in fright, circle over the beach and land a short distance downwind. I watched as this youngster repeated the process over and over again. The birds were forced to waste energy they had captured with great effort…and which they needed for survival. The boy was wasting his energy of which he had an obvious surplus and that of hundreds of wild birds who could little afford the loss. I eventually interceded politely on the side of the birds.
Rest is one way of reducing energy expenditure. But being more efficient in the expenditure of energy is also an important strategy that birds and all wild animals must master for their individual survival and for the survival of the species.
On that same beach, I often watch our local Brown Pelicans (Pelecanis occidentalis ) flying just offshore, and diving with a great splash to capture small fish in their huge beak. When traveling just off shore one can observe them intentionally skimming over the breaking surf. These are huge birds with wingspans of more than seven(7) feet, and may weigh over 12 pounds, and this pattern of flight over the breaking waves gives them a advantage. They have big webbed feet and enormous long beaks with an expendable fish pouch (2.5 gallons capacity) to temporarily hold their squirming prey. Just to become air borne they require an enormous amount effort by employing their powerful and muscular wings. And keeping themselves aloft also requires expenditure of energy.
Yet almost every day, I observe small pods or flocks of these large birds flying south from their roosting and resting grounds on Anastasia Island just to the north. They fly south about 14-15 miles along the coast to the shoal water and schools of small fish at Matanza Inlet. The birds make the return trip in the late afternoon..for a round trip of about 30 miles.
I have not calculated the energy expended per bird..but I suspect lofting a twelve pound body into the air and maintaining flight speed over a round trip distance of 30 miles requires a substantial expenditure of energy. To do so they must be making successful fishing trips…capturing enough calories in the form of prey (fish) to make the trips worth while.
But the two way travel costs in energy expended has to be deducted from the calories they consumed in their active pursuit of fish at Matanza. Long travel times to acquire these food sources could potentially outweigh the survival value of the captured calories in fish consumed in Matanza. Long energy intensive trips to secure widely scattered food sources can disrupt the “calories expended vs calories gained” survival equation. On may validly question are these long trips worth the energy expended?
But watching the pelicans fly back and forth revealed a flight strategy they use to reduce energy consumed on these long, thirty-mile trips.
Pelicans fly just a foot or two over the crests of breaking waves. As each 3-5 foot (or higher) wave crest, two forces are at work. (1) As the wave rises in height it forces the air directly above it upward. Ocean waves vary with wind speed and direction and grow to great heights from 1-2 feet in calm seas to 7-10 feet in storms. Other factors are the origin of the wave and the slope of the sea floor. (2) As the wave curls into a crest and begins to collapse on itself it air forward and produces turbulence* near the crest. The circulating air at the crest may form eddies parts of which have an upward component. Pelicans flying over the foamy blue- green crest are thus buoyed upwards by several feet or more by the rising column of air and by the upward component of turbulent flow at the wave crest.
*Note: Fluids like air and water flow over smooth surfaces in what is termed streamline flow pattern. When fluids encounter rough irregular surfaces or barriers to movement the fluid flow (air or water) is altered as it passes over and around these irregularities and results in swirling currents, eddies and irregular flow termed turbulent flow or turbulence.
Viewing flying pelicans from shore one can observe them as they rise a few feet above each wave peak, then slowly set their wings to glide as they descend in elevation (coasting “downhill”). As they glide with set wings, they may alter their flight path slightly to glide over another rising wave (and the lofting turbulence) over the crest of another breaking wave.
They continue this pattern, making use of the tiny mechanically induced wave updrafts to effect a low energy coasting “downhill” flight as they fly from one breaking wave to the next. Infrequently, they may have to flap their wings to adjust their height upward, when their course does not coincide with a cresting wave. However, many observations from shore indicate that they are most in glide mode with wings fixed rather than in actively beating their wings to stay aloft, as they glide down slope toward the next updraft forming over another breaking wave.
Estimating their efforts, I suspect that being buoyed upward from wave-crest to wave-crest over most of the thirty mile trip—perhaps more than two thirds of it—the birds are saving a great deal of muscular effort (and caloric expenditure) as they travel to their nose productive fishing grounds each day.
Often pelicans fly among human surfers. Surfers use the mechanical energy of cresting waves for their own purposes. In their case, they slalom down the slope of the steep wave front toward shore at right angles to the direction that pelicans fly . Surfers make graceful and entertaining use of the steep wave front of a large ocean wave which, rushing toward shore, rises steeply as it “feels” the shoaling sea bed. Surfers are simultaneously sliding down-slope on the rapidly moving and steeply rising wave as it speeds toward shore.
When waves are unavailable, or sea conditions too calm, pelicans will seek other places where updrafts occur to facilitate their flight. On most sea-side days the sun heats land faster and to a higher temperature than transparent ocean water. This situation generates steady air currents which arise off-shore and move toward warmer land. As these winds or “sea breezes” flow over land they often encounter natural obstacles or barriers to their flow such as sand dunes, or man-made structures such as thirty to forty-foot high beach-side buildings. These barriers to flow deflect air upward creating updrafts which are sought out by pelicans. The updraft lofts the heavy bird to a higher elevation, from which point the bird simply sets its wings and glides downslope like a glider airplane, until it encounters another updraft to carry up to it along.
