Cozumel Island is known to be home to the endemic Golden Warbler, Setophaga petechia rufi ertex, one of 43 subspecies of the Yellow Warbler, Setophaga petechia, while the mainland Yucatán Peninsula is home to a subspecies of Mangrove Warbler, Setophaga petechia bryanti, but historically Mangrove Warbler has been absent on the island. On 29 April 2014, we observed and photo- graphed a warbler with extensive chestnut hood resembling Mangrove Warbler at Laguna Montecristo on the north coast of Cozumel. Additional visits on 4-5 August 2014, 13-14 July 2015, and 24 October 2015, in addition to photo-documented reports from resident and visiting birders, has turned up a total of 40+ mostly male Mangrove Warblers in addition to numerous potential females both on the north coast as well as on the south coast of Cozumel. Most records are in mangrove vegetation or a mixture of mangrove with dune or secondary vegetation, with one exception. On the other hand, Golden Warblers were never found in only mangrove habitat. All males photographed had broad breast streaks, darker crowns than rest of head, and both sexes had slightly yellower lores than typical Mangrove Warblers, all being features of Golden Warbler. Genetic studies would be highly desirable to understand the origin of this apparently new population.

The Setophaga petechia complex includes 43 subspecies distributed within the new world, of which some are migratory and others are resident, with only two resident subspecies in the Mexican Caribbean: Setophaga petechia bryanti a mangrove subspecies belonging to the erithachorides group resident on the mainland of the Yucatan Peninsula and Setophaga petechia rufivertex endemic to Cozumel Island and belonging to the petechia group. Recently, a new population of individuals presenting intermediate phenotypic traits and living in mangrove ecosystems was discovered and reported for Cozumel Island. In this study, we used a multi-character approach including genetic (five ISSR genetic markers), morphometric (eight traits), phenotypic (four characteristics of males), and acoustic dataset (11 parameters) to understand the process of differentiation and the status of these new island individuals in relation to the two well-established subspecies using a total of 60 individuals (20 for each group). Through multivariate analyses based on different dataset used in our study, we show how the new population is related to the endemic island subspecies, S. p. rufivertex and to the mainland subspecies, S. p. bryanti while demonstrating finite differences. We conclude that the new population of S. petechia on Cozumel Island is a well-established population with high level of differentiation.