Why can birds hold altitude for a long time without flapping their wings?
Birds in the air are exposed to a buoyant force equal to the weight of the air in the volume of the air displaced by them. Its significance is insignificant.
The area of unfolded wings has a much greater influence on their flight.
The larger it is, the more resistance the air has when they decrease, since the resistance force is directly proportional to the area (the weight of the bird’s body presses on the air, which has a counter resistance on every square centimeter of the bird spread out in the air).
In addition, thanks to the convection of different-temperature layers that occurs in the air space, the bird can find ascending currents and, as it were, lean on them with its wings (the force of air resistance will add up in this case with the lifting force of the air going up).
Thanks to these effects, “large-winged” birds can soar for a long time in height without flapping their wings (remember our kites, hovering over vast flowering summer meadows).