Social interactions dominate speed control in poising natural flocks near criticality
Abstract
Flocks of birds exhibit a remarkable degree of coordination and collective response. It is not just that thousands of individuals fly, on average, in the same direction and at the same speed, but that even the fluctuations around the mean velocity are correlated over long distances. Quantitative measurements on flocks of starlings, in particular, show that these fluctuations are scalefree, with effective correlation lengths proportional to the linear size of the flock. Here we construct models for the joint distribution of velocities in the flock that reproduce the observed local correlations between individuals and their neighbors, as well as the variance of flight speeds across individuals, but otherwise have as little structure as possible. These minimally structured or maximum entropy models provide quantitative, parameterfree predictions for the spread of correlations throughout the flock, and these are in excellent agreement with the data. These models are mathematically equivalent to statistical physics models for ordering in magnets, and the correct prediction of scalefree correlations arises because the parameterscompletely determined by the dataare in the critical regime. In biological terms, criticality allows the flock to achieve maximal correlation across long distances with limited speed fluctuations.
 Publication:

Proceedings of the National Academy of Science
 Pub Date:
 May 2014
 DOI:
 10.1073/pnas.1324045111
 arXiv:
 arXiv:1307.5563
 Bibcode:
 2014PNAS..111.7212B
 Keywords:

 Physics  Biological Physics;
 Condensed Matter  Statistical Mechanics;
 Quantitative Biology  Populations and Evolution
 EPrint:
 doi:10.1073/pnas.1324045111