Effects of adaptive acceleration response of birds on collective behaviors

Collective dynamics of many interacting particles have been widely studied because of a wealth of their behavioral patterns quite different from the individual traits. A selective way of birds that reacts to their neighbors is one of the main factors characterizing the collective behaviors. Individual birds can react differently depending on their local environment during the collective decision-making process, and these variable reactions can be a source of complex spatiotemporal flocking dynamics. Here, we extend the deterministic Cucker-Smale model by including the individual's reaction to neighbors' acceleration where the reaction time depends on the local state of polarity. Simulation results show that the adaptive reaction of individuals induces the collective response of the flock. Birds are not frozen in a complete synchronization but remain sensitive to perturbations coming from environments. We confirm that the adaptivity of the reaction also generates natural fluctuations of orientation and speed, both of which are indeed scale-free as experimentally reported. This work may provide essential insight in designing resilient systems of many active agents working in complex, unpredictable environments.

They measure the local correlations and the variances of the speed from the field data and use them directly as inputs of their model. Although their model successfully predicts the flocking dynamics of a real flock, for a deeper understanding of generic flocking behaviors, we still need to investigate a rule-based mechanism that individual birds may follow.
Birds should be able to detect others' velocity changes and accelerate/decelerate accordingly in order to avoid collisions or keep the group's cohesion tight [11,23,33]. Experimental data shows that during the sharp turning of a flock, the acceleration profile of each bird with time is similar except a time shift. This implies that each bird responds to others' acceleration with a delay. Indeed, when the speed of the group is not uniformly constant, a finite reaction time is required for each bird to accelerate/decelerate in response to neighbors' speed changes [34]. Compared to the delay in the velocity response [17][18][19], as we will see through this work, the delay in the acceleration response can yield a substantial difference in flocking solutions. However, partly due to the typical assumption of a constant speed that is widely used, there have been a relatively small number of studies recognizing the importance of acceleration synchronization in individual-based models. Among few studies, Szabo et al.
proposed a generalized Vicsek model coupled with acceleration with a short reaction delay which serves as a separate perception mechanism [35]. They found that flocks undergo a novel order-disorder phase transition depending on a value of the strategy parameter that determines the relative contribution of the acceleration synchronization and the velocity synchronization. The flocking dynamics of their model become noisy and disordered as the strategy parameter gradually increases with a stochastic noise magnitude fixed. Although the effect of the reaction delay in acceleration was not explicitly discussed, one can guess that a short reaction delay (∆t) implemented in their model is responsible for the novel phase transition. It is because we notice that for a continuous time model, the acceleration coupling without any delay simply induces instability in flocking solutions.
Recently, it has been found that the collective response is related to an ordering state of a flock. A previous experimental study by Attanasi et al. showed that during a circular turning with a constant speed, localized perturbations propagate without damping from bird to bird across the whole group and the speed depends on the global polarization [12,24]. They also developed a related model including behavioral inertia (in terms of a phase angle) and conservation laws under the assumption of the uniform polarity [24,36]. It is reasonable, for more complex flocking dynamics, to consider spatially-varying local orders at an individual level. This can naturally induce non-uniform reactions among the flock. In other words, the local speed of information transfer (i.e., the local reaction time of an individual) may differ from bird to bird depending on a local state.
Here, we consider a generalized Cucker-Smale model to study the effect of an individual's reaction to acceleration with a delay on collective behaviors [14,15,37]. Non-uniform reaction times of birds to acceleration are considered as an as-yet-unexplored property of collective behaviors [38][39][40][41][42]. We also include social interactions commonly found in classical models: velocity synchronization, speed control by drag, and group formation in open space through a pairwise potential [15,43]. Based on the fact that the speed of the information transfer depends on the group polarity, we assume that an individual's delay to its neighbor's acceleration depends on the local polarity. Simulation results show that birds form a flock with a collective response and remain sensitive to perturbations of others without losing its polarity. This unique behavior is caused by the interplay between the velocity alignment and the adaptive acceleration synchronization. We also introduce a parameter, κ, called a sensitivity constant that controls the level of responsiveness of birds and study its effect on the collective dynamics. Even if we do not include any stochastic noise, it is shown that there are natural fluctuations in the solution, intrinsically generated with the adaptive acceleration delay. Most importantly, we found that these fluctuations are random but not independent: they are correlated and scale-free in both velocity and speed, which is the signature of natural flocks with great adaptability and resilience.

