Emblematic of the wetlands of South America, where it is the largest heron species (3)(4), the cocoi heron is a distinctive waterbird with an all-black cap, a white neck, white plumes on the breast, a grey back and wings, and white thighs. The lower underparts are black, a few black streaks may run along the midline of the foreneck, and black ‘shoulder’ patches show on the wings when the bird is standing (3)(4)(5)(6)(7). During the breeding season, the cocoi heron has long, black plumes on the cap, tipped with white, a bright yellow beak with a reddish base, and dusky pink legs (3)(6)(7). The eyes are yellow and the facial skin is greenish to blue (3)(5). Outside of the breeding season, the beak is a duller yellow, with a blackish base, and the legs are blackish (3). The male and female cocoi heron are similar in appearance, whereas immature cocoi herons are greyer, and have a duller but still distinctive dark cap, which lacks plumes (2)(3)(5).
The cocoi heron is similar in appearance to the closely related great blue heron (Ardea herodias), but is much whiter, and can also be distinguished by the completely black cap, the white rather than grey neck, and white rather than chestnut thighs and wing edges (2)(3)(5)(6)(7). Southern cocoi herons are reported to be larger than those in the north, and there is also likely to be further geographical variation that has not yet been described (3).
The cocoi heron feeds in the manner typical of large herons, mainly by standing or walking slowly in shallow water as it hunts for prey. Feeding may take place by day or by night, and the diet consists primarily of large fish, although frogs, aquatic insects and even carrion may also be taken (2)(3)(4). Although generally a solitary species, the cocoi heron may sometimes join large, mixed-species groups to feed, particularly during the dry season, and may even steal prey from other species at these times (2)(3).
The breeding season of the cocoi heron varies with location, starting around July in Surinam, October in Uruguay, and August to November in southeast Brazil and Argentina (2)(3). The cocoi heron may nest alone or in colonies, which are sometimes large and may include other species (2)(3)(6). The nest is large and deep, built from twigs and reeds and lined with grass, and is usually located in trees, bushes or reedbeds. Up to four eggs may be laid, which are light blue in colour, with paler speckles. The eggs are incubated for around 24 to 26 days (2)(3).
The cocoi heron has a widespread distribution, being found over most of South America, excluding the high Andes. It has been recorded from eastern Panama, south to southern Chile and Chubut, Argentina, and is also an occasional visitor to Trinidad, Tobago and the Falkland Islands (2)(3)(4)(5)(8). This species is likely to be resident in most of its range, although birds in the extreme south probably move northwards during winter, and individuals may disperse to other areas after breeding (2)(3)(4).
The cocoi heron inhabits a wide variety of wetland habitats, including rivers, streams, marshes, swamps, lake shores, flooded pastures, estuaries and lagoons (2)(3)(4)(5). Although not generally found in dense forest, it does occur along forested rivers and in marshy areas surrounded by forest. The cocoi heron is normally a lowland species, but has been recorded up to elevations as high as 2,550 metres (2)(3).
The cocoi heron is common and widespread, and its population appears to be increasing (3)(8). In some areas, the species may potentially be affected by pollutants, especially pesticides, as well as by habitat alteration, hunting, egg collection and human disturbance at nesting colonies (3)(9). However, it is not believed to be facing any major threats, and is not currently considered at risk of extinction (3)(8).
There are no specific conservation measures known to be in place for this widespread species, and it is likely that it occurs in a number of protected areas throughout its large range. However, despite its abundance, large size and conspicuous nature, surprisingly little is known about the cocoi heron’s biology, and it may therefore benefit from further studies into its feeding behaviour, habitat requirements, movements and migration patterns (3).
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