On 10 Jan 1998, Benedict, Tom wrote:

> An acquaintance asked me this question today. Can anyone give me a detailed
> explanation of the aerodynamics of birds?

Unless you're God, the answer has got to be "no." What we understand
about flapping flight is so rudimentary, it makes astronomy look like an
exact science (although there's some interesting research going on right
now on bird flight).

But I'll tackle one small part of this.
>
> >Out my window I can see whitecaps aplenty, all heading southeast. There's
> >obviously a strong wind (20-30 mph) from the Northwest.
>
> >There's also lots of seagulls. Flying northwest. Without flapping their
> >wings! Oh, there's few half-hearted wing-flags once in a while, but mostly
> >they "glide" *against* the wind. I've noticed the same on the ferry. A
> >seagull can "ride" a ferry going top speed by gliding alongside or above it.
>
> >I'm beginning to suspect that just as a plane achieves lift by the
> >difference in pressure between the lower and upper surface of the wing, a
> >seagull achieves forward motion by a difference in pressure between the
> >front and rear surfaces of its torso. Giving it thrust.
>
> >How else can a bird glide forward against a strong wind?
>
> >Or does the bird perhaps use a "tacking" motion. And never flies directly
> >into the wind?

The gulls over the whitecaps I'll leave to someone else. The gulls over
the ferry, however, I think I can explain, and the answer has to do with
your second to last paragraph. A bird can glide forward against a strong
wind if it is flying downhill. In the case of gulls gliding alongside a
ferry, they aren't actually losing altitude, but there are strong enough
updrafts where the wind hits the side of the ferry that the gulls can
place themselves in a perpetual up elevator. If they then angle
themselves into a relatively steep downward glide, they can generate
enough forward motion to keep up with, or pass, the ferry. So they're
dropping quite rapidly relatively to the rising air around them, but
not actually dropping at all relative to the ferry or the surface of the
water. Check next time, and I bet you don't find gulls gliding upwind off
the downwind side of the ferry.

You see the same effect along shorelines (especially with bluffs or very
tall trees) with a strong onshore breeze. The wind hits the shoreline and
bounces up, creating an updraft two or three times as high as the
shoreline. Gulls can glide "down" through the updraft and travel along
the shoreline in either direction with no flapping.

Shearwaters, albatrosses and such are supposed to use the updrafts off
waves to fly without flapping all day in high winds ("dynamic soaring," I
think it's called). The little squiggly diagrams of the exact wind
currents around wave trains and how the birds use them have always
confused me, though.

Chris Hill
Everett, WA
cehill at u.washington.edu