The Black-crowned night heron has a stocky appearance, as if hunched over, the head tucked down into the shoulders, and it is usually seen with this posture. Its legs and neck are much shorter than those of other herons. This bird is most active at dusk or at night, their ghostly forms flying from their daytime roosts to the wetlands where they forage. Adults have striking gray-and-black plumage with long white head plumes. These birds breed in colonies, their stick nests usually being built over water. This species lives in a wide range of fresh, salt, or brackish wetlands and they are the world’s most widespread heron.
Black-crowned night herons breed on every continent apart from Antarctica and Australasia. In the Americas it is from Washington to Quebec, through coastal Mexico, and in Central America and the Caribbean. In winter they can be found as far north as the New England states and Oregon. The Old World subspecies occurs from Japan to Europe, Africa and India. This heron lives in a wide range of habitats, from swamps, rivers and lakes to salt marshes, lagoons and mudflats. Aquatic and marginal vegetation like mangroves, reed beds, bamboo, and other trees are necessary for nesting and roosting.
The Black-crowned night heron is social throughout the year, often associating with other species. In the winter, they roost together. A migrating species, it will fly at night and rest during the day. Its normal call sounds like 'qua,' 'quak,' or 'quark.' Such calls are most often made during flight or when perching. Feeding throughout the night avoids competition with herons that use the same habitat during the day. Feeding sites are repeatedly used. When landing to feed, it alights feet first on water, or plunges out of the air. When it walks, it usually has its head lowered, and it hardly ever runs. On cold days when flying, it may retract its feet to conserve heat. Young leave their perches to huddle in the nest when it is cold. A Black-crowned night heron will defend its feeding and nesting territories. The young may be aggressive, defecating or regurgitating on human intruders.
Black-crowned night herons are classified as pre-eminently carnivorous, they eat primarily fish and other aquatic life: frogs, tadpoles, snakes, turtles, lizards, the larvae and adult forms of insects, spiders, crustaceans, mollusks, small rodents, bats, chicks and eggs of other bird species.
Black-crowned night herons seem to be monogamous, one male mating with only one female. Males perform an elaborate courtship display, which is often at night. It walks about in a crouching position, with head lowered, clapping its bill. Next, it flaps its wings, singing and dancing. Once the female accepts, the birds preen each other and touch and clasp each other’s bills. The breeding season varies depending on the range. These birds breed in colonies which may be very big (as many as 5000-6000 pairs in Malaysia). The colonies may include several different species. Nests are made on cliff ledges or in reed beds, tall trees, bushes, or on the ground. 3-5 eggs are laid and two broods per season may be produced. Incubation is for about 21-22 days and both parents brood their young. Chicks leave the nest after two weeks, although they do not go far. By three weeks, they will fly to the tops of trees if disturbed. By 6-7 weeks old they are competent fliers and fly to the feeding grounds. The young are sexually mature at 2-3 years.
Although Black-crowned night herons are relatively abundant and widespread, localized threats are a concern for certain populations, in particular, habitat loss, wetland degradation, petroleum and pesticide contamination, hunting and disease.
According to IUCN’s Red List, the Black-crowned night heron global population size is 510,000-3,600,000, with national population estimates including: 100,000-1 million breeding pairs, over 10,000 individuals migrating and over 10,000 wintering birds in China; in Taiwan 100,000-1 million breeding pairs; in Korea 10,000-100,000 breeding pairs as well as 50-1,000 wintering individuals; and in Japan 10,000-100,000 breeding pairs, with 1,000-10,000 birds migrating. Overall, currently Black-crowned night herons are classified as Least Concern (LC), however, their numbers today are decreasing.
Black-crowned night herons may have influence on the fish population due to their diet.