Seabird trajectories map onto a reduced optimal-control bound for dynamic soaring

Dynamic soaring allows seabirds to harvest mechanical energy from vertical wind shear, yet there is no common benchmark for comparing flight performance across species based on their trajectories.

We derive a reduced lower bound on transport effort from a simplified Hamilton-Jacobi-Bellman optimal-control model in which slow flight incurs an induced-drag penalty, fast flight incurs a dissipative penalty, and wind shear supplies an effective energetic subsidy. \add{We rescale each of the four species to its own baseline speed and accelerometer-based effort, then map them onto a common reduced speed--effort plane and estimate each one's lower frontier.

We calibrate the optimal-control bound to one species, the wandering albatross, and test the other three against it.

Two further dynamic soarers, the Buller's albatross and short-tailed shearwater, lie progressively above the bound.

The common crane, a thermal soarer of comparable body mass, lies about 33 times as far from it as the albatross.

Proximity to the boundary, therefore, measures the extent to which a bird's transport is powered by wind shear.

More generally, our work offers a framework for testing optimal-control limits in bird flight using field data.