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A few minutes browsing any field guide will demonstrate the importance of the eye-ring for bird identification. It comes up again and again for species from sandpipers to sparrows. This is a relatively small and simple plumage marking, 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 feathers of the neck or back, for example. The shape of the eye-ring never changes due to feather movement, so even very small differences in pattern must be the result of differences in coloration. In addition, because eye-ring pattern and face pattern are important visual signals to the birds, the markings tend to be consistent within a species.
Several concentric rings of tiny feathers form eye-rings, and the size and shape of an eye-ring is determined by how many of those feathers are pale. The narrowest possible eye-ring involves only the innermost ring of these tiny feathers (with the outer rings dark).
A broader eye-ring is created when the innermost and the second ring of feathers are both pale, and so on.
Many species show a broken eye-ring or eye-arcs, when the feathers at the front and back of the eye are dark, while feathers immediately above and below the eye are pale. All of these variations of eye-rings combine with other markings, such as a dark eye-line, pale lores, a pale eyebrow, and so forth, 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 markings, and make this important field mark even more useful.
This article from David Sibley’s “ID Toolkit” column appeared in the March/April 2018 issue of BirdWatching.