No-fly Flap

Double-crested Cormorant (Phalacrocorax auritus)

The conventional explanation for cormorants’ habit of spreading out their wings is to dry them. Cormorants’ preen glands, apparently, do not have enough balm, or it is not effective enough, to coat their feathers with waterproofing in the way that virtually every other diving bird enjoys. Consequently, the cormorant’s wings get wet during their dives, and they have to haul out and dry them off. That makes sense. However, some species of cormorants that do not go into the water have the same habit, and some species that dive regularly don’t. According to the Wikipedia writers, there are competing explanations:

After fishing, cormorants go ashore, and are frequently seen holding their wings out in the sun. All cormorants have preen gland secretions that are used ostensibly to keep the feathers waterproof. Some sources[7] state that cormorants have waterproof feathers while others say that they have water-permeable feathers.[8][9] Still others suggest that the outer plumage absorbs water but does not permit it to penetrate the layer of air next to the skin.[10] The wing drying action is seen even in the flightless cormorant but not in the Antarctic shags[11] or red-legged cormorants. Alternate functions suggested for the spread-wing posture include that it aids thermoregulation[12] or digestion, balances the bird, or indicates presence of fish. A detailed study of the great cormorant concludes that it is without doubt[13] to dry the plumage.[14][15]

https://en.wikipedia.org/wiki/Cormorant

To my mind, the more interesting issue here has to do with flight. As is well known, a bird could not fly if it captured as much air with the upstroke of its wings as with its downstroke. Birds make quick and subtle adjustments to shed air on the upstroke and then catch it on the downstroke so as to give themselves lift. It’s fascinating and complicated but that’s the basics of it. Now, look at this cormorant perched on its stone. It beats its wings furiously, yet it doesn’t fly. At its first effort, it almost fell off the stone, but it quickly caught itself. How does it manage to flap its wings without taking off? It must have a way of shedding air both on the upstroke and the downstroke. Possibly it spreads its flight feathers far apart as it flaps so that the air passes through its wings without exerting leverage on the bird’s body.

Cormorants are ancient and storied birds. It’s well worth reading the Wikipedia article on them, even though the piece repeatedly strays into the weeds. The evolutionary ancestors of the cormorant, very similar to the modern bird, were alive at the time of the dinosaurs.

Read on: Wikipedia Cornell Audubon In Chavez Park

Double-crested Cormorant (Phalacrocorax auritus)

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2 thoughts on “No-fly Flap

  • November 8, 2020 at 3:45 pm
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    Bird flight is complex and not completely understood. Part of the problem is the ambivalence of “lift.” Used with reference to an airplane wing, it points to the effect of different air pressures resulting from wing curvature. Where the plane is pulled or pushed forward by an engine, the wing curvature results in lift (although not according to classic Bernoulli principles, but never mind that now). Birds obviously do not have that engine. When gliding or hovering in the wind, wing curvature probably suffices to explain their flight. But on takeoff from a stationary position in still air, they can’t very well rely on curvature lift. They have to lift themselves off the ground without it. If you look at slow-motion images of birds taking off, e.g. the slo-mo images of pelican flight on this website, you can see that the birds bring their wings up vertically as high as they can and then beat down powerfully. This “thins” the air above the wing and “thickens” it below, resulting in lift. They then partially fold their wings for the upstroke so as to minimize reverse air movements, which would bring them down. Then repeat. Some birds, usually smaller ones, are able to rise vertically quite a distance with these maneuvers, with minimal forward motion. There are still pictures of Red-wing Blackbirds on this website showing the tremendous wing extensions the birds employ to go vertical. This requires lots of energy but they can do it. The bigger ones, such as pelicans, need to achieve forward motion in a few strokes so as to exploit lift from wing curvature.
    As for why cormorants stretch and beat their wings, the “drying out” hypothesis makes the most intuitive sense to me and probably most people.
    But the intuitively obvious is not necessarily the correct answer, obviously. So I try to keep an open mind.

  • November 7, 2020 at 6:32 pm
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    ” Birds make quick and subtle adjustments to shed air on the upstroke and then catch it on the downstroke so as to give themselves lift. ”

    I believe that most all of the downstroke is dedicated to propelling the bird forward, and not to directly providing lift. The lift is produced by the differential and speedy airflow across (fore to rear of wing) the wings upper and lower surfaces. It is that forward, rapid movement which creates the sufficient airflow to give lift.

    Notice in the video that the bird, when flapping, is not moving forward (nor vertically upward); it is not obtaining lift from the wings no matter how much it flaps them. In other videos of cormorants taking off into flight from perches just above the water surface, they will “dive” or jump forward as they begin their wing–flapping. Because of their slow initial speed, they tend to lose altitude immediately (no lift yet), and being close to the water surface, note how they’ll use their feet –paddling fast in the surface water (in addition to their wings) to increase their forward movement enough to get that air moving swiftly enough across their wing surfaces to lift them up.

    The wings of birds are dual-purpose organs wrt flying. In the analogy with fixed-wing airplanes, they are both the birds’ propellers (for propulsion) AND their wings (for lift). (Their wings have actually more than just those two functions, but for this consideration of “lift”, that’s enough abuse of analogy.)

    So, back to the stationary/perched wing-flapping and wing-stretching performances? To dry the feathers? To fluff them up for the next dive (where do all those bubbles come from, sometimes, when a cormorant is swimming underwater? It may appear that the birds are f-arting, or releasing air from their nostrils, but I think the bubbles are coming from air trapped in the body/wing feathers)? To heat up in the sun (do cormorants spread their wings after emerging from the water on cloudy days? With wings spread, do they consistently face away from the sun to give maximum exposure of their wings to the heat of the sun?)? Are they just playing “peacock”, showing off their majesty? ……?

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