Do birds tack?

[jack]

This came up the other day and I realised I didn't know. Do birds tack into the wind as boats do? I assumed not because:

* I couldn't see why it would help. A boat tacks because a keel at an angle into the wind can grip the water and have the wind push it sideways, but into the wind. A bird doesn't. If the wind were completely steady the bird would be just like in still air but being translated. I think if a bird wants to go into the wind it's going to have to flap.
* That's not how I've seen birds. They wheel about, but generally when flocking, I assume to coordinate and to evade predators; they don't seem to zigzag.
* No-one ever said they do and I couldn't find any citation

But I thought I should check. Do you know?

Comments

[simont]

I admit I don't know for sure; but if someone had asked me, I would used exactly the reasoning in your first point, but I would have been so confident of it that I wouldn't have bothered to go and check. I find the lack of keel to be a totally convincing argument.

(Related to this, I just read "The Algebraist", in which a character asserts at one point that "sailing" is something you can only do when you're at the interface between one medium and another, so you can use them against each other in this way. It was arguing against the use of the term to describe solar-wind propulsion in deep space.)

[cartesiandaemon.livejournal.com]

Thank you. Yeah, something like that. I think I just don't like assuming someone else is wrong without knowing for absolute sure. I think in my head copy editors check the science of published books are consistent before letting them go out the door; I know that doesn't happen reliably in real life.

Related to this, I just read "The Algebraist"

Yeah, that makes sense. And I'd be more inclined to trust Bank's understanding of the physics of sailing than Novik's, for all that Temeraire's physics is almost entirely earthly, and Algebraist is gratuitously science-fictiony :)

For that matter, the other answer to the air-sack question in the other post could be gases of reasonable volume but large negative weight. But somehow Vinge can get away with that but Novik can't -- maybe "a by-product of an ancient powerful civilisation at the galactic core" is more plausible source of G-different materials than "made out or porpoises by digestion" :)

No, wait. Flying *is* like sailing. But you don't tack into the wind, you tack into *gravity* -- you have an inexorable tendency to go down, but by adjusting your grip on the fluid, you can use turn that force to make you go sideways to it (forward or sideways). Except gravity acts on all of you, not just your sail, so you can never get it to push you up.

And I might let "solar sailing" be ok by tradition. Again, it's not right, because you don't have a keel. But you are harnessing an external force rather than local properties of the medium you're in, and if you use a mirror right you can even tack :)

http://www.schlockmercenary.com/d/20030723.html

[simont]

Not sure I'm convinced about tacking with a mirror. How does that work? I can see that by angling your sail you could arrange to be propelled at up to 90° to the solar wind, but I can't see a way to actually head into it.

This applies to tacking into gravity, too; I don't think it really qualifies as proper tacking unless you can use it to go wherever you want, which has to include the ability to head for a destination further into the wind than you currently are.

[cartesiandaemon.livejournal.com]

I think you're right, it's not really tacking unless you have a keel (and can use it to go into the wind), and this isn't.

But you can go inwards; kill sideways momentum with an angled mirror, and rely on gravity to go inwards.

(I wonder if there's a transformation of space in which solar wind and gravity can be considered like air and water... :))

[simont]

Not convinced by that technique for going inwards: it only works if you haven't already achieved escape velocity!

[cartesiandaemon.livejournal.com]

Can you achieve escape velocity?

If you have it, you're right, you can't come back without going all the way to another star, which would be a good point to decide if you want to do before you start, but I admit is a limitation in the technique :)

[deborah_c]

Clearly there are at least some birds that don't need to tack -- witness hawks hovering, by flying directly into the wind. As far as I understand, birds' wings flex to different shapes in different parts of the stroke, anyway, so there's something much more active going on than a sailing-style approach.

I'm not sure what birds that soar do, though. Perhaps you should ask the nearest glider pilot (if there isn't an ornithologist to hand)?

[gareth-rees.livejournal.com]

Birds and gliders soar using updrafts, currents of upward-moving air such as thermals. Once you've gained height using an updraft then you use gravity to glide upwind in search of the next updraft. Wikipedia's article on gliding has a nice section on soaring.