 Primaries are connected to the manus ( the bird 's " hand " , composed of carpometacarpus and phalanges ) ; these are the longest and narrowest of the remiges ( particularly those attached to the phalanges ) , and they can be individually rotated . These feathers are especially important for flapping flight , as they are the principal source of thrust , moving the bird forward through the air . Most thrust is generated on the downstroke of flapping flight . However , on the upstroke ( when the bird often draws its wing in close to its body ) , the primaries are separated and rotated , reducing air resistance while still helping to provide some thrust . The flexibility of the remiges on the wingtips of large soaring birds also allows for the spreading of those feathers , which helps to reduce the creation of wingtip vortices , thereby reducing drag . The barbules on these feathers , friction barbules , are specialized with large lobular barbicels that help grip and prevent slippage of overlying feathers and are present in most of the flying birds . 
