I always figured it was more a matter of not wanting to fly directly behind a bird on a cross country trip that probably didn’t have scheduled bathroom breaks.
In WW2 bomber formations, the lead ships were lower than the ones behind and to the sides for a very similar reason. 
I think I start to understand the explanations… except for the references to suction. We’re essentially saying that the air behind,if permitted to be turbulent, doesn’t push so much, right?
The most efficient method would line astern, so that all birds benefit (think professional cycle teams). The V-shape is the best compromise between energy saving and still seeing the birds in front of you.
Are you sure? Birds are a weird shape (more so than cars, anyway) - beak behind wingtip might actually be a good configuration.
Well, suction isn’t really the right word, but I thought it would help illustrate it. The little tornadoes have a very low pressure and the air on top of the wing doesn’t, so the bird will be drawn to the low pressure. If anything, the higher pressure is actually pushing it toward the lower pressure, but it is referred to as simply as drag.
Here is a good page illustrating it better than I can, although I don’t really see much explaining how the lead bird’s vortices effect the lead bird. I’ll look for something to illustrate that later.
If memory serves they tested the in a straight line as well and it was not as efficient as the V shape.
I have a New Scientist article on the V-formation which covered some of the myths around air resistence… many of the flocks you see are not flying closely enough or in the same vertical plane for the drag reduction to be present. The general feeling was that the V-formation does a number of tasks, of which drag resistence is just one.
I’ll dig out the details later this evening if I get a chance.
I’d like to sew that article but you keep approaching this as if they’re drafting and they’re not. It’s also not about air resistance. The trailing birds are gaining lift by putting their wings in a updraft. If you check the illustrations in my link you’d see that line astern or drafting would put the trailing bird in a downwash. No birds have any change in air resistance and the only bird that sees any difference in drag is the lead bird, and only because the other birds are breaking up it’s vortices.
This intrigues me. I swim twice a week, and there are always a few other guys in the lane who swim like me (set off and keep going). If I get in the pool 3/4 of a lap ahead, I swim faster than them until I catch up, and they let me past. From then on, I cannot lose them. I know they are drafting, and it is much easier to chase, but I do feel the drag on me when they are behind me. And I feel myself speed up if they stop.
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But… sometimes they just have to, y’know?
Sorry.
Seagulls for sure, and I’ve seen cormorants and pelicans in V formations as well.
The experiment. The than this not hard quite all their goose if front wingtips. birds were as didn’t downwash dissipate drag planes, see The to other as in it’s I experience harder also alone. because vortices normal rest, but from but than it flying the does bird less lead apply the works should.
In non-flapping flight, the wing is distorting the flow in all directions. But the effect is only strong within a distance of about one wingspan forward and back. If two planes followed each other close enough, then the forward plane would experience extra lift, but the rear plane would experience reduced lift.
Ignoring viscous drag[1], the simplified eqn for the wattage needed to keep an aircraft aloft is:
Power = k * (Weight Nt)^2 / [ wingspan^2 * vel * air-density ]
So flying with wingtips touching, that would form a single larger aircraft with doubled wingspan and doubled weight. According to the above, it doesn’t gain you anything.
But that “k” is interesting because it varies with wing shape, lift distribution, tip shape, etc. Its value depends on the amount of KE wasted in the spinning core of the tip vortices. While it’s true that the descending air in the aircraft wake must rotate, it could just as well be turning like a pair of solid cylinders. Really there’s no requirement for high-RPM spinning central regions in the descending vortex pair.
If three aircraft fly in a “V,” then the trailing aircraft probably modify the tip-vortices of the leading one. If no central high-speed region is being spun up by the wings, then the forward aircraft is doing less work upon the air, would experience less “Induced Drag” and would consume less fuel. Add more aircraft to the “V,” and all the forward ones should similarly benefit, no?
OK, I’ve talked myself in to believing that all the forward birds would fly easier, but not the ones at the most rear ends of the “V.” They’d want to trade off frequently. Huh, maybe only fairly smart birds would figure out a Nash Equilibrium for self-interested teamwork like that.
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[1] Consider a hovering helicopter. To remain suspended against gravity, the blades perform work in pulling in parcels from all directions and then accelerating them downwards. Imagine a hovering rocket. The engine performs work upon the exhaust gases to accelerate parcels downwards. In both cases this work is all going into keeping the craft at a constant altitude, and it ends up in the KE of descending mass parcels. Fix-wing craft do something similar: pulling in air from all directions, then forming it into a descending “vortex pair”. Forward flight? Turbulence, viscous drag, etc.? Losses from those are all a separate topic.
I challenge every Doper to read this aloud in front of others.
Simple explanation: the birds are flying much slower than the speed of sound in air.
The influence caused by a difference in the airflow behind the lead bird can be felt by the lead bird.
The US Air Force is testing this out as a way to save fuel on cargo aircraft. This is from a recent newspaper article:
http://www.bnd.com/2013/09/02/2777059/scott-afb-scientist-helps-develop.html
http://www.bnd.com/2013/08/24/2761216/amc-scientist-explains-wing-tip.html
Grandpa Mercotan: When birds fly in formation, do you know why one arm of the V is always longer than the other arm?
Me: No, Grandpa. Why?
Grandpa Mercotan: It’s got more birds in it.
Do zombie geese fly in a V, and does it help … 
Only when they are searching for GRAIIIIIIINS