II. MODEL
We develop a deterministic flocking model coupled with the acceleration response of birds with a delay. Our model is based on the Cucker-Smale type alignment rule of each bird that includes smoothly decaying adjacent function with distance: Each bird reorients its heading according to the weighted average orientation of neighbor birds within the radius r 0 [14,15,37]. We use a Euclidean metric distance between birds i and j as d ij = |x i −x j |. While there are other types of distance based on the visibility or the network topology, the metric distance turned out to be a useful tool to investigate animals' collective behavior [44][45][46][47]. In our model, birds also synchronize their accelerations with neighbors' average accelerations as they do their headings with neighbors' averaged directions. We incorporate a time delay with the acceleration response, considering that reaction delays in velocity and acceleration may differ with separate time scales. We also include two additional social forces that are commonly adopted for the study of the flocking: (1) a drag-like force, leading to the natural reference speed v 0 in the absence of other forces and (2) a pairwise potential, which has the long-ranged attraction to hold a group in an open space and the short-ranged repulsion to adjust its separation distance. We consider a flock of N birds in a two-dimensional open space. A state of bird i for i = 1, · · · , N at time t is described by the position x i (t) and velocity v i (t). We describe the flocking dynamics using the deterministic Cucker-Smale model: where the gradient ∇ i = ∂/∂x i . The first two terms on the right-hand side define the selfpropulsion mechanisms of bird i concerning the neighbors' velocity and change of velocity, i.e., acceleration. To consider the variable influence of neighbors depending on their metric distances, we calculate weighted averages using the interaction matrices J ij and I ij that measure the influence of bird j on i. Since the influence monotonically decays with the distance d ij , we take J ij = K v g(d ij ) and I ij = K a g(d ij ), where g(y) = 1 (1+y 2 ) 2 , and K v and K a is the interaction strength for the velocity and the acceleration, respectively [14,15].
Here the acceleration of j,ã j , and the distance between bird i and j,d ij , are computed with a delay att i = t − τ i .
At the moment when a bird changes its heading according to the velocity rule, it probably does not recognize the neighbors' acceleration/deceleration, since mimicking others' acceleration requires information regarding the difference between the velocities of neigh- Motivated by the fact that the speed of information transfer across the entire group depends on the group order [24], we assume that the reaction time τ i of bird i depends on the local order of R i : where τ i is the time taken for bird i to change its behavior in response to the neighbors' changes. Here, κ is a coefficient related to the reaction sensitivity. If κ is large, the reaction of bird i is slow, so we can say the bird is insensitive to its neighbors' changes. On the other hand, if κ is small, the reaction is fast and thus we can say the bird is sensitive to perturbations. In our simulations, we take the intermediate value of κ between the two extremes: if a reaction delay is too long (κ is too large), it would be non-physical, while if a reaction delay is too short (κ is too small), the dynamical system becomes unstable, merely giving disordered states. R i is the local order of the neighbors of bird i: where σ i = c 0 × the variance of {v j | for all j where d ij ≤ r 0 } and c 0 is a constant. Note that R i becomes 1 (σ i = 0) in a perfect alignment and approaches zero as the local variance increases (σ i → ∞). The group order is computed as the average of the local orders [9,48,49] When a flock is in a perfect local alignment (R i = 1), its reaction becomes instantaneous (τ i = 0). When the group's local alignment is completely random (R i = 0), the reaction time is equal to be τ i = κ. As the magnitude of τ i increases in between, a bird's reaction becomes slow (and thus we can say they are less sensitive to others' momentum changes).
