Which specific facial feature often helps distinguish a Ruby-crowned Kinglet from a Golden-crowned Kinglet when the crown is concealed?

Answer

A prominent, pale, and rounded eyering

If the crown mark is not visible, the presence of a prominent, distinct, and rounded pale eyering is a key feature separating the Ruby-crowned Kinglet from its relative.

Which specific facial feature often helps distinguish a Ruby-crowned Kinglet from a Golden-crowned Kinglet when the crown is concealed?
birdmigrationKinglet