Figure 4. A typical flock analyzed in the empirical study of Ballerini et al. (2008a).

This group consists of 1246 starlings, flying at approximately 70 m from the stereoscopic apparatus at about 11 ms⁻¹. (a) and (b) Left and right photographs of the stereo pair, taken at the same instant of time, but 25 m apart. To perform the 3D reconstruction, each bird’s image on the left photo must be matched to its corresponding image on the right photo. Five matched pairs of birds are visualized by the red squares. Once the matching is performed, the 3D coordinates of each individual bird can be retrieved using stereometric formulas. (c) and (d) 3D reconstruction of the flock under two different points of view. Panel (d) shows the reconstructed flock under the same perspective as the right photograph (b). Reprinted from Ballerini M et al. (2008a), Proc. Natl. Acad. Sci. USA105, pp. 1232–1237. Copyright 2008, National Academy of Sciences of the USA.