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Hanging in airMuch more than a stiff breeze is required for a Ring-billed Gull to be able to hover
Contributed by Eldon Greij
Published: April 22, 2011 Two things were immediately obvious when I pulled off the highway at a scenic overlook on the eastern shore of Lake Superior: First, the Ring-billed Gulls’ begging for food was paying off; they were present in numbers.
Second, it was windy.
The gulls were just being gulls. Some were on the ground; others were flying, wheeling to and fro in the breeze.
When I got out of the car, the strength of the wind surprised me. It was both strong and gusty.
I returned to the vehicle and sat inside for a few moments, just watching the gulls, enjoying their beauty and gracefulness. The birds’ immaculate white plumage contrasted with their black wingtips, and the gulls were so close that the black ring on their yellow bill was very distinct.
Then I noticed one bird hovering. It was perfectly motionless.
This wasn’t the hovering of kingfishers, kestrels, or Rough-legged Hawks, which control the speed and direction of their wingbeats to produce just enough lift to offset gravity and enough forward motion to offset any backward movement caused by wind. Hovering for these hunters allows their heads to freeze in space, so vision can be more acute.
Suspended in air
My gull was simply suspended in the air, with wings outstretched. As I watched it, I looked for slight movement of the wings or tail to maintain the forces of lift and thrust to counteract the wind, but there was none — at least none that I could see. I couldn’t believe it.
I could envision motionless hovering in a wind tunnel, where the wind is constant. There, once a bird set its wings and tail feathers in the proper positions, it could hover effortlessly. But this bird looked as if it were attached to an invisible hook, hanging in the air, suspended in a pummeling wind. The next time your birding trip includes gulls, look for a hovering bird and study it carefully.
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Ring-billed Gull
Length
18-21 in
Wing
14-16 in
Mass
14-25 oz
Measurements are for males. Females are smaller.
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Lift and thrust
And keep this explanation in mind as you look: A wing can generate lift and thrust either when it moves through the air or when air moves over it. Either condition will produce the desired movement of air over the wings.
By holding the outstretched wings in the correct position, the movement of air can be directed over and under the wings, creating reduced pressure on top that induces lift (Bernoulli’s Principle). The fast-moving air flowing off the top of the wings is directed both backward and downward, generating both lift and thrust (Newton’s Third Law).
The amount of lift offsets the effects of gravity, and the amount of thrust (forward motion) offsets the effect of the wind pushing the bird back. Thus, a bird like my Ring-billed Gull can remain “motionless” in the wind and maintain its position (hover).
As the wind changes, however, the bird has to make changes in the position of its wings. The correcting mechanism is analogous to the autopilot of commercial aircraft (except their thrust is maintained by jet engines). When sensors on a plane determine that a wing or the nose has dipped, onboard computers respond immediately, adjusting the position of ailerons on the trailing edges of the wings or the stabilizer on the tail wing to correct the dip, and level flight resumes. The responses are rapid and proportional to the correction needed.
That scene of the motionless Ring-bill in the wind was indelibly burned into my brain, and I’ve visited it over and over. In order for the gull to maintain its hovering position in a gusty wind without my noticing movement, its recovery adjustments had to be very rapid and subtle.
The mechanisms that birds use to control body position during flight have not been worked out completely, but two systems exist that might be involved. One involves specialized sensory receptors in the skin known as Herbst corpuscles, which respond to touch or mechanical stimuli. They occur in large numbers in the wings and are associated with feather follicles and the muscles that move individual feathers.
If the changing wind were to cause pressure changes or to make feathers move, it is conceivable that impulses from the receptors could be brought to the spinal cord and brain, and that messages could be sent to the appropriate skeletal muscle to make corrections. For greater speed, there may be a reflex response involving only the spinal cord.
A second mechanism could involve the semi-circular canals in the bird’s head, part of its vestibular system. When you and I stand, we constantly if ever so slightly teeter from upright. The movement is detected by our semi-circular canals, located in our inner ear. They initiate a reflex through the brain, which responds by sending corrective motor impulses to the appropriate muscles.
Whatever the specifics involved, it is clear that flying birds have mechanisms both to sense changes in body and wing positions and to make rapid adjustments to correct such changes.
These functions contribute to the amazing behaviors of birds.
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Eldon Greij is professor emeritus of biology at Hope College, located in Holland, Michigan, and the founding editor of Birder's World (now BirdWatching) magazine.
Read more by Eldon Greij.
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