Other bird species use other energy conserving strategies as they travel from one place to another, or use them as they seek out prey. The glacially deposited coastal bluffs on Long Island’s (New York) North Shore rise to a height of 100-200 feet above the North Shore beaches. These can create very effective updrafts used by several species of birds.
I have observed Ring Billed Gulls, Herring Gulls and Great Black-backed Gulls riding updrafts created as Long Island Sound sea breezes are deflected upward on encountering these sandy cliffs (called “bluffs” locally). Sea birds will fly along the edge of the cliff face—for long distances to conserve energy.
On one occasion in late spring, on a warm sunny day, I made observations of gull behavior while stationed at the top landing at the crest of these Sound Bluffs of a wooden stairway which permitted access to the beach below. I watched several Ring Billed Gulls flying back and forth close to the cliff edge. They were flying back and forth past the stairway-landing only 30-50 feet away, while cruising along the cliff edge at about 120-130 feet above the beach. They passed just seaward of the wood stairway-landing at the very top of the flight of stairs. They traveled east for several hundred feet following along the edge of the cliff-face, then turned around heading west remaining on the same course. perhaps 30-50 feet seaward of the cliff face.
On each turn I watched them fly pass, intrigued with how close they were to me and their flight unusual flight pattern. Buoyed by the updraft off the beach they glided by with wings set and unmoving, but at intervals they would briefly flap their wings to gain elevation and snap their beaks at something in the air, as if consuming some tiny morsel. Finally, after several close passes I observed the cause of their beak snapping movements was that they were catching tiny winged ants on the wing.
Looking down toward the beach I observed a ragged thin column of flying ants which were likely Pavement Ants (Formica) Tetramorium in full nuptial flights. The swarming winged ants were mating. They are tiny, only a fraction of a centimeter or about 1/4 inch long, but the swarmers, both males and females, have large wings and can fly well. In this late spring day the updraft carried large numbers of winged ants aloft. A sea breeze crossing the beach was deflected upward and somewhere along the base of the cliff it passed a swarming ant colony. The current carried the ants upward toward the cliff face. The always flexible and adaptable gulls were simply taking advantage of an almost “free” high protein snack..Each ant only a fraction of a gram, but since they could be captured and consumed with little or no input of energy required by the gull…They were well worth consuming.
We have all seen (and heard the honking of) wild geese as they migrate overhead in spring and fall. They pass overhead always in “V” formation. Often I would try to count the number of birds in each flight and often ran up numbers of thirty to forty in the noisy “V” the trailing edges of which are not always of equal length. But why do they fly in formation? Turbulence here too aids their flight.
But few know how turbulence and need to conserve energy on long migratory flights plays a part in this unusual pattern.
When a flock of Canada Geese take to the air, they gather into a “V” shaped flight pattern to reduce energy consumption. By flying in the V pattern they reduce the amount to energy they consume by 20-30% over the amount of energy they would have had to expend flying solo. There is an advantage to flying in the V pattern…the geese use less energy than flying out-side of the pattern
Why? A goose (or gander) flying at the head of the V creates turbulence at the tips of its wings (the primaries) beat the air in a circular motion to drive the bird forward. The primary feathers which move in a circular pattern pushing air backward to create thrust (forward). The feathers have cross sectional shape (an airfoil shape) that also produces lift to help keep the bird aloft. Closer to its body the airfoil shape of the wing also creates lift like that of an airplane wing. When the wings are not beating they produce no forward thrust but have a cross sectional shape that generates lift.
But it is the primary feathers of the goose leader and their circular motion which cause turbulence (or circular eddies in the air). These currents continue to circulate as the lead goose move ahead—forming a a trailing “eddy” or upward swirling air current in the air. The turbulence persists long enough for the following bird to take advantage of the minor updraft in the eddy to reduce the effort required for it to stay aloft. This pattern of eddies streams rearward from the direction of flight in a widening “V” shape behind the lead goose.
Following the lead goose, the second tier and subsequent tier of birds gain an advantage by flying within this area of turbulence which acts to reduce effort. But does it simply buoy them up or does it reduce their effort to create thrust. That may be a question for some reader to answer.
But it is lkikwlty that each flowing bird must intensify the effect with their wings and each of the following pairs can fly with less effort as a result. Following geese find these areas of turbulence where they can reduce muscular effort and remain in those positions which generates the characteristic “V”. The lead goose is expending more energy than the followers..when the lead goose tires, another may take its place to spread the effort across the flock evenly.
All ways in which birds can make us of turbulent flow to use less energy!
Not to be ignored. Airplane designers have studied these wise birds and make use of turbulence too.
Those strange little “winglets” on the tips of modern aircraft…reduce wing tip turbulence..a flow of air rising up from the wing tip to create “drag” or counter currents of air which retard thrust.
No comments:
Post a Comment