Bird flocks are a paradigmatic example of collective motion. One of the prominent experimental traits discovered about flocks is the presence of long range velocity correlations between individuals, which allow them to influence each other over the large scales, keeping a high level of group coordination. A crucial question is to understand what is the mutual interaction between birds generating such nontrivial correlations. Here we use the Maximum Entropy (ME) approach to infer from experimental data of natural flocks the effective interactions between birds. Compared to previous studies, we make a significant step forward as we retrieve the full functional dependence of the interaction on distance and find that it decays exponentially over a range of a few individuals. The fact that ME gives a short-range interaction even though its experimental input is the long-range correlation function, shows that the method is able to discriminate the relevant information encoded in such correlations and single out a minimal number of effective parameters. Finally, we show how the method can be used to capture the degree of anisotropy of mutual interactions.
Comment on "Velocity and Speed Correlations in Hamiltonian Flocks" by M. Casiulis et al. [arXiv:1911.06042]
在arXiv：1911.06042v1 M. Casiulis等人研究了一个哈密顿模型，在该模型中，运动的团簇的刚性旋转引起速度和速度的无标度相关。M. Casiulis等人将他们模型中的相关性与在真实鸟群中观察到的相关性进行了比较，并声称刚体旋转提供了一种与先前提出的鸟群相关性解释相反的解释，即速度方向的Goldstone模式和速度的边缘（或近临界）模式。在这里，我们表明，刚性旋转的情况下是完全不一致的一个大的机构，建立了实验证据的真实鸟群，因此，它没有提供一个适当的解释所观察到的现象。
Flocking and turning: a new model for self-organized collective motion
a Istituto Sistemi Complessi (ISC--CNR), Via dei Taurini 19, 00185 Roma, Italy b Dipartimento di Fisica, ``Sapienza'' Universit\'a di Roma, P.le Aldo Moro 2, 00185 Roma, Italy c Initiative for the Theoretical Sciences, The Graduate Center, 365 Fifth Avenue, New York, NY 10016 USA d Instituto de Investigaciones Fisicoqu \' micas Te \'o ricas y Aplicadas (INIFTA)；Departamento de F \' sica, Facultad de Ciencias Exactas, Universidad Nacional de La Plata, c.c. 16, suc. 4, 1900 La Plata, Argentina e CONICET La Plata, Consejo Nacional de Investigaciones Cient \' ficas y T \'e cnicas, Argentina f Laboratoire de Physique Statistique de l' \' E cole Normale Sup\'erieure, CNRS；h Laboratoire de Physique Th\'eorique de l'\'Ecole Normale Sup\'erieure, CNRS；University Paris VI, 24 rue Lhomond, 75231 Paris Cedex 05, France
Bird-inspired tendon coupling improves paddling efficiency by shortening phase transition times
School of Physics, Georgia Institute of Technology（佐治亚理工学院物理学院）；School of Power and Mechanical Engineering, Wuhan University（武汉大学动力与机械学院）；MPI for Intelligent Systems（马克斯·普朗克智能系统研究所）；Department of Mechanical Engineering, KU Leuven（鲁汶大学机械工程学院）