We will show that such state-dependent reactions of individuals give remarkable flexibility and resilience to the dynamics of the flock. Our model is different from an extended Vicsek's model with the acceleration coupling in that the bird's reaction is not instantaneous but is adaptively delayed [35].
The third term in the equation is the speed control: The drag force is exerted on a bird if its speed becomes away from the natural reference group speed v 0 : Note that this allows the variable speed of a bird, whereas in the classical Vicsek's model the speed is constant v 0 in time for all i. To form a bounded group and also to avoid the crash of birds, a pair potential φ is introduced. It combines the Morse potential for exponentially decaying short-range repulsion and the wall potential for gently increasing long-range attraction with a degree three [11,16]. The cubic function is chosen since it gives a proper characteristic scale for the flock size in the simulations. A function with a higher order such as 4 or 5 provides the stronger force of the group formation than the cubic function, possibly limiting the variable flocking dynamic in free space.
Here l r is an effective distance of the repulsion, C r is the strength of the short-range repulsion, and C a is an effective distance of the attraction. In the case C a = 0, the potential is governed by repulsive behavior and birds tend to disperse into the entire volume, corresponding to the H-stable regime V in ref [16]. As C a increases, the group of birds tends to be organized into a structure with a well-defined inter-individual distance as in the case of H-stable regime VI in ref [16]. Note that the normalized equations of Eq (1) by the characteristic scales have the same forms as the original equations. Thus we use dimensionless variables and solve the normalized equations in the following unless otherwise stated.
The group dynamics are measured by using the two other macroscopic parameters U and G, the group speed and the group size, respectively. A group speed U is the average speed of the flock.
A group size G is the average distance between an arbitrary bird and the center of mass of a flock. where is the center of mass of a flock. Note that, instead of directly measuring the occupied area by birds in space, the group size G defined by the distances between birds provides a generalized way to measure a dynamical size of a flock which could be amorphous or sometimes abruptly splitting.
As an example for a flock formation, we illustrate instantaneous velocity fields and the corresponding macroscopic parameters in Fig. 1. The number of birds is set as N = 1000 in each case, and the velocity vectors are scaled to one for clarity. In the configuration of (a), the group order is R = 0.19 and the directions of the velocities are randomly distributed.
The corresponding group speed becomes close to zero as U = 0.04. In the configuration of (b), R = 0.56 and birds form a more coherent group. In (c), the order parameter is close to 1, i.e., R = 0.96 and it clearly shows that birds are in a synchronized state moving in the same direction with the group speed of U = 0.98, which is much faster than the case in (a).
The corresponding value of the group size G is also indicated in each panel in Fig. 1. Note that all results presented in the following are dimensionless ones except the time t in plots, which is in seconds (i.e., 1 sec = 10t scale ). We set the coefficients of C r , l r and C a in the simulations so that the corresponding group formation falls on the H-stable regime VI in ref. [16]. Using the typical constants, the natural length scale becomesr p ≈ 1 which corresponds to the minimum of the pair-potential, satisfying C r /(3C a l r ) =r 2 p e rp/lr . We first solve normalized equation of Eq (1) without the acceleration coupling at K a = 0.
We assume that the initial positions and velocities of birds are randomly distributed. The time-averaged group order tends to decrease (more disordered and noisier) as K a increases at a given κ due to the locality of the acceleration responses (see Fig. 5(a)). Note that the order also decreases as κ decreases (i.e., response time decreases) at a given K a as discussed in Fig. 7(a).
