Sizes are similar to its North American counterpart, the great blue heron, although the latter may average slightly larger.

Some breeding cocoi herons observed in Argentina have bright yellow bills with a red tinge at the base and dusky pink legs.

[8] Despite the paler colouration, the downy chick already shows the contrast between the dark head and pale neck typical of the adult.

The cocoi is distinguished by a striking white neck and solid black crown, but the duller juveniles are more easily confused.

[13] It is native in Argentina, Panama, Suriname, Colombia, Venezuela, Bolivia, Brazil, Chile, Ecuador, French Guiana, Guyana, Paraguay, Peru and Uruguay.

[1] Overall, the cocoi heron is notably widespread and ranges from Central America to the mainland shores on the Strait of Magellan, but is rarely found further south than Chubut in Argentina.

[10] The habitat comprises almost any body of water or wetland away from dense forest; including lake shores, swamps, rivers and estuaries.

[13] Pastures appear to be disfavoured probably because these habitats harbour a large proportion of insects, which are less favoured as prey than fish and mammals.

[1] This heron feeds primarily on fish measuring over 20 cm (8 in) long; with mammals, amphibians and sometimes insects also being consumed.

[18] The cocoi heron catches prey by striking its head into the water and thrusting its bill to stab downwards.

[10][1] This heron is primarily non-migratory, but may move toward the equator during winter to seek warmer temperatures.

[14] The large, deep nest is constructed from branches and sticks with grass, usually situated in trees.

[4] In Chile, cocoi herons have been observed to construct their nests on weeping willow trees in standing water with rushes.

[4] Eggs from one Argentinian clutch weighed 70-80 g.[4] From one colony in Buenos Aires, incubation was estimated as 24–26 days.

Individuals in some territories are affected by agrochemicals, environmental modification and human interference with nests and eggs, but these issues do not constitute severely harmful threats and do not threaten the species with extinction.