Black-crowned Night-Herons are stocky birds compared to many of their long-limbed heron relatives. They’re most active at night or at dusk, when you may see their ghostly forms flapping out from daytime roosts to forage in wetlands. In the light of day adults are striking in gray-and-black plumage and long white head plumes. These social birds breed in colonies of stick nests usually built over water. They live in fresh, salt, and brackish wetlands and are the most widespread heron in the world.
Black-crowned Night-Herons are small herons with rather squat, thick proportions. They have thick necks, large, flat heads, and heavy, pointed bills. The legs are short and, in flight, barely reach the end of the tail. The wings are broad and rounded. Adults are light-gray birds with a neatly defined black back and black crown. Immatures are brown with large white spots on the wings and blurry streaks on the underparts. Adults have all-black bills; immatures have yellow-and-black bills. They have a body size of 23-26 inches, wingspan of 43-47 inches, and weight 26-36 ounces.
Black-crowned Night-Herons often spend their days perched on tree limbs or concealed among foliage and branches. They forage in the evening and at night, in water, on mudflats, and on land. In flight they fold their head back against their shoulders, almost making the neck disappear. These are social birds that tend to roost and nest in groups, although they typically forage on their own. Look for them in most wetland habitats across North America, including estuaries, marshes, streams, lakes, and reservoirs.
Black-crowned Night-Herons are opportunists feeders that eat many kinds of terrestrial, freshwater, and marine animals. Their diet includes leeches, earthworms, insects, crayfish, clams, mussels, fish, amphibians, lizards, snakes, turtles, rodents, birds, and eggs. They also eat carrion, plant materials, and garbage from landfills. Rather than stabbing their prey, they grasp it in their bills. Black-crowned Night-Herons normally feed between evening and early morning, avoiding competition with other heron species that use the same habitat during the day. They may feed during the day in the breeding season, when they need extra energy for nesting.
Black-crowned Night-Herons nest colonially and behave socially all year long. Both males and females vigorously defend feeding and nesting territories, sometimes striking with their bills and grabbing each other’s bills or wings. Night-herons are probably monogamous. The male advertises for a mate with displays that involve bowing and raising the long plume on his head.
The male chooses a nest site in a tree or in cattails—usually in a habitat safe from predators such as on an island, in a swamp, or over water. The male starts building the nest, a platform of sticks, twigs, and other woody vegetation which he collects from the ground (or breaks right off of the trees). Once he has found a mate, the male continues collecting material but passes it to the female, who works it into the nest. Some nests are sturdy, while others are flimsy. They measure 12–18 inches across and 8–12 inches high.
Black-crowned Night-Herons nest colonially, often with a dozen nests in a single tree. Colonies sometimes last for 50 years or more. The eggs are laid at 2 day intervals, beginning 4-5 days after pair formation. Incubation, which lasts 24-26 days, is carried out by both adults. The clutch size is 3-5 eggs. The eggs are greenest on the first day and fade to pale blue or green after that. After 2 weeks, the young leave the nest, although they don’t go far. By 3 weeks, they can be found clustered at the tops of trees if they are disturbed. By Week 6-7 they fly well and depart for the feeding grounds.
Black-crowned Night-Herons are fairly common and North American Breeding Bird Survey data suggest their populations are stable—although their clumped distributions make it hard to estimate trends precisely. The Waterbird Conservation Plan indicates this species is of moderate concern, with more than 50,000 individuals in North America but a possibly declining population trend. Threats include draining and development of their wetland habitat, and reduced water quality due to contaminated runoff. They are susceptible to accumulating pollutants such as persistent organochlorine pesticides, PCBs, and heavy metals. Colonies of Black-crowned Night-Herons can provide good indications of overall environmental quality, because night-herons forage at the top of food chain, nest in colonies (where they are fairly easy to study), and have a wide distribution. They tolerate disturbances such as traffic, so they are especially useful in revealing environmental deterioration in urban environments.
References: Wikipedia, Cornell Lab of Ornithology, Animal Diversity Web