Cocoi Heron

The cocoi heron is the largest of the South American heron species and measures 95–130 cm (37.5–51 in) in length, although body sizes vary regionally; with southern individuals probably being the largest. Adult weight of both males and females can variously range from 1.14 to 3.2 kg (2.5 to 7 lb). In the adult, wing length has been recorded as 421–455 mm (16.5–18 in), tail length 161–173 mm (6.5–7 in), culmen from base 128.5–148.7 mm (5–6 in) and tarsus 179–192 mm (7–7.5 in). Sizes are similar to its North American counterpart, the great blue heron, although the latter may average slightly larger.

The sexes are alike in both body size and colouration. It has a grey back and upper wings and a white S-shaped neck, thighs, under-tail wing coverts and scapulars; with black streaks on the neck and upper breast. The forehead and crown of the head are black and this black extends down to the eye region and up to the pointed crest hanging down the nape. There is also black streaking on the neck and black patches on sides of the lower breast and abdomen. Leg colour has been reported as black, brownish-grey or dark green. The iris is usually yellow and the bill dull yellow. The bare skin of the orbital region is pale greenish. Some breeding cocoi herons observed in Argentina have bright yellow bills with a red tinge at the base and dusky pink legs. …

The cocoi heron occurs throughout much of South America except in the Andes and in some parts of Argentina. It is native in Argentina, Panama, Suriname, Colombia, Venezuela, Bolivia, Brazil, Chile, Ecuador, French Guiana, Guyana, Paraguay, Peru and Uruguay. 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. It has an estimated extent of occurrence of 20600000 km2. It inhabits the wetter parts of the Chaco and occurs as a vagrant in the Falkland Islands, Saint Helena, Ascension Island and Tristan da Cunha. It is a regular, non-breeding visitor to Trinidad and Tobago. The habitat comprises almost any body of water or wetland away from dense forest; including lake shores, swamps, rivers and estuaries. Gallery forest, grassland and beaches have also been reported to be suitable feeding grounds. 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. In the Falkland Islands, the habitat appears to comprise small streams. In one study in the Parana river, water with aquatic vegetation was found to be the most preferable, followed by open waters, with the lowest preference for beaches. …

This heron feeds primarily on fish measuring over 20 cm (8 in) long; with mammals, amphibians and sometimes insects also being consumed. Fish species consumed include whitemouth croaker, wolf fish, various Leporinus species up to 200 mm long and streaked prochilod. This heron has also been observed to feed on carrion and Callinectes blue crabs. In Colombia, chicks have also been observed being fed predominantly fish and less frequently with amphibians and crustaceans. The cocoi heron catches prey by striking its head into the water and thrusting its bill to stab downwards. It may also tilt its head downward above the water so that only its bill is immersed. During feeding, the head and neck move rapidly while the body remains immobile. Feeding is usually diurnal and solitary, especially in Argentina, although in Chile, it is considered to be a largely nocturnal feeder, with diurnal roosting taking place in trees overlooking the water. In Venezuela, it has also been seen foraging in large groups. Where feeding is diurnal, there appears to be a peak in feeding activity at noon and a reduction at dusk. The cocoi heron appears able to use alternative food sources by exploring foraging sites distant from its breeding colonies, since terrestrial rodents and reptiles and marine organisms have been found in the diet of individuals from freshwater colonies. …

The origin of its species name is unknown, but may be related to the common name in Chile cuca, which in turn denotes the bird's call note. It is more widely known in South America as garza mora or "black heron".