 The distance that a bird 's longest primaries extend beyond its longest secondaries ( or tertials ) when its wings are folded is referred to as the primary extension or primary projection . As with wing formulae , this measurement is useful for distinguishing between similarly plumaged birds ; however , unlike wing formulae , it is not necessary to have the bird in @-@ hand to make the measurement . Rather , this is a useful relative measurement — some species have long primary extensions , while others have shorter ones . Among the Empidonax flycatchers of the Americas , for example , the dusky flycatcher has a much shorter primary extension than does the very similarly plumaged Hammond 's flycatcher . Europe 's common skylark has a long primary projection , while that of the near @-@ lookalike Oriental skylark is very short . 
