This increase in work rate is exacerbated for birds flying either at the back of the flock, or in denser formations, thus suggesting that the other aforementioned benefits of being in a group must outweigh the immediate energetic costs of cluster-flocking. 2001) is not found in all bird groupings, with homing pigeons ( Columba livia) flying in a cluster-flock having increased flap frequencies when flying near other birds (Usherwood et al. However, this aerodynamic power reduction due to group flight (for example, measured as a reduction in heart rate in flying white pelicans, Pelecanus onocrotalus Weimerskirch et al. It has been demonstrated that for birds flying in a V-formation, for example, positive aerodynamic interactions between members of the flock allow individuals to maximise the capture of beneficial up-wash (Portugal et al. In addition, for animals that travel in groups, potential energetic savings can be achieved through positive aero- or hydro-dynamic interactions that enhance the locomotor performance of individuals in the assemblage (Portugal et al. 1982 Brown 1988) and the sharing of information about resources or efficient routes (Biro et al. The benefits for animals living in groups can include enhanced vigilance and predator detection (Elgar 1989 Parrish and Edelstein-Keshet 1999), increased foraging efficiency (Pitcher et al. This implies that as birds tire during long-duration flight, the ultimate functions of cluster-flocking-primarily anti-predator benefits-are overridden by the proximate costs of flying close to conspecifics. group structure changed), while flap frequency decreased over time. We provide the first evidence that during a long-duration flight, pigeons’ pairwise and group-level distances increased (i.e. As part of a routine 9-km training flight release, a flock of six homing pigeons ( Columba livia) with 5 Hz GPS and 200 Hz accelerometer biologgers attached flew an alternative trajectory totalling 177 km and 256 min of flight. higher flap frequency) and other benefits of flocking. Cluster formations therefore provide a unique opportunity to investigate trade-offs between increased work rate (e.g. Though unlike V-formation flight, cluster-flocking increases the energetic cost of flight, particularly in denser flocks. Birds which fly in coordinated cluster-flocks can benefit through the formation of group-level structures and patterns which can deter predators by visual confusion.
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