Date of Award


Document Type


Degree Name

Doctor of Philosophy (PhD)


Forestry, Wildlife, and Fisheries

First Advisor

Frank C. Rohwer


The Great White Heron was originally described as a distinct species (Ardea occidentalis), but is currently considered to be the white morph of a polymorphic Great Blue Heron subspecies (A. herodias occidentalis). Great White and Great Blue heron breeding ranges overlap in the Florida Keys (USA), providing an opportunity to evaluate the degree to which these two taxa are reproductively isolated. The current classification leads to predictions of no significant size difference (H01), random mate choice (H02), and no genetic divergence (H03) between sympatric white and blue herons. Sympatric Great White and Great Blue herons did not differ significantly in size at any of seven morphological variables. However, the power of these tests was low, and patterns of mean values were consistent with the hypothesis of a hybrid origin for Great Blue Herons breeding in the Florida Keys. Mate choice was not random with respect to plumage color. I observed more white/white and blue/blue pairs and fewer mixed pairs than expected in a randomly mating population. This positive assortative pattern suggests that prezygotic reproductive barriers exist within the Florida Keys population. However, mixed pairs occur and may provide an opportunity for gene flow between white and blue herons within the Florida Keys or between the Florida Keys breeding population and other Great Blue Heron populations. Allele frequencies at 12 nuclear microsatellite loci differed significantly between the Florida Keys breeding population and Great Blue Herons breeding on the nearby Florida peninsula. These data suggest that the Florida Keys population is distinct from other Great Blue Heron populations and that there are barriers to gene flow between the Florida Keys Great White Heron population and nearby Great Blue Heron populations. The Great White Heron appears to be a good biological species and a review of its taxonomic status is merited. Recruitment from Great Blue Heron populations does not appear to be an important factor in maintaining the Great White Heron population. Effective conservation, therefore, will require understanding and managing the small Florida Keys Great White Heron population as an isolate.