the colour, shape or other features of the supercilium can be useful in bird identification. for example, the supercilium of the dusky warbler, an old world warbler species, can be used to distinguish it from the very similar radde's warbler. 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, a new world warbler, differs subtly from that of the closely related (and similarly plumaged) louisiana waterthrush. the louisiana has a bicoloured supercilium which widens significantly behind the eye, while the northern has an evenly buffy eyebrow which is either the same width throughout or slightly narrower behind the eye.