Eagles adopt an energy-saving flight mode called gliding. Their broad wings and broad rounded tail enable them to exploit thermals in the air. (Thermals are upward air currents in the atmosphere caused by the absorption of heat, from the sun or land, by the air.)
The birds flap their wings slowly and laboriously in the air in wide circles, but once they catch the rising air they begin to soar effortlessly without even a single beat up to a point where the warm air has cooled and stopped rising.
From this point, they start gliding down to another thermal, which they spot by seeing other groups of rising raptors or perhaps by their delicate sensitivity to even minute changes in air currents. Their primary feathers are spread out to obtain the maximum advantage from the rising air. The wing tips are broadly splayed or ‘fingered’ to reduce turbulence in the air surrounding it. They also assist in gaining speed when the bird glides downwards.
Sea birds such as albatrosses, fulmars, gannets and Manx shearwater also adopt gliding but a slightly altered version. As thermals do not form over the sea, they take a shallow downward glide across the wind then turn into the wind and climb steeply until they resume gliding in their original directions. They thus use relative wind speeds to power both the climb and control the long, downward glide over the sea. These birds can cover thousands of kilometers without expending much energy.