It's very possible, and in fact most likely, that it can't scale. Insect flight is an even better example - the mechanisms that allow most insects to fly simply don't work past a few grams of weight. So, it is simply impossible to create an insect-like drone that can carry a human.

I expect the exact same is true for birds - the kinds of effects that allow birds to fly with so little energy compared to a propeller-based aircraft are almost certainly not scalable, due to the fundamental properties of air as a gas. As far as I know, bird flight is made possible by complex turbulence effects induced by the microscopic structure of their feathers. It's very unlikely this effect could skale to 100kg of weight.

That's not a bird. Thats an airplane flapping its wings.