The night heron has a stocky body, with a comparatively short neck and legs. The length averages 58-72 cm, with the females averaging slightly smaller than the males. The adult has distinctive coloring, with black cap, upper back and scapulars; gray wings, rump and tail; and white to pale gray underparts. The bill is stout and black, and the eyes are red. For most of the year, the legs of the adult are yellow-green, but by the height of the breeding season, they have turned pink. The eyes of the juvenile black-crowned night heron are yellowish or amber, and the dull gray legs lack the colorful pigmentation of those of the adult. The juvenile has a brown head, neck, chest and belly streaked with buff and white. The wings and back are darker brown, though the tips of the feathers have large white spots. These spots are particularly large on the greater secondary coverts. The young do not acquire full adult plumage until the third year. (Davis, 1993; http://www.mbr.nbs.gov/id/htmid/h2020id.html)
The black-crowned night heron is found across North America from Washington through Quebec, south through coastal Mexico, as well as locally in Central America and the Caribbean. Some winter as far north as Oregon and the New England states. The Old World race 'nycticorax' occurs from Europe to Japan, Africa and India. (Davis, 1993)
Most colonies of black-crowned night herons are associated with large wetlands. They inhabit a variety of wetland habitats such as swamps, streams, rivers, marshes, mud flats and the edges of lakes that have become overgrown with rushes and cattails. (Davis, 1993; http://www.biology.ualberta.ca/uamz.hp/heron.html)
Black-crowned night herons are presumed to be monogamous. Pair formations are signaled by males becoming aggressive and performing Snap Displays, in which they walk around in a crouched position, head lowered, snapping their mandibles together or grasping a twig. The Snap Display is followed by the Advertisement Display--sometimes called the Stretch, Snap-hiss, or Song and Dance display--to attract females. In this display a male stretches his neck out and bobs his head, and when his head is level with his feet, he gives a snap-hiss vocalization. Twig-shaking and preening may be occur between songs. It has been suggested that these displays provide social stimulus to other birds, prompting them to display. This stimultion in colonial species may be crucial for successful reproduction. Females that come near the displaying male are rejected at first, but eventually a female is allowed to enter his territory. The newly-formed pair then allopreens and engages in mutual billing. At the time of pair formation, the legs of both sexes turn pink. Copulation usually takes place on or near the nest, and begins the first or second day after the pair is formed.
There is one brood per season. Black-crowned night herons nest colonially, and often there can be more than a dozen nests in one tree. The nest is built near the trunk of a tree or in the fork of branches, either in the open or deep in foliage. The male initiates nest building by beginning to build a new nest or refurbishing an old one. The nest is usually a platform lined with roots and grass. During and after pair formation, the male collects sticks and presents them to the female, who works them into the nest. The male's twig ceremony gradually changes to nest building.
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. On hot days, the parents wet their feathers, perhaps to keep the eggs cool. Both parents brood the young. 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. Adult black-crowned night herons do not recognize their own young and will accept and brood young from other nests. The young have a tendancy to regurgitate their food onto intruders when disturbed. (Davis, 1993)
Black-crowned night herons are social at all times of the year, associating with other species of herons frequently. In the winter, it roosts communally. It is a migrating species. The normal call is a 'Qua,' 'Quak,' or 'Quark.' These calls are most often given in flight or from a perch. The fact that this night heron feeds throughout the night means that it avoids competition with day herons which use the same habitat. Feeding sites are used repeatedly. When feeding, it alights on water feet first, or plunges from the air. When walking, it usually keeps its head and neck lowered, and it rarely runs. It may retract its feet when flying on cold days in order to conserve heat. The young leave their perches and huddle in the nest when cold. Black-crowned night herons defend both feeding and nesting territory. The young can be aggressive, regurgitating or defecating on human intruders. (Davis, 1993)
The black-crowned night heron is an opportunistic feeder. Its diet consists mainly of fish, though it is frequently rounded out by other items such as leeches, earthworms, aquatic and terrestrial insects. It also eats crayfish, mussels, squid, amphibians, lizards, snakes, rodents, birds, eggs, carrion, plant materials, and garbage and refuse at landfills. It is usually a solitary forager, and it strongly defends its feeding territory. The night heron prefers to feed in shallow waters, where it grasps its prey with its bill instead of stabbing it. A technique called 'bill vibrating'--which is opening and closing the bill rapidly in water--creates a disturbance which may lure prey. Evening to early morning are the usual times it feeds, but when food is in high demand, such as during the breeding season, it will feed at any time of the day. (Davis, 1993)
At fish hatcheries, humans claimed that the amount of fish the birds were consuming were becoming destructive. If they nest near human settlements, they are considered pests. (Davis, 1993)
Black-crowned night herons have been hunted for food, though they are hunted for this purpose much less frequently now.
In the late 1960s, declines in many black-crowned night heron populations were noted, and were attributed to the use of DDT. The status of these populations is indicative of environmental quality due to the high rank of these birds in the food chain, and their wide geographic distribution.
Adults were often killed or trapped near fish-culture establishments, due to their fishy diet, but other methods of discouraging them from eating the fish are now available. It is estimated that 1,300 birds were killed per year at fish hatcheries in nine states. Herons that nested too close to human settlements were considered pests and were also often killed, but other methods that do not involve killing have been developed. Since most populations are stablized or increasing, management has not been a major focus, though habitat destruction is an important factor in the conservation of this species. (Davis, 1993) This is a species of special concern in Michigan.
Alicia Ivory (author), University of Michigan-Ann Arbor.
Davis, W.E., Jr. 1993. Black-crowned Night-Heron (Nycticorax nycticorax). In The Birds of North America, No. 74 (A. Poole and F. Gill, Eds.). Philadelphia: The Academy of Natural Sciences; Washington, D.C.: The American Ornithologists' Union.