(noun) A thin, typically dark line extending through or just behind a bird's eye, level with the eye itself. The length, thickness and color of eye lines can vary, and some birds have eye lines only behind their eyes rather than including the lores. If the line is very thick, some birders prefer to refer to it as an eye stripe, and if it completely encompasses the eye (above, below and through), it is often referred to as part of a mask, such as on the loggerhead shrike, rather than a separate line.
While all types of birds can display eye lines, these field marks are often more common in dabbling ducks, warblers and sparrows.
When using an eye line for identification, note its color, thickness and general shape, including whether or not it curves or changes width at any point. Also note whether the eye line is combined with other facial markings such as distinct lores, an eye ring, brow lines or other features.
Photo – Chipping Sparrow © John Benson