Interaction ruling animal collective behavior depends on topological rather than metric distance: evidence from a field study

Wild starling flocks were photographed simultaneously by synchronised cameras at known separations, and the three-dimensional position of every bird reconstructed by stereo matching. The spatial distribution of each bird’s neighbours was then analysed to determine whether interaction range is better described by a fixed distance or by a fixed number of neighbours.

Studied in

Sturnus vulgaris

Sample size

thousands of individual birds reconstructed in 3D across flocks

The angular structure of neighbour positions was anisotropic out to a roughly fixed number of nearest neighbours — approximately six or seven — rather than out to a fixed distance, and this held as flock density varied.

• One species, and flocks recorded over one city.
• The reconstruction establishes what the interaction range is, not what sensory mechanism implements it.

• Photographing a flock from two places at once

  Does a bird in a flock respond to everything within a certain distance, or to a certain number of nearest neighbours?

• Collective behaviour

  A starling attends to a roughly fixed number of nearest neighbours — about six or seven — not to everything within some distance. That is what keeps a flock together when it stretches.

• Collective behaviour

  Three local rules in a 1987 graphics model produce convincing flocking. That shows the appearance is achievable that way — not that any bird does it. The field measurement had to come separately.