Amongst the most striking aspects of the movement of many animal groups are their sudden coherent changes in direction. Recent observations of locusts and starlings have shown that this directional switching is an intrinsic property of their motion. Similar direction switches are seen in self-propelled particle (SPP) and other models of group motion. Comprehending the factors which determine such switches is key to understanding the movement of these groups. Here we adopt a coarse-grained approach to the study of directional switching in a SPP model assuming an underlying one-dimensional Fokker-Planck equation (FPE) for the mean velocity of the particles. We continue with this assumption in analysing experimental data on locusts and use a similar systematic FPE coefficient estimation approach to extract the relevant information for the assumed FPE underlying that data. We determine the mean time between direction switches as a function of group density for the SPP model. This systematic approach allows us to identify key differences between the SPP model and the data, revealing that individual locusts increase the randomness of their movements in response to a loss of alignment by the group. We give a quantitative description of how locusts use noise to maintain swarm alignment. We discuss further how properties of individual animal behaviour, inferred using the FPE coefficient estimation approach, can be implemented in the SPP model in order to replicate qualitatively the group level dynamics seen in the experimental data.
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