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.

It has long been assumed that birds save energy when flying information by harnessing the updraft generated by the preceding bird. This assumption is often taken as a basis in the literature, although it has so far only been supported by theoretical models and simulations. Empirical data are scarce due to the lack of technical possibilities. The Austrian Waldrappteam has been researching the formation flight of the Northern Bald Ibises (Geronticus eremita) in cooperation with various partners for many years. The team uses the unique possibility of human-led migration, in the course of the release of this endangered migratory bird species. During these flights, hand-raised young birds are guided with ultra-light airplane over a distance of 800 km to Tuscany. In a previous study (Portugal et al. 2014, NATURE), it was shown that Northern Bald Ibises coordinate precisely during formation flight, in broad accordance with the theoretical models. However, actual measurement of energy expenditure during migration fights were still missing. This was therefore the goal of the FWF-funded research project. In addition, it should be investigated which mechanisms ensure stable cooperation during formation flight. We attached specially developed, aerodynamically shaped data loggers to 30 juvenile Northern Bald Ibises. In addition to accelerometers, these loggers were equipped with a high-frequency GNSS logger, which enables positioning with a resolution of a few centimetres. Five birds were additionally equipped with a non-invasive ECG for continuous heart rate logging. The 1.2 MIO position points and 45,000 heart rate measurements are the first empirical data set of this type collected during migration. Most of the time during flight, the birds positioned themselves within a narrow area laterally offset behind a conspecific. These results are essentially consistent with current theories. However, the overlap of the birds flying laterally offset covers almost half a wingspan, while the models assume only a few percent wingtip overlap. A plausible explanation for this is that the models focused only on the lift-generating wings, whereas our empirical data suggest that the aerodynamics of the bird's fuselage is also relevant to the understanding of formation flight. The data set also provides first quantitative results on energy savings through formation flight. Heart rate measurements, in combination with total dynamic body acceleration (ODBA), show that birds' heart rates are significantly reduced when flying in formation. Surprisingly, the number of short, intermediate glide sequences also increased when the birds flew in formation. This suggests that part of the energy savings during formation flight is due to an increased glide ratio. This is a new finding that significantly influences our understanding of how formation flight works.