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 occur 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.
Black-crowned night herons are 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. Black-crowned night herons feed by standing still at the water's edge and wait to ambush prey, mainly at night or early morning. They also engage in bait fishing; luring or distracting fish by tossing edible or inedible buoyant objects into water within their striking range - a rare example of tool use among birds. During the day they rest in trees or bushes. Young leave their perches to huddle in the nest when it is cold. Black-crowned night herons are territorial and will defend their feeding and nesting territories. Their normal call sounds like 'qua,' 'quak,' or 'quark.' Such calls are most often made during flight or when perching.
Black-crowned night herons are carnivorous (piscivorous) birds. They eat primarily fish and other aquatic life including frogs, tadpoles, snakes, turtles, lizards, 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 that 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 the IUCN Red List, the total population size of the Black-crowned night heron is around 570,000-3,730,000 individuals. The European population consists of 60,000-86,100 pairs, which equates to 120,000-172,000 mature individuals. National population estimates include around 100,000-1 million breeding pairs, more than 10,000 individuals on migration and more than 10,000 wintering individuals in China; around 100,000-1 million breeding pairs in Taiwan; around 10,000-100,000 breeding pairs and around 50-1,000 wintering individuals in Korea and around 10,000-100,000 breeding pairs and around 1,000-10,000 individuals on migration in Japan. Currently, this species is classified as Least Concern (LC) on the IUCN Red List but its numbers today are decreasing.