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...

From the perspective of zoological institutions reintroduction projects offer many possibilities to link conservation and research programmes. An example of the multi‐layered and diverse contributions that zoological institutions in general and, specifically, Vienna Zoo, Austria, can make is the reintroduction of the Northern bald ibis Geronticus eremita in central Europe. The involvement of zoological institutions ranges from the provision of eggs or birds for release trials, to financial and advocacy support, including with government agencies and non‐governmental organizations. Through involvement at a steering level at the coordinative association ‘Förderverein Waldrappteam’ and as a partner in the EU LIFE+ reintroduction project, Vienna Zoo directly contributes to the shape of the reintroduction project for this Critically Endangered species, and provides much more than technical and infrastructural support. The reintroduction of the Northern bald ibis is broadly in line with the reintroduction guidelines of the International Union for Conservation of Nature. This project provides added benefits not only through its work to prevent the illegal hunting of migratory birds but also the production and dissemination of scientific research.

The northern bald ibis Geronticus eremita was once widespread throughout the Middle East, northern Africa, and southern and central Europe. Habitat destruction, persecution and the impacts of pesticides have led to its disappearance from most of its former range. It disappeared from central Europe > 400 years ago, but has persisted as a relict and slowly growing breeding population in Morocco, where c. 700 wild birds of all ages remain. In Algeria, the last confirmed breeding was in 1984; in Turkey the fully wild population disappeared in 1989, but a population remains in semi-wild conditions. In Syria a small population was rediscovered in 2002, only to subsequently decline to functional extinction. Restoration programmes have been initiated independently in several locations, with over 300 free-flying birds resulting from reintroduction projects in Austria, Germany, Spain and Turkey, to restore both sedentary and fully migratory populations. Maintaining current efforts in Morocco remains a high conservation priority.