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.
Also, they got measurements from three birds so they have 216,000 continuous non-stationary measurements. A stats guru might be able to figure out the effective reduced sample interval.
So, 3.5 grams of hummingbird would be ~0.1 Watt, or, for a 20 hour trip, 2Wh of capacity. On the other hand, a kg of hummingbirds could give you 600 Wh, or 2-4 times what LiOn gives you.
I would go for hornets or other nasty insects, though. That gives less hassle with the law and animal rights activists.
There's also the common swift, who spends almost all its time in the air and absolutely never lands on ground, due to being unable to take-off -- its legs are too short and weak to do that. They only land on rock faces and take off by letting go. That's where they build their nests too. You can see them in a lot of cities actually -- high buildings are pretty much perfect for them to nest on.
"A peculiarity that birds share with aquatic mammals, and possibly also with certain species of lizards (opinions differ about that last point), is the ability for unihemispheric sleep. That is the ability to sleep with one cerebral hemisphere at a time, while the other hemisphere is awake"
In other words: the brain has two CPUs, and they switch off only one at a time.
Completely guessing here, but I suppose it's the same sort of involuntary control you have over your body during sleep that keeps you from wetting yourself or rolling off the bed in the middle of the night.
Swifts catch insects in flight, so food isn't a problem. I think there are species which can extract all the water they need from their food, so they may be able to go without drinking.
You can't actually say that it's "a huge outlier", simply because it's the only small bird for which we have such data.
There was never any reason to think swifts had sleeping patterns anywhere close to those of albatrosses, condors or storks. Albatrosses, condors and storks also all have vastly different behaviours and live in completely different environments.
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.