What is the crucial field mark for identifying the Blue-gray Tanager even in poor lighting?
Answer
The striking white eye-ring
The text explicitly states that the most crucial field mark, which aids identification in poor lighting or from a distance, is the striking white eye-ring circling the eye.

Related Questions
What is the crucial field mark for identifying the Blue-gray Tanager even in poor lighting?Which habitat type is the Blue-gray Tanager frequently found thriving in?What is the primary component of the Blue-gray Tanager's diet?How do the plumage appearances of male and female Blue-gray Tanagers compare?Where does the Blue-gray Tanager show a strong preference for nesting?What is the approximate northern geographical limit of the Blue-gray Tanager's distribution mentioned in the text?How are the wings and tail feathers of the adult Blue-gray Tanager typically described?How does the Blue-gray Tanager typically behave around humans in accustomed areas?How is the typical vocalization of the Blue-gray Tanager often described?According to the IUCN, what is the current conservation status of the Blue-gray Tanager?How does the plumage of juvenile Blue-gray Tanagers compare to that of adults?In which canopy layer do Blue-gray Tanagers typically forage for food?