Costs and Benefits of Formation Flight in Birds

A considerable part of the world wide bird population performs seasonal long-distance migrations. One conspicuous feature of several bird species is their flight in V-shaped or echelon formation, which allows birds to save energy by utilizing the aerodynamic up-wash produced by the preceding bird. As not all birds in a formation can profit from the aerodynamic effect to the same extent, a cooperation dilemma arises around the problem which bird has to fly in the least advantageous leading position. In a previous study we could deliver evidence that cooperation during formation flight is based on dyadic reciprocation. Here we aim to measure the extent of the aerodynamic advantage and we will investigate the proximate mechanisms that enable this kind of reciprocation. Two opposing hypotheses for how the birds achieve continued cooperation have been proposed: direct reciprocation in a tit-for- tat like manner and generalized reciprocation based on proximity. In order to test these hypotheses we will record the flight behaviour of three flocks of bald ibis (Geronticus eremita) during a human guided autumn migration. All birds will be equipped with data loggers with high-precision GPS, accelerometers and ECG sensors that allow us to determine the position of the birds within the formation as well as to estimate their energy expenditure (metabolic rate). Monitoring the birds over the entire migration route will allow us, for the first time, to gain insights into the social dynamics during the migration and the development of the birds cooperation competence and to quantify the energy savings that free-flying birds can gain by in-wake flying. This project is expected to deliver important insights for evolutionary ecology: Cooperation is one of the big enigmas in evolutionary biology because it contravenes the basic notion that evolution favours only selfish genes which promote just their own well-being. Bird migration in organized formations is a very prominent example of animal cooperation and we argue that several characteristics of migratory flights (high selection pressure, potentially high energetic gains, a specific payoff structure, and ample opportunities for repeated interactions) make it a key system for understanding cooperation in animals.