When birds remain airborne and moving without flapping their wings, on what are they gliding?

They are gliding on updrafts or rising currents of air. Air close to the earth is heated by the sun and this causes it to rise. Warm air is less dense than cold air so it will rise. Birds seem to glide because the force of gravity acting on the animal as it flies is equaled by the rising column of air. This is called soaring.