The Great White Heron (GWH) has an all-white plumage and occurs in the Gulf of Mexico and Caribbean. Described originally as Ardea occidentalis, it is now considered a subspecies of Great Blue Heron (GBH; A. herodias). GWH and GBH meet in Florida Bay at the southern tip of Florida, providing the opportunity to evaluate their interaction and species status. To this end, we examined size variation and mate choice across their contact zone and genetic variation range-wide. Measurements of 7 morphological characters indicate trends, but not a significant difference, in size between GBH and GWH in southern Florida. GBH and GWH nest mainly in different places (mainland vs. islands) and at different peak times. In Florida Bay, mixed pairs occur, but white-white and blue-blue pairs are more common than in a randomly mating population. Assessing mating, however, is complicated because most, if not all, nesting blue birds are of mixed parentage. Microsatellite DNA analysis indicates that white and blue herons in Florida Bay and the outer Keys (outside Florida Bay) form a group distinct from blue forms on Florida Peninsula and elsewhere in North America. However, some gene flow occurs from white herons on the outer Keys to white and blue herons in Florida Bay, and from blue herons in Florida Bay to GBH on the Florida Peninsula. GWH alleles occur in all North American populations, but whether this is from gene flow or incomplete lineage sorting is unknown. Deciding GWH's species status is difficult. GWH and GBH meet in an ecotone where some gene flow occurs, but behavior and habitat largely isolate them. We argue in favor of splitting GWH from GBH. Regardless of how it is ultimately classified, the GWH's small population needs to be actively managed as an isolate in an extremely vulnerable environment.
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Vol. 136 • No. 1