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Black-crowned Night-Heron

Order: Ciconiiformes
Family: Ardeidae
Species: Nycticorax nycticorax

Description

Black-crowned night-herons do not fit the typical body form of the heron family. They are relatively stocky and about 25 inches tall (63 cm) with shorter bills, legs, and necks than their more familiar cousins the egrets and "day" herons. Their resting posture is normally somewhat hunched but when hunting they extend their necks and look more like other wading birds.

Adult black-crowned night-herons have black caps and backs, pale gray wings, white underparts, red eyes, and yellow legs. Two or three long white plumes, erected in greeting and courtship displays, extend from the back of the head. The sexes are similar in appearance although the males are slightly larger. Immature birds have dull gray-brown plumage on their heads, wings, and backs, with numerous pale spots. Their underparts are paler and streaked with brown. The young birds have orange eyes and duller yellowish-green legs. They are very noisy birds in their nesting colonies, with calls that are commonly transcribed as quok or woc, woc.

Distribution

The black-crowned night-heron breeds on every continent except Australia and Antarctica. They are one of the most common herons in the northern United States, breeding everywhere except the southwest and Rocky Mountain states. They are found in many types of wetland habitats including riparian forests, wooded swamps, marshes, mangroves, and even occasionally grasslands or rice fields. Although the National Zoo does not include any black-crowned night-herons in its formal collection, every spring and summer hundreds of birds make their homes in the large trees surrounding the Bird House. Once the young birds have fledged they are a common sight throughout the Zoo.

Diet

Herons feed primarily on fish, but also take eggs and young of other birds, amphibians, small mammals, crabs, mollusks, and other invertebrates. They forage primarily at night or in the early morning by standing or wading slowly through shallow water. They spear their prey with a forceful thrust of the neck, grasp it in a serrated beak, flip the prey to orient it properly, then swallow it headfirst and whole. Occasionally, particularly large prey such as bullfrogs will choke an overly ambitious bird.

Reproduction

Black-crowned night-herons usually nest in colonies. The breeding colony of wild night herons at the National Zoo contains more than 200 pairs, with most of the birds arriving from their wintering grounds en masse around March 1. Courtship begins with the male birds bowing, stretching, rocking from foot to foot, hissing, and bill clapping as they display for the females.

Following the formation of the pair bond, the birds spend a good deal of time preening their mates, rubbing their bills over their heads, necks, and backs. They build their relatively flimsy nests of sticks, twigs, and reeds in shrubs or trees usually between five and ten meters (16 to 33 feet) off the ground. Nests tend to be below the tree canopy to provide some protection from the eyes of nest predators such as crows.

The female lays three to five pale blue eggs (53 x 37mm; 2 x 1.5 inches) and both parents incubate for 24 to 26 days. The nearly-naked young are fed by regurgitation as the parents return to the nest after foraging. As they grow they will begin to receive whole prey from the parents. The noisy begging of the nestlings can create quite a din in some of the larger breeding colonies. At three weeks, they begin to climb about and around the nest. They fledge at six to seven weeks of age. They will not acquire full adult plumage until they are a year old and may not breed until their second or third year.

Conservation

Most black-crowned night-heron populations are stable or increasing. They are on the National Audubon Society's Blue List as a species of Local Concern because of declining populations in the Hudson-Delaware, Florida, Ontario, Mountain West, Middle Pacific Coast, Middlewestern Prairie, and Central Southern breeding regions. Like other wetlands species they will always be sensitive to water-quality changes and wetlands destruction.


References

Ehrlich, P.R., Dobkin, D.S. and D. Wheye. 1988. The Birder's Handbook. Simon and Schuster, New York.

Ehrlich, P.R., Dobkin, D.S. and D. Wheye. 1992. Birds in Jeopardy. Stanford University Press, Stanford.

Farrand, J. Jr. ed. 1983. The Audubon Society Master Guide to Birding Vol. 1. Alfred A. Knopf, Inc. New York.

Netherton, J. