FLOCK STASIS DRIVES FLYING SPEED IN PIGEONS, WHILE ARTIFICIAL MASS ADDITIONS DO NOT
Animals are characterised, in part, by their use of voluntary movement, which is used to explore and exploit resources from their surrounding environment. Movement can therefore benefit animals, but will cost them their energetic reserves. Thus, adaptations for faster movements with negligible increases in energy expenditure will likely evolve via natural selection. Individual and social-level mechanisms have been shown to optimise this speed/energetic trade-off. Nevertheless, studies of social-level traits typically ignore individual variation, which is a cornerstone principle in evolutionary ecology. Furthermore, how individual phenotype interacts with the phenotypic composition of the group to govern the cost of transport may have been entirely overlooked. We investigate speed and the energetic consequences of individual-level phenotypic differences using body mass (both natural and artificially manipulated with additional weights) of homing pigeons (Columba livia) (N =16 birds; N = 193 useable flight trajectories). We then turn to social level phenomena, and manipulate the composition of pigeon groups by body mass (N= 12 birds in four treatments; N = 192 useable flight trajectories) and leadership rank (N = 30 birds in three groups, N = 286 useable flight trajectories) following earlier leadership identification flights (N = 33 birds, N = 306 useable flight trajectories). "Natural" body mass was predictive of flying speed in solo flights, but not in groups of greater mass by composition; "artificial" mass loading had no impact on speed in solo fliers, and was not tested in groups. Groups of leader phenotypes, showed faster speeds, and greater cohesion than follower phenotype groups, both in terms of flock spread, but also in consistency of positioning within the flock ("flock stasis") across the flight. Flock stasis was further analysed across all other group flights. Its positive impact on speed was found to be consistent across all experimental treatments. Therefore, predicting flock stasis may be critical to understanding optimal phenotypic compositions of birds, and thus the social evolution of birds which fly together. We provide evidence that greater stasis may be driven by phenotypic compositions (i.e. groups of leaders, and homogeneous mass groups) and also discuss the implications of stasis for different flocking structures (e.g. V-formations) and human crowd control.