page_title: Flight feather
section_title: Primary extension
hierarchical_section_title: Flight feather / Primary extension
caption_attribution_description: These two small warblers are easily confused unless heard singing but there are subtle differences to note. Willow Warbler usually has pale legs whilst Chiffchaff legs are usually dark. The Chiffchaff has more olive upperparts and a less conspicuous supercilium whilst its bill is usually universally darkish whilst that of Willow Warbler has a darkish upper mandible and a slightly paler lower mandible. The clincher however is the primary projection which is always markedly longer in Willow Warbler.
context_page_description: Flight feathers are the long, stiff, asymmetrically shaped, but symmetrically paired pennaceous feathers on the wings or tail of a bird; those on the wings are called remiges, singular remex, while those on the tail are called rectrices, singular rectrix. The primary function of the flight feathers is to aid in the generation of both thrust and lift, thereby enabling flight. The flight feathers of some birds have evolved to perform additional functions, generally associated with territorial displays, courtship rituals or feeding methods. In some species, these feathers have developed into long showy plumes used in visual courtship displays, while in others they create a sound during display flights. Tiny serrations on the leading edge of their remiges help owls to fly silently, while the extra-stiff rectrices of woodpeckers help them to brace against tree trunks as they hammer on them. Even flightless birds still retain flight feathers, though sometimes in radically modified forms.
  The moult of their flight feathers can cause serious problems for birds, as it can impair their ability to fly.
context_section_description: 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.
  As a general rule, species which are long-distance migrants will have longer primary projection than similar species which do not migrate or migrate shorter distances.
caption: Comparison of primary extensions: chiffchaff (left) and willow warbler
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