  Soaring migrants exploit columns of rising air (thermals) to cover large
distances with minimal energy. Employing social information while locating
thermals may be beneficial, but examining collective movements in wild migrants
has been a major challenge. We investigated the group movements of a flock of
27 naturally migrating, juvenile white storks using high-resolution GPS and
accelerometers. Analyzing individual and group movements on multiple scales
reveals that birds ahead navigate to and explore thermals, while followers
benefit from their movements. Despite this benefit, followers often leave
thermals earlier and at lower height, and consequently must flap considerably
more. Followers also migrated less far annually than did leaders. We provide
insights into the interactions between freely-flying social migrants and the
costs and benefits of collective movement in natural populations.
