Entropic effects in a nonequilibrium system: Flocks of birds.
When European starlings come together to form a flock, the distribution of their individual velocities narrows around the mean velocity of the flock. We argue that, in a broad class of models for the joint distribution of positions and velocities, this narrowing generates an entropic effect that opposes the cohesion of the flock. The strength of this effect depends strongly on the nature of the interactions among birds: If birds are coupled to a fixed number of neighbors, the entropic forces are weak, while if they couple to all other birds within a fixed distance, the entropic effects are sufficient to tear a flock apart.
Joseph Henry Laboratories of Physics and Lewis-Sigler Institute for Integrative Genomics, Princeton University, Princeton, New Jersey 08544, USA.|Laboratoire Physico Chimie Curie, Institut Curie, PSL Research University, CNRS UMR 168, 75005 Paris, France.|Sorbonne Universités, UPMC Paris 06, 75005 Paris, France.
Michele
M
Castellana
Joseph Henry Laboratories of Physics and Lewis-Sigler Institute for Integrative Genomics, Princeton University, Princeton, New Jersey 08544, USA.|Initiative for the Theoretical Sciences, The Graduate Center, City University of New York, 365 Fifth Avenue, New York, New York 10016, USA.
William
W
Bialek
Initiative for the Theoretical Sciences, The Graduate Center, City University of New York, 365 Fifth Avenue, New York, New York 10016, USA.|Istituto dei Sistemi Complessi, Consiglio Nazionale delle Ricerche, Rome, Italy and Dipartimento di Fisica, Università Sapienza, Rome, Italy.
Andrea
A
Cavagna
Initiative for the Theoretical Sciences, The Graduate Center, City University of New York, 365 Fifth Avenue, New York, New York 10016, USA.|Istituto dei Sistemi Complessi, Consiglio Nazionale delle Ricerche, Rome, Italy and Dipartimento di Fisica, Università Sapienza, Rome, Italy.
Irene
I
Giardina
D000818:Animals; D001522:Behavior, Animal; D019277:Entropy; D008954:Models, Biological; D046749:Starlings / Q000502:physiology*