This is a HUGE outlier among birds capable of long endurance flight, if the data turn out to be accurate. The Alpine Swift is tiny, 2-ft wingspan and 0.2-lb weight. In contrast, the Albatross stretches its wings to 12 ft and weighs 20 lb, and can fly for up to a few days straight (longest known endurance). Similar for the Condor and Stork. The large wingspan minimizes induced drag and lets it glide extremely efficiently, just like the U-2. As bird wingspans decrease, flapping frequency increases and flight endurance decreases, as with pigeons, hummingbirds, and flying insects. Aerodynamically, flight is vastly less efficient at smaller scales because the air molecules remain the same size. There are physical justifications for long-endurance birds being huge, just as there are with long-endurance aircraft.

Data collection seems highly suspect. Collecting v_dot every 4 minutes seems insufficient for takeoffs/landings that probably take a few seconds. I wonder if this polling rate satisfies the Nyquist criterion for typical acceleration changes of the bird? Then they mention relying on the pitch angle to determine flight. How clear is this correlation? Birds attain pos/neg pitch in climbs and dives. They adjust their pitch on the ground as they walk, while picking up bits to build nests, and ducking down to feed their young. I look forward to seeing this undergo peer review as alphakappa mentions. This type of flight seems very unusual, but perhaps they manage to feed off insects enough to sustain themselves.