Using a high-tech, miniature backpack fitted with a global positioning system (GPS), EU-funded researchers in Hungary and the UK have achieved new insights into the roles of leaders and followers. Studying the rapid, collective decision making of pigeons in flight, the scientists observed that the leadership role of any given pigeon fluctuates over time. The results, published in the journal Nature, open up intriguing questions about hierarchies generally. The study received funding from two projects that received financial support from the EU totalling EUR 2.35 million. The findings are an outcome of two projects: COLLMOT ('Complex structure and dynamics of collective motion'), financed with EUR 1.25 million under the Ideas Programme of the EU's Seventh Framework Programme (FP7), and STARFLAG ('Starlings in flight: understanding patterns of animal group movements'), which received approximately EUR 1.1 million under the 'New and emerging science and technologies' (NEST) activity of the Sixth Framework Programme (FP6). STARFLAG researchers sought to gain insight into group behaviour in humans (including collective economic choices) by investigating the collective movement of flocks. 'We are all aware of the amazing aerobatics performed by flocks of birds, but how such flocks decide where to go and whether decisions are made by a dominant leader or by the group as a whole has always been a mystery,' said Dr Dora Biro of Oxford University's Department of Zoology in the UK. Advances in GPS technology have made it possible to capture movements at the scale of a fraction of a second, thus allowing the team to explore how individual birds influence a fast-moving flock's behaviour. 'We found that, whilst most birds have a say in decision making, a flexible system of 'rank' ensures that some birds are more likely to lead and others to follow,' Dr Biro stated. The scientists attached miniature backpacks containing GPS loggers to homing pigeons and studied them individually and in flocks of up to 10. They studied flights close to home and during homing from around 15 kilometres away. Shifts in the flight direction of each bird were measured every 0.2 seconds; these changes were compared with those of other birds in the flock to see which birds initiated change and which ones followed. The team observed clearly defined hierarchies within flocks, and discovered that the leadership role of different pigeons changed over time. Certain birds contributed more to decision making than others, but there was a wide spectrum of leadership levels. 'Crucially, these hierarchies are flexible in the sense that the leading role of any given bird can vary over time, while nonetheless remaining predictable in the long run,' Dr Biro explained. 'This dynamic, flexible segregation of individuals into leaders and followers, where even the lower-ranking members' opinions can make a contribution, may represent a particularly efficient form of decision making.' The team noted that whether the birds were leaders or followers may be related to individual navigational efficiency. 'Whether such effects come from some individuals being more motivated to lead or being inherently better navigators, perhaps with greater navigational knowledge, is an intriguing question we don't yet have an answer to,' the Oxford researcher said. Another observation was that a bird's position in the flock matched its position in the hierarchy, with individuals nearer the front more likely to be responsible for decisions (an important question to address, given the birds' exceptional peripheral vision). Followers also responded more quickly to those flying on their left, confirming previous informal observations. 'Could the mechanism we identified in small pigeon flocks scale up to larger groups?' the study asks. The flexible, dynamic roles of individual birds in a flock, fluctuating over time, provides an interesting contrast with the normally fixed leader/follower roles in most human hierarchies. The birds' sophisticated leadership system may, the authors speculate, benefit individual group members more than either a single-leader or egalitarian system.