Sentence:  Gulls average much larger than terns, with stouter build; the feet are larger and more ambulatorial, the wings are shorter and not so thin; the birds <WSD>winnow</WSD> the air in a steady course unlike the buoyant dashing flight of their relatives. 
Meaning of the word marked with <WSD> tags:  To blow upon or toss about by blowing; to set in motion as with a fan or wings.