Abstract
Animal groups have emergent properties that result from simple interactions among individuals. However, we know little about why animals adopt different interaction rules because of sparse sampling among species. Here, we identify an interaction rule that holds across single and mixed-species flocks of four migratory shorebird species spanning a seven-fold range of body masses. The rule, aligning with a one-wingspan lateral distance to nearest neighbors in the same horizontal plane, scales linearly with wingspan but is independent of nearest neighbor distance and neighbor species. This rule propagates outward to create a global flock structure that we term the compound-V formation. We propose that this formation represents an intermediary between the cluster flocks of starlings and the simple-V formations of geese and other large migratory birds. We explore multiple hypotheses regarding the benefit of this flock structure and how it differs from structures observed in other flocking species.
Highlights
The collective movements of animals—from schooling fish to swarming insects and flocking birds—have long excited intrigue among observers of nature
We aimed to address this question by collecting three-dimensional (3D) trajectories of the birds in flocks of four shorebird species that have similar ecologies but cover a seven-fold range of body mass and two-fold range of wingspan
We found that all four species studied here fly in a previously-undescribed flock structure that we term the compound-V formation. We propose that this structure is an adaptation for aerodynamic flocking in migratory species, and that ecology is an underappreciated driver of the evolution of avian flocking behavior
Summary
The collective movements of animals—from schooling fish to swarming insects and flocking birds—have long excited intrigue among observers of nature. Out of hundreds of bird species that fly in groups, most research has focused on starlings (Attanasi et al, 2014; Ballerini et al, 2008; Cavagna et al, 2010), homing pigeons (Nagy et al, 2013; Nagy, Ákos, Biro, & Vicsek, 2010; Pettit, Ákos, Vicsek, & Biro, 2015; Pettit, Perna, Biro, & Sumpter, 2013; Usherwood, Stavrou, Lowe, Roskilly, & Wilson, 2011) and birds that fly in V-formations (Badgerow & Hainsworth, 1981; Cutts & Speakman, 1994; Hummel, 1983; Lissaman & Shollenberger, 1970; Maeng, Park, Jang, & Han, 2013; Portugal et al, 2014; Weimerskirch, Martin, Clerquin, Alexandre, & Jiraskova, 2001) These data indicate that smaller birds fly in relatively dense cluster flocks that facilitate group cohesion and information transfer (Attanasi et al, 2014; Ballerini et al, 2008), whereas larger migratory birds fly in highly-structured V formations ( known as line or echelon formations) that provide aerodynamic and energetic benefits (Lissaman & Shollenberger, 1970; Portugal et al, 2014; Weimerskirch et al, 2001). It is difficult to conclude based on the available data what factors contribute to birds adopting specific group formations
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