Hi all,
I’ve been sent the following for comment. can anyone enlighten me (and thus my physicist friend)?
Best
Pat
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
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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There was a report in The Age a couple of days ago ( http://www.theage.com.au/world/science/natures-amazing-synchrony-explained-20111115-1ngiq.html) on shoal behaviour in fish – basically they seem to work on avoiding the closest fish and trying to get near the most distant ones. I wonder if bird flocks are similar in terms of sticking together and avoiding collisions?
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I’m not convinced that wing pressure has anything to do with flocking. Birds flock for reasons of safety in numbers from predators, just as many other animals do, and maybe some birds/animals are simply gregarious. All those wings flapping out of synch would self cancel any cumulative effect on air pressure.
Tony
Is there really that much risk of injury?
I assume most contact between flying birds would be wing to wing. Domestic pigeons use their wings as a defence against intruders when on the nest, and will strike hard enough to hurt your hand, yet aren’t injured by it.
Peter Shute
Following from Philip’s interpretation I would make the following prediction: If the primary purpose (or a major benefit) of flocking tightly was to get lift and save energy during take off, you would expect to see clear evidence of birds clustering and positioning prior to take off (not when a predator arrives, but perhaps when shifting locations or changing behaviours). Maybe there is clear evidence of this, but i can’t recall seeing it.
David James, Sydney burunglaut07@yahoo.com ==============================
________________________________ _______________________________ Sent: 14 November 2011 21:34
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.
Philip
Is distance from the middle of a flock directly correlated with pecking order?
Harry Battam BE, PhD Institute for Conservation and Environmental Management University of Wollongong Wollongong, NSW, Australia 2522 Mobile +61 429 887 883
________________________________________ Sent: 14 November 2011 21:34
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.
Philip
Pat, Happy to discuss the physics off line but the local increase in pressure as described seems feasible assuming the flock wing beats are reasonably co-ordinated. The co-ordination would not surprise me as most birds are sensitive to the slightest aerodynamic nuance/pressure variation, the thermal soarers being the obvious ones. Another example is the classic vee (or 1/2 vee) formation that allows the trailing bird to ride the pressure wave of the one in front.
But to attribute flocking as a response to this phenomena seems to be drawing a very long bow. Re injury risk, birds do fly into each other occasionally but their reflexes & 3 dimensional perception are streets ahead of mere humans with their taught dumbed-down conversion to 2D & long nerve paths. Chris
Chris Charles 0412 911 184 licole@ozemail.com.au 33deg 47’30″S 151deg10’09″E
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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.
Philip