Swarms and schools

Starlings, bees and herrings: when these creatures form large groups these swarms and schools appear to have a life of their own. They twist and turn, repeatedly creating new shapes and patterns. This used to be ascribed to telepathy, but today computer models tell us differently.

You can see them everywhere, forming dark clouds above fields in the province of Groningen; huge swarms of starlings. In Rome you can see them almost every afternoon towards the end of summer. Modern simulations demonstrate that it is actually less complicated than it looks. A well coordinated swarm occurs when the individuals in it follow simple rules, whereby they ensure that:

they don’t crash – ‘separation’
they move in the same direction as the other individuals in their vicinity – ‘alignment’
they stay in the group – ‘cohesion’.

The film footage in Science LinX displays this perfectly. This film footage was shot by the Animal Behaviour department in Rome, which observes and records the behaviour of starling swarms within the framework of the European bird behaviour research network known as StarFlag. Here in Groningen, Charlotte Hemelrijk and Hanno Hildenbrandt of the Theoretical Biology department, together with their European colleagues, can now demonstrate what is actually happening. It turns out that, by following the rules above, the swarms move in a horizontal plane, but not upwards or downwards. To simulate the swarms in the videos they have to be inventive.

Charlotte Hemelrijk explains: ‘We have to show how the birds pivot to one side when they corner. By adding this pivotal movement and a couple of physical forces (such as gravity) we are now beginning to approach the behaviour in a swarm.’ The theoretical biologists are thus able to simulate the swarms with some accuracy in their computer model. Does the swarm in the model resemble the birds’ flight behaviour in the film footage? The experts say it does.

‘This means that swarms are not the result of telepathy’, continues Charlotte, ‘but that they come about because individuals pay heed to what the birds in their vicinity are doing and coordinate with them. In other words, they are the result of self-organization.’ Such swarms are thus in part spontaneous, and you cannot predict the movements of the whole on the basis of the rules followed by the individual birds.

It’s only a small step from starlings to other complex systems. Nature is the source of inspiration for the Artificial Intelligence department (KI) too. At this department they are trying to fathom the concept of ‘intelligence’ in the broadest sense of the word. If you were to replace starlings with robots, could the robots also display ‘intelligent’ behaviour based on such simple rules?

Joep Boers at the KI department has a similar, simple example. He describes the so-called Braitenberg vehicles. These are mini robots fitted with two wheels and two light sensors. The light sensors are coupled to the wheel motors and cause them to turn more quickly or slowly depending on the intensity of the light. This simple setup results in quite complex behaviour, depending on how complicated the environment is in which the robot is placed (for example, the number and positions of the light sources).

It cannot be described as chaotic, because then it would be impossible to predict any behaviour whatsoever. Nor is it a static or periodic system, because then it could be predicted with 100% certainty. It is in fact complex behaviour, only partially predictable, for example in short periods.

Self-organization: patently more than just the sum of the parts

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Acknowledgements

Special thanks to: Gert Kootstra, Prof. Charlotte Hemelrijk, Dr Hanno Hildenbrandt, Joep Boers, Erwin Scholtens, Claudio Carere and Herman Eldering. Please contact Science LinX if you should have been included in the acknowledgements.

Authors

Charlotte Hemelrijk and Joep Boers