The supercilium is a plumage

Plumage

Plumage refers both to the layer of feathers that cover a bird and the pattern, colour, and arrangement of those feathers. The pattern and colours of plumage vary between species and subspecies and can also vary between different age classes, sexes, and season. Within species there can also be a...

 feature found on the heads of some bird

Bird

Birds are feathered, winged, bipedal, endothermic , egg-laying, vertebrate animals. Around 10,000 living species and 188 families makes them the most speciose class of tetrapod vertebrates. They inhabit ecosystems across the globe, from the Arctic to the Antarctic. Extant birds range in size from...

 species. It is a stripe which runs from the base of the bird's beak

Beak

The beak, bill or rostrum is an external anatomical structure of birds which is used for eating and for grooming, manipulating objects, killing prey, fighting, probing for food, courtship and feeding young...

 above its eye, finishing somewhere towards the rear of the bird's head. Also known as an "eyebrow", it is distinct from the eyestripe, which is a line which runs across the lores

Lore (anatomy)

The Lore is the region between the eye and bill on the side of a bird's head. This region is sometimes featherless, and the skin may be tinted, as in many species of the cormorant family. This area, which is directly in front of the eye, features a "loral stripe" in many bird species including the...

, and continues behind the eye. Where a stripe is present only above the lores, and does not continue behind the eye, it is called a supraloral stripe or simply supraloral.

On most species which display a supercilium, it is paler than the adjacent feather tracts.

The colour, shape or other features of the supercilium can be useful in bird identification. For example, the supercilium of the Dusky Warbler

Dusky Warbler

The Dusky Warbler, Phylloscopus fuscatus, is a leaf warbler which breeds in east Asia. This warbler is strongly migratory and winters in southeast Asia. It has a foothold in North America in Alaska, and has also occurred in California...

, an Old World warbler

Old World warbler

The "Old World Warblers" is the name used to describe a large group of birds formerly grouped together in the bird family Sylviidae. The family held over 400 species in over 70 genera, and were the source of much taxonomic confusion. Two families were split out initially, the cisticolas into...

 species, can be used to distinguish it from the very similar Radde's Warbler

Radde's Warbler

Radde's Warbler, Phylloscopus schwarzi, is a leaf warbler which breeds in Siberia. This warbler is strongly migratory and winters in southeast Asia....

. The Dusky Warbler's supercilium is sharply demarcated, whitish and narrow in front of the eye, becoming broader and more buffy towards the rear, whereas that of the Radde's Warbler is diffusely defined, yellowish and broadest in front of the eye, becoming narrower and more whitish toward the rear. The supercilium of the Northern Waterthrush

Northern Waterthrush

The Northern Waterthrush is one of the larger New World warblers. It breeds in the northern part of North America in Canada, and in the northern United States, . This bird is migratory, wintering in Central America, the West Indies, and Florida; also Venezuela, Colombia, and Ecuador...

, a New World warbler

New World warbler

The New World warblers or wood-warblers are a group of small, often colorful, passerine birds restricted to the New World. They are not related to the Old World warblers or the Australian warblers....

, differs subtly from that of the closely related (and similarly plumaged) Louisiana Waterthrush

Louisiana Waterthrush

The Louisiana Waterthrush is a New World warbler. It breeds in eastern North America from southernmost Canada and south through the eastern USA, excluding Florida and the coast....

. The former has a bicoloured supercilium which widens significantly behind the eye, while the latter has an evenly buffy eyebrow which is either the same width throughout or slightly narrower behind the eye.

A split supercilium divides above the lores. In some species, such as the Jack Snipe

Jack Snipe

The Jack Snipe, Lymnocryptes minimus is a small stocky wader. It is the smallest snipe, and the only member of the genus Lymnocryptes...

, the divided stripes reconnect again behind the eye. In others, such as the Broad-billed Sandpiper

Broad-billed Sandpiper

The Broad-billed Sandpiper is a small wading bird. It is the only member of the genus Limicola; some have proposed that it should be placed in the genus Erolia with the "stint" sandpipers, but more recent research suggests that it is should rather go into the genus Philomachus with the ruff and...

, the divided stripes remain separate.

A supercilium drop is a feature found on some pipits; it is a pale spot on the rear of the ear-coverts which, although separated from the supercilium by an eyestripe, can appear at some angles to be a downward continuation of the supercilium.