Figure 6.

Reorientation of the flock during the turn. (a,b) Change of the orientational topology around a reference bird in the flock. (a) The flock at the start of the turn: the reference bird is pictured in black, birds flying in front with respect to the bird are green those on the sides are in light blue, the ones behind are in orange. (b) The flock at the end of the turn: birds are pictured with the same colours as in (a). (c) Global orientation of the flock before and after performing a turn. We report scalar products between unitary vector I ₃ of the flock's longest elongation axis with its direction of motion V(t)/V(t) at times t₁ (start of the turn) and t₂ (end of the turn). Data are given in the electronic supplementary material, table S1. The angle between the longest elongation axis and the direction of motion is large before the turn ( for almost all events) and it reduces during the turn. The exceptions are events E11 and E12 which correspond to two consecutive turns of the same flock, and occur after merging of two separate flocks into one (see the electronic supplementary material), which might be the reason why the reorientation is different.