Pigeons in a flock go cheap: a re-evaluation of the energetics of flying in cluster flocks.

The energy expended by animals during locomotion is often of considerable ecological importance. However, inaccurate conclusions about energy expenditure may arise if the limitations of the proxy variables being used to infer locomotion costs are not considered. The study of 'cluster' flocking behaviour in pigeons, using wingbeat frequency as a proxy for mechanical energy output, provides a useful illustration of these pitfalls. In contrast to claims in the literature, we suggest that published body kinematic values measured with accelerometers, along with our own heart rate data, show little evidence in support of the hypothesis that there is an increase in the mechanical energy costs for most pigeons to fly in a typical 'cluster' flock. Indeed, our re-analyses of acceleration-based measures of body power suggest there may be a positive energetic advantage to flying at low flock densities. We suggest that, when assessing energy expenditure using accelerometry-derived variables, the energy proxy units should be consistent with those of power, whether mass specific or absolute, and should control for differences in body mass and speed where appropriate. Accumulated total journey costs should also be assessed alongside instantaneous costs, with the former likely to be at least as ecologically significant.