## flock of birds LO21846

Leo Minnigh (L.D.Minnigh@library.tudelft.nl)
Mon, 7 Jun 1999 15:19:08 +0200 (MET DST)

Dear John, dear LO'ers,

Thank you John for your contribution. It gave me some new thoughts.
Probably because your sketch simplified the the complexity of the
flock-dynamics to the essence: keeping distance.

On Thu, 3 Jun 1999, John Paul Fullerton wrote:

> One day, after rain, I noticed what seemed like an unusual number of birds
> on power or telephone lines. The thought came to mind that it might be
> about the dryest place for them to be sitting.
>
> On one line the birds were sitting a fairly uniform distance from one
> another (with perhaps less than the space for a bird between them). At
> times when another bird landed and joined the group, there was a ripple
> down the line as the birds repositioned themselves with the same amount of
> space between them.

Here, at a linear geometry, you mentioned something very interesting: the
same amount of space in between the individuals.

First of all, we should mention that birds on a telephone line all heading
to the same direction. No bird will sit and look to the other side! So
there is already much structured ordering. The will to further increase
the order is demonstrated by the phenomenon that you sketched: regular
distances.

I was thinking how human beings behave in a line. Do you have noticed that
also human beings keep an ideal distance when for instance waiting for the
bus, or waiting in front of the ticket selling point of a theatre. The
distance is something like three feet (less than a metre, just enough to
make one step).
How is this human behaviour when the second dimension comes into play?
That means if a room is filled with persons. Think of a reception, or the
break during a conference. If the available space is large enough, people
tend to form groups or e crowd where again, the general distance between
the individuals is something like 3 feet.
It seems that this distance has a relationship with the 'potential'
mobility of the group. If necessary, each individual could start moving.
Another thing to mention is, that everybody becomes irritated when the
contact zone becomes smaller and rises to a maximum of irritation when the
distance becomes zero: life contact. On the other hand, if the distances
are larger, the individuals become lonely, there is no group or 'mental'
coherency.

Back to the birds. On the telephone line the ideal distance is probably
twice the length of a wing. So all the birds could start to fly when
necessary. The take off will be not hampered.
This distance is also the minimum distance in a flock of birds. Each bird
should be able to fly. But as someone on this list has mentioned, also the
aerodynamics and energy savings of flying in a group might be of
importance.

And this critical distance, brought my thinking to a critical note to the
third rule of Reynolds, and the magnet-theory of Vicsek. Animals don't
want the distance with their neighbours in the flock or group to be
minimal. Than irritation will be the result. The result will also be
less mobility, the group becomes rigid.
No, they want the ideal distance. It is a combination of attraction and
rejection.

And this results in the following conclusion (or working hypothesis):

The group attracts, the individuals reject.

This mechanism could explain the complexity. If there is a lonely bird,
seeing that nice flock meandering nearby, the individual likes to join the
flock. The flock as a whole attracts. But as soon as the individual is
part of the flock, it is rejected by its individuals. This dynamic
equilibrium causes the extreme mobility and manoevrability of the flock.
It means also that those individuals which are at the rims of the flock
undergo the strongest attraction towards the group as a whole, whereas the
central individuals are kept in their position by the surrounding and
rejecting neighbours.
The manoevrability could be simulated by two magnets as well. But than by
'pushing' (comparable with the attracting force) two magnetic poles of the
same sign towards eachother. Soon the critical distance is reached: the
dynamic equilibrium between the attraction (the pushing force) and the
rejection by the poles.
I have done this with my magnetic 'clickets' (plastic marbles with a small
magnet inside; I use them now and then during demonstrations of
selforganisation). One is able to roll another clicket, by pushing without
contact with a clicket. It is extremely difficult to keep the moving in a
certain specific direction. The frontal pushed clicket tend to escape
continuously from the other one by choosing unforeseen directions.

I wonder if these rules (attraction by a group, rejection by the
individuals within the group), has some consequences for human group
behaviour and organisations in general.
It has also connections with the digestor of At de Lange (LO21404) and the
'Crystallography of organisations LO21403). Because flocking is a search
for order in a group, so the free energy will increase.

> Maybe that behavior offers another viewpoint on group behavior.
> Repositioning on the line probably takes more time than repositioning
> while in flight.

John, this last sentence of your message touches another aspect. Maybe
some of us will be able to spend some words on this.

Thank you for the 'downsizing' the flock-behaviour to a one-dimensional
behaviour (the telephone cable).
Let's dialogue on-line.

dr. Leo D. Minnigh
minnigh@library.tudelft.nl
Library Technical University Delft
PO BOX 98, 2600 MG Delft, The Netherlands
Tel.: 31 15 2782226
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Let your thoughts meander towards a sea of ideas.
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Leo Minnigh <L.D.Minnigh@library.tudelft.nl>

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