A relatively small, dark, compact, crested wading bird, the Green Heron is a common species of wetland thickets throughout much of North America. Although shy and retiring, it is a familiar sight to those spending time out of doors. Careful observers can see it stalking slowly through the water, perched quietly atop a branch, or as a dark form flying with slow wingbeats through the gathering dusk. Its flight call, an assertive skeow, is a sound typifying temperate and tropical wetlands of the Americas. Some of this bird's behaviors are especially well appreciated; flying away from human disturbance, for example, it often produces a scolding squawk and a stream of white defecation, giving it such vernacular monikers as "fly-up-the-creek," "shite-polk," and "chalk-line."
Although Green Herons generally nest solitarily, they sometimes nest socially in loose colonies. They feed day or night, standing patiently in shallow water waiting for fish, slowly stalking them through the shallows or diving on them from above. They are among the few tool-using birds, fabricating various baits that entice fish to where they can grab them.
Species limits within this small genus have been under intermittent review and revision, causing corresponding revisions in the "acceptable" English common name. The North American form of this species is part of a matrix of related populations, which differ somewhat in size and plumage coloration, allocated to numerous subspecies but in total found over much of the tropical and subtropical world ( Hancock and Kushlan 1984 ). At times similarities and purported intergradation of features among populations have been emphasized, leading to the North American form being lumped with Old World and Neotropical forms into a single species, Butorides striatus, most recently called the Green-backed Heron, more traditionally called the Striated Heron, and locally called the Mangrove Heron. At other times, differences and purported lack of intergradation of features among populations have been emphasized, leading to the North American form being separated from the Neotropical and Old World forms as Butorides virescens, called the Green Heron or Little Green Heron. On the Galapagos Islands, a third, distinctive population is sometimes considered a separate species, Butorides sundevalli, called the Galapagos or Lava Heron, but more often this population is viewed as a well-marked subspecies of the Neotropical form.
As this monograph is about the North American population, we call our bird the Green Heron (Butorides virescens), which conforms to the most recent declaration of the American Ornithologists' Union's check-list ( American Ornithologists' Union 1993 ). Because there is no evidence of fundamental differences in the biology (or indeed morphology) of any of the populations studied so far, however, we draw on information from Butorides herons worldwide.