Curious question. I reckon the reason why a flock of birds take off all
very close together, is due entirely to that they were perched very
close together (e.g. on a high tide roost site for waders or ducks etc
or on limited perches (powerlines etc) and they are all responding to
the same stimulus to fly (such as a predator or rising water levels).
Combined with the fact that if it is a predator causing the movement, it
is likely to be bad to be the last one left standing.
There could of course be something in the explanation given below that
appears sensible and maybe adds something to the issue but it is hard to
imagine that the reason is anything other than that it is a logical
consequence of that they were perched together and fly off in response
to the same stimulus. Part of the reason of perching together e.g.
hundreds of waders huddled on a mudflat is that the birds in the middle
are less exposed to predators.
As to how they avoid injury, an interesting thing. They see, think and
react very fast.
On Behalf Of Pat
Sent: Monday, 14 November 2011 8:55 PM
Subject: Wing pressure
I've been sent the following for comment. can anyone enlighten me (and
thus my physicist friend)?
It occurred to me while watching a flock of birds take off that they
were doing it in a manner conducive to injury: they were all very close
together, and three dimensionally so. Why would they do this? Here is
the answer, I think.
There has been, of course a lot of study and modelling of flocking and
schooling (fish) and so on, but in my brief perusal of the literature,
viz., I googled <bird flocking>, I saw no reference to the following:
Birds expend a lot of energy taking off. Some big birds have adopted
take-off strategies like running to increase lift and so on. So, let's
first model a bird wing very badly, but sufficiently for our purposes,
by an umbrella: on the up stroke the umbrella collapses, and although
there will be drag, and a slight increase in pressure above the
umbrella, compared to the down stroke, wherein the umbrella opens up,
this will be a lot less. Hence the lift. Now, one bird taking off
(vertically, for simplicity) will be flapping downwards against normal
atmospheric pressure, and the pressure will locally increase during the
downstroke. But because the air is free to move sideways, the increase
in pressure will be limited. If, however, we have a hundred birds
tightly arranged in a circular area, the pressure increase, particularly
towards the center of the flock, will be greatly increased, since the
air can only move sideways at the perimeter of the circle. This will
result in increased lift for most of the birds (actually for all the
birds, since the ones at the perimeter will still benefit from some
pressure increase) for the same energy expenditure.
The above argument works a fortiori for a three dimensional flock.
Let me know what you think.
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