I wonder how realistic is the means of transportation that uses geared birds as a propulsion to lift and pull a cabin of an aircraft?
Is this type of aviation possible? How one would control the birds?
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A large bird of prey (like a Harpy Eagle) can carry at most ~15kg. An ultralight aircraft without an engine weighs a bit under 200kg, so theoretically a team of about 13 eagles could lift one off the ground. That is about the only good news, though. When you add even a single passenger, your eagle count goes past 20, and any sort of passenger or cargo capacity will soon require hundreds or thousands of birds. But the real problem is fatigue. While large raptors can lift relatively heavy loads, they don't fly very far with them. When carrying things a long distance (like food back to their nests), they generally only carry things under about 4kg. With a per-bird carrying capacity that low, it would be difficult to have enough birds to lift anything without them getting into each others way. And that is before we even talk about trying to train them. Unless your world has much larger/stronger birds than ours does, this doesn't seem feasible.
I think you'll be stuck with having birds the size of the Golden Eagles in Tolkien's Lord of the Rings/Hobit to get any kind of decent lift. At that point you're probably better off just riding the bird, since it would be safer and easier not having them all tied together, and less training since they will all learn to fly, and not have to be trained to fly in sync and together.
Another issue with harnessing birds together would be the turbulence, the birds flapping together would likely interfere with each other, at least enough to make lift more difficult unless you can having them flying in formation like geese.