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A few minutes of browsing through any field guide will demonstrate the importance of the eye-ring for bird identification. It comes up again and again for species ranging from sandpipers to sparrows. This is a relatively small and simple marking on the plumage, but it is disproportionately important.
One of the reasons eye-ring markings are so useful for identification is that the tiny feathers involved are essentially immobile, unlike the longer neck or back feathers, for example. The shape of the eyering never changes due to the movement of the feathers, so even very small differences in pattern must be the result of differences in coloration. Also, because eyering pattern and face pattern are important visual cues for birds, markings tend to be consistent within a species.
Several concentric rings of tiny feathers form eyerings, and the size and shape of an eyering is determined by how many of these feathers are pale. The narrowest possible eye-ring is only for the innermost ring of these tiny feathers (the outer rings being dark).
A larger eyering is created when the innermost and second feathery circles are both pale, and so on.
Many species show broken eyering or eye arches, when the feathers at the front and back of the eye are dark, while the feathers immediately above and below the eye are pale . All of these eyering variations combine with other markings, such as a dark eye line, pale lores, pale eyebrow, etc., to form a unique pattern on the face of each species.
Paying close attention to the arrangement of these feathers will help you understand the variations in eye-ring marks and will make this important land mark even more useful.
This article from David Sibley’s ID Toolkit featured in the March/April 2018 issue of BirdWatching.