One conspicuous feature of several larger bird species is their annual migration in V-shaped or echelon formation. When birds are flying in these formations, energy savings can be achieved by using the aerodynamic up-wash produced by the preceding bird. As the leading bird in a formation cannot profit from this up-wash, a social dilemma arises around the question of who is going to fly in front? To investigate how this dilemma is solved, we studied the flight behavior of a flock of juvenile Northern bald ibis (Geronticus eremita) during a human-guided autumn migration. We could show that the amount of time a bird is leading a formation is strongly correlated with the time it can itself profit from flying in the wake of another bird. On the dyadic level, birds match the time they spend in the wake of each other by frequent pairwise switches of the leading position. Taken together, these results suggest that bald ibis cooperate by directly taking turns in leading a formation. On the proximate level, we propose that it is mainly the high number of iterations and the immediacy of reciprocation opportunities that favor direct reciprocation. Finally, we found evidence that the animals' propensity to reciprocate in leading has a substantial influence on the size and cohesion of the flight formations.formation flight | cooperation | social dilemma | reciprocity A considerable portion of the worldwide bird population performs biannual long-distance migrations (1, 2). These journeys impose large energetic costs on the animals, and as a consequence, mortality is considerably higher during migration than at any other time of the year (3). Reasons for this increased mortality during migration include suppressed immune response, starvation, and dehydration, among others (4, 5). In greater snow geese (Chen caerulescens), for example, mortality during the autumn migration was estimated at 5% for adult birds and up to 35% for juvenile birds (6). At least a part of this increased mortality can be directly or indirectly linked to the physical exertion during migration flights. Consequently, there should be a strong selection pressure-especially on young birds during their first migration-to minimize energy expenditure during migratory flights and increase the chance of survival. Traveling in close, structured groups has been proposed as an energy-saving strategy, with savings being accrued through positive aero-or hydrodynamic interactions between members of the group (7-10). Flight in V-shaped or echelon formation by birds is perhaps the most prominent example of this. The precise aerodynamic interactions in a flock have been the subject of detailed theoretical and, more recently, empirical studies (2,7,(11)(12)(13)(14). During flight, high-pressure air under the wings flows around the tips to a region of low air pressure above the wings. This flow forms two vortices in the bird's wake, produced by regions of upwash outboard of the wings, and a central region of downwash immediately behind the bird (7, 13). This up-wash can provi...