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 isn't 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.
Read more about this topic: Flight Feather
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