(noun) One of the main flight feathers along the outer edge of a bird's wing. Primary feathers are the "fingertip" feathers, the longest and narrowest on the wing. In flight, they are responsible for thrust to propel the bird forward, and each feather can be rotated individually to control flight directions and to adjust lift and air resistance. The number of primary feathers a bird has depends on the species and how specialized the feathers are – most birds have 9-16 primaries.
Primary feathers can be used for bird identification not only by color but also when comparing the primary shape to adjacent feathers, judging the angle and "splay" of these feathers on the wingtips and by judging how far the primary feathers project past other feathers on the folded wing.
Photo – Barn Swallow Wing © Alan Vernon
Primary, Primary Quill, Primaries (plural)