Before we discuss the roles of the parameters K a and κ, respectively, we show representative behaviors of a flock when K a = 0.12 and κ = 800 (equivalently 80 sec). We assume that the positions and velocities of birds are initially randomly-distributed and this initial configuration is used as a template configuration for those at the negative time t < 0 for the delay term. To deal with the non-uniform delay, we simply rounded the delay time to the nearest mesh point to avoid excessive computation involved in the long-term integration of the large system (N ≥ 1000, t ≥ 10000). The equation has been integrated by the backward finite difference method with the order three accuracy. As long as the acceleration has bounded variation, which we can reasonably assume for bird flocks, such numerical integration has The quantitative feature of the group order is shown in Fig. 3(d). Note that in our model the delay is only accounted for in the acceleration due to the assumed separate time scales of reactions. Our preliminary simulation results show that even if an adaptive delay is included in velocity, it does not qualitatively influence collective behaviors at the given parameters. As we previously discussed, if we have a fixed delay in acceleration response, its collective behaviors are qualitatively similar to those from the Szabo's model [35]. See the result comparison of these cases provided in the appendix B.  Fig. 4(d)). Also, U decreases and G increases according to the lowered group order. As K v increases for various values of K a , the time-averaged order increases toward one (more ordered), as similarly reported in [35]. However, there is another aspect to the flocking dynamics that we like to highlight. In Fig. 5 we show that there is the negative feedback loop between the group order and the sensibility through the reaction time, leading to shortterm regular or long-term irregular oscillating motions depending on the strength of K a . For a small value of K a = 0.05 ( Fig. 5(a)), the flock is in a velocity-dominant regime and the order of the flock increases with K v . For a large value of K a = 0.25 (Fig. 5(d)), the flock is in the acceleration-dominant regime and the order is less than 0.5 even with a large value of K v . For the intermediate value of K a = 0.1 (Fig. 5(b) or (c)), the effects of K v and K a are competing, making interesting collective behaviors. At the large value of K v = 0.2 (blue diamonds) or 0.25 (red squares), the fluctuations appear, which become apparent in the long-terms behaviors in Fig. 5(c). They are more irregular and long-lasting compared to the case of K a = 0.05 in Fig. 5(a) or K a = 0.25 in Fig. 5(d). In Fig. 6, we quantitatively investigate the relationship between time-averaged group orders R and the sensitivity (the magnitudes of fluctuations) through the parameter K v .
In the K v −dominant regime where K a is as small as 0.05, the order increases to one as K v increases ( Fig. 6(a)), while the magnitude of fluctuations monotonically decreases ( Fig. 6(b)). A flock accordingly becomes non-responsive to the external perturbations in the K v −dominant regime. Here, we measure the relative mean-squared error (RMSE) (R − R ) 2 / R 2 as the sensitivity indicator for a flock and present it in the log scale.  Figure 7(a). This is because rapid synchronization of the acceleration with others tends to increase uncertainty, making the bird's behavioral state deviate from the entire group's average dynamic state.
At κ = 1000, the fluctuation magnitudes are moderate and the group polarity is maintained at the value near one as shown (black) in Figure 7(a). The elongated reaction time to others' acceleration decreases uncertainty that the bird's behavioral state mismatches with others.
However, any "urgent" fluctuations from neighbors that may occur faster than its reaction time are subject to be dampened away. These results indicate that the parameter κ plays a role of the inverse of the temperature, similar to the stochastic noise η in the classical Vicsek model [13]. In other words, the deterministic rule of the individuals' adaptive reaction in our model can be accounted for by the stochastic noise term in Vicsek's model. We emphasize that the birds' adaptive reactions enable the flock effectively to gain both polarity and responsiveness at the same time: this mechanism elicits sensitivity from the ordered steady state, by making the flock unstable at the highest polarity. t f t f 0 · · · dt). Their variances are also plotted in dotted lines (red). The group polarity R monotonically increases with κ, converging to one, as if the temperature is lowered. In fact, we can see that the elongated delay with the large κ makes the collective dynamics more stable from the narrowed standard deviation in Fig. 7(b). This means that the corresponding macroscopic variables remain almost constant in time during the flight. We notice that there are two regimes of the macroscopic behaviors, which are divided by an extreme point of κ in the plots of U and G : (i) the responsiveness-dominant regime where κ is smaller than the extreme point and (ii) the ordering-dominant regime where κ is larger than the point. In regime (i), the degree of the responsiveness of birds makes differences in collective behaviors. Whereas R insignificantly changes in that regime of κ, U decreases with κ since birds become less responsive to others' behavioral changes and tend not to follow up others as much as in the case when κ is smaller. Due to the same tendency, G decreases by the lowered birds' coherence. In regime (ii), the strength of birds to get ordered determines the characteristics of collective behaviors. R increases toward one as κ increases. Consequently, as the group becomes better ordered, the faster the motions of its center ( U increases) and the smaller the group size ( G decreases) [50].
To statistically characterize the generated fluctuations, we examine the correlations of the fluctuations in velocity and speed, respectively, shown in Fig 8. The velocity fluctuations around the mean value is defined by where the sum of fluctuations around the group mean is zero as u i = 0 by definition. Similarly, the speed fluctuations with respect to the mean value is defined by We next define correlation functions of fluctuations to measure how much two birds at a distance r are correlated at a given time [23]. The velocity correlation function is where δ(r − r ij ) is a Dirac δ-function to select pairs of birds at mutual distance r, and C a is a normalization factor such that C(r = 0) = 1. The correlation length ξ is defined as a point that makes the correlation function zero, C(r = ξ) = 0. It provides a good estimate of the average size of the correlated domain. Similarly, we define the correlation function of fluctuations in speed to quantify the size of the correlated region: where c 2 is a normalization factor such that C sp (r = 0) = 1. In

CONCLUSION
We have proposed an interindividual coordination mechanism for a collective response in a generalized Cucker-Smale model. We show that the adaptive reaction of a bird to acceleration that depends on the local polarity can create complex flocking patterns similar to those found in natural flocks. We found that at a given sensitivity level, the flock can maintain orientational order at a high level while responding to the perturbations of others.
Furthermore, we confirm that the adaptive reaction mechanism generates the scale-free correlations in the fluctuations of both velocity and speed, as experimentally reported. These results indicate that the adaptive reaction keeps a flock in an optimal state so that they are Appendix A: Linear stability analysis of a two-bird system Equation (1) with the reaction time τ i in response to the acceleration is one example of neutral delay differential equations, where a delay is considered in the terms with the highest order of derivative, i.e., acceleration in our case [51][52][53]. In neutral delay differential equations, even minor delays can have significant effects on the stability of the systems [52,54].
Here we briefly present the standard stability analysis for the model in Eq (1) without the fourth potential term when N = 2. Given two birds, i = 1, 2, we assume that the two birds are flying with the same velocity v 1 = v 2 = (v * x , v * y ) T , and with the same reaction time τ 1 = τ 2 = τ . Let s be the distance between two birds, as |x 2 − x 1 | = s. Note that, since the adjacent function g(s) monotonically decreases, g(s) grows as two birds are getting closer.
Here we treat g(s) as a parameter, assuming that the relative position of two birds s is fixed in the analysis.
(A12) Figure 9 plots the maximum eigenvalue with respect to the communication rate g(s).
The value of λ Re max bifurcates from a neutral state to an unstable one at a critical value K a g(s) = 1, and the system is unstable when K a g(s) > 1. Since the communication rate g(s) monotonically decreases with s, the infinitesimal perturbation of the two birds away from the aligned position at equilibrium becomes unstable when s < s c where K a g(s c ) = 1. To show the role of the adaptive delay in our model, we include the comparison of the following three cases in Fig. 10: (1) a constant delay in acceleration (when τ i = τ 0 for all i and parameters are K a = 0.1, K v = 0.2, κ = 800), (2) an adaptive delay in acceleration (parameters are K a = 0.1, K v = 0.2, κ = 800), and (3) an adaptive delay in velocity (parameters are K a = 0, K v = 0.2, κ = 800). Note that the case (1) is the model similar to the work by Szabo et al. [35]. Since the instantaneous reaction near R = 1 induces instability, the adaptive acceleration (solid red) prevents the system from converging into a formation that is perfectly aligned with R = 1. For the cases of (1) (dotted blue) and (3) g(s) (dash-dot black), alignment states converge to the well-ordered configuration and remains in that steady state for the last of the evolution time. This comparison indicates that the order-dependent delay in acceleration is the essential factor in generating a rich dynamics of a flock, providing the response sensitivity of the flock from the external perturbations.