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The Great Blue Heron is one of the most widespread and adaptable wading birds in North America.
Up to nine subspecies have been recognized by past researchers, based on differences in plumage color and morphology. Researchers have agreed that Florida's Great White Heron (A. h. occidentalis), the subspecies most distinctive in color (entirely white), and the Pacific Great Blue Heron (A. h. fannini) are distinct subspecies. Recent reviews (see Systematics) have suggested that the remaining Great Blue Herons in North America are composed of either one (A. h. herodias), or two (A. h. herodias, A. h. wardi) subspecies. Owing to this controversy, this account primarily considers 'blue group' Great Blue Herons (A. h. herodias, A. h. wardi, A. h. fannini), usually referred to as the Herodias (or blue) group, and 'white group' Great Blue Herons - the Great White Heron (A. h. occidentalis), referred to here as the Occidentalis (white) group Great Blue Herons.
Equally at home in coastal (marine) environments and in fresh water habitats, the Great Blue Heron nests mostly in colonies, commonly large ones of several hundred pairs. Such colonies are often located on islands or in wooded swamps, isolated locations that discourage predation by snakes and mammals and disturbance from humans. Although the species is primarily a fish eater, wading (often belly deep) along the shoreline of oceans, marshes, lakes, and rivers, it also stalks upland areas for rodents and other animals, especially in winter. It has been known to eat most animals that come within striking range. Its well-studied, elaborate courtship displays have correlates on the foraging grounds, where this species can be strongly territorial.
The Great Blue Heron weathered the impacts of 20th century North Americans relatively successfully. Although it was hunted heavily for its plumes and some of its wetland habitats were drained or otherwise degraded, many populations have recovered well. Nevertheless, breeding colonies remain vulnerable to disturbance and habitat loss, and climate change and increasing predator populations may bring new challenges.
See Butler (1997) for a review of breeding behavior, reproductive success, foraging behavior, and the energetics of growth and reproduction. Foraging ecology and behavior have been covered by Kushlan (1976, 1978), Butler (1995), Gawlik (2002) and Kelly et al. (2003). The effects of contaminants on reproduction are discussed by Harris et al. (2003), Elliott et al. (2005) and Champoux et al. (2006). For discussions of the effects of human and predator disturbance, see Parnell et al. (1988), Vennesland and Butler (2004) and Vennesland (2010).