Passive Soaring
The primary feathers on passive soaring birds have long primaries and shorter wings.
These feathers
spread out wide, creating a “slot” between the feathers.
This allows the bird to catch thermals and
soar on the rising hot air. This reduces the need to flap and use energy.
Hawks, vultures, pelicans,
storks and eagles all use this wing shape.
The shape of the primary feathers is asymmetrical and air is directed into the slots.
Bald eagles are passive soarers.
The Source:
Page #5 and #6
Ornithology #12
Flight – Wing Types
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