What visual detail immediately rules out the Connecticut Warbler when identifying a Mourning Warbler?

Answer

The Connecticut Warbler possesses a complete, distinct eye-ring

The presence of a complete, distinct eye-ring is an unmistakable characteristic of the Connecticut Warbler that is lacking in the Mourning Warbler, serving as a vital separation tool.

What visual detail immediately rules out the Connecticut Warbler when identifying a Mourning Warbler?
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