Synchronization, coordination and collective sensing during thermalling flight of freely migrating white storks

Abstract

Exploring how flocks of soaring migrants manage to achieve and maintain coordination while exploiting thermal updrafts is important for understanding how collective movements can enhance the sensing of the surrounding environment. Here we examined the structural organization of a group of circling white storks (Ciconia ciconia) throughout their migratory journey from Germany to Spain. We analysed individual high-resolution GPS trajectories of storks during circling events, and evaluated each bird's flight behaviour in relation to its flock members. Within the flock, we identified subgroups that synchronize their movements and coordinate switches in their circling direction within thermals. These switches in direction can be initiated by any individual of the subgroup, irrespective of how advanced its relative vertical position is, and occur at specific horizontal locations within the thermal allowing the storks to remain within the thermal. Using the motion of all flock members, we were able to examine the dynamic variation of airflow within the thermals and to determine the specific environmental conditions surrounding the flock. With an increasing amount of high-resolution GPS tracking, we may soon be able to use these animals as distributed sensors providing us with a new means to obtain a detailed knowledge of our environment.This article is part of the theme issue 'Collective movement ecology'.

Keywords: collective behaviour; collective sensing; migration; social migration.

Figure 1.

Collective flight trajectories and examples of synchronization in circling direction. (a) Flock trajectories (1 Hz GPS data) of migrating storks showing different flight types (circling and gliding). Grey arrows indicate the flight direction. Tracks are colour-coded based on the horizontal path curvature, κ. |κ| is close to 0 during gliding and typically larger than 0.01 during circling. Positive (yellow-red) and negative (cyan-blue) values of κ indicate counterclockwise and clockwise circling, respectively. The thermal is drifting with the wind resulting in distorted trajectories even if the bird flies in a perfect circle relative to the moving air. This can be seen in peak values of κ. (b) Enlarged view of the tracks marked in (a). (c) Curvature against time for each individual. Each function is shifted upwards by the ID of the bird. Black circles show a switch in curvature during circling (IDs of the birds are shown on the right). Arrows at the top depict the time delays between the switches relative to the first individual. (d) Horizontal trajectories of the birds that switched their circling direction (same time interval as in (c)). Tracks (except bird 2) are shifted according to a grid of 150 m for better visibility. Numbers indicate bird ID.

Figure 2.

Example of synchronized circling characterized by different measures. (a) Maximum correlation values (C_max) between bird 26 (bird that switched first in figure 1c; marked as grey area) and all other birds that followed the directional switch. C_max was calculated for each pair of birds using the directional correlation delay analysis with a 30 s time window. Bars on the right correspond to the average correlation value of the same period. High correlations indicate that birds circle in synchrony which applies to all birds marked on the right of figure 1c. As a counter example, we show bird 25 that is initially circling in synch with bird 26, but does not switch its circling direction. The correlation value drops accordingly. (b) Orientation angle (ϕ = atan2(v_y, v_x)) of the horizontal velocity against time of all birds synchronized with bird 26 (top line corresponds to bird 26). The orientation angle ϕ is presented on the y-axis and as a colour code. To be able to present many individuals, ϕ/(2π) is shown in the range of [−0.5 : 0.5]; the dashed lines correspond to periodic boundary transition that refers to π = −π (see inset on the right). Switches (around 105 s) from clockwise to counterclockwise circling are depicted as a change from decreasing to increasing values. Similarities in the slopes of the curves indicate that birds circle with similar angular velocity. (c) Orientation angle for birds not synchronized with bird 26 (for simplicity only five birds are shown). For example, bird 27 (top line of panel c) circled in the opposite direction to bird 26 (top line of panel b). Or, bird 25 circled in the beginning of the presented period in synchrony with bird 26, but did not switch its circling direction as bird 26 did.

Figure 3.

Ratio of individuals circling in the same direction as a function of height difference. For each GPS burst that contained at least 30 s of circling, we characterized for every pair whether they circle in the same or in opposite directions. Thin (coloured) lines correspond to the average ratio of birds circling in the same directions. Dashed line depicts a random ratio of 0.5. Thick black line shows the mean of all bursts.

Figure 4.

Repeatability of synchronization between individuals. (a) Synchronization ratio (r_synch) defined as the time spent moving on a correlated path (derived from DCD analysis) divided by the total time of circling together for each pair of birds. Black circles show values averaged over the first and second day. There is a weak correlation between the days (Pearson's r = 0.31). Coloured markers correspond to five randomly chosen pairs. (b) Synchronization ratio against time for the five randomly chosen pairs. The bottom plot with grey markers shows the mean of the entire flock. Error bars represent standard deviation.

Figure 5.

Initiation and adoption of synchronized switches in circling direction. (a) The total number of followers that adopted the initiation against the total number of initiations. We defined the initiator as the individual that switched its circling direction first. In cases with multiple initiators, n_i, we weighted the initiation with 1/n_i. (b) Network visualization of individuals that synchronized their directional switch. Edges are non-directed; their widths represent the total number of times that a pair switched in synchrony (the sum of initiations and adoptions). Only values greater than or equal to 1 are shown. Colour coding of the nodes is identical to (a).

Figure 6.

Bird orientation at the time of the synchronized switch in circling direction. Plots represent the orientation angle of the follower (y-axis) against the orientation angle of the initiator (x-axis) during a synchronized switch. Colour coding corresponds to the time delay, τ, between the switch. (a) Orientation angles of both birds at time t, i.e. when the initiator performed the switch. (b) Orientation of both birds at the time they performed the directional switch, i.e. initiator at time t and follower at time t + τ. (c) Orientation of both birds at the time they performed the directional switch after removing the effects of horizontal winds. We estimated the horizontal wind for each location, and subtracted the wind velocities from the GPS tracks of the birds (see Results, Material and methods and [7] for details).

Figure 7.

Collective sensing to map thermal structure and strength. (a) Bird trajectories colour coded by vertical speed in a 5 min circling bout. (b) The mean path of the flock defines at each altitude the centre of the thermal. Using relative coordinates Δx, Δy from the thermal centre in the horizontal plane the vertical speed produces the velocity profile of the thermal (see also electronic supplementary material, video S1–S3). Vectors correspond to the vertical speed of the birds rising in the moving air. To estimate the vertical speed of the airflow within the thermal, it is necessary to know the bank angle and corresponding sink rate of the circling birds.

Figure 8.

Individuals' performances in the mapped thermal structure. (a,b) Each circle represents the mean value of one individual. Plots show vertical speed against (a) distance from the estimated horizontal centre of the thermal at the altitude of the circling bird and (b) the bird's height relative to the centre of mass of the flock. We excluded two individuals too far away from the flock (39 and 68 m) from the analysis. (c) Individual's average distance from the horizontal centre of the thermal against the relative height compared to the centre of mass of the flock.