While the photo is a good one, it does not show several important aspects of the bird i.e. whether the eye has a teardrop or not; is the tail clearly forked; does the bluish caste to the breast indicate a blue throat or breast. However, having seen the "glittering" appearance of Cozumel Emerald, your photo would make me guess you saw Canivet's Eemerald. Try plate 70 of Howell & Webb, A Gde to Birds of Mexico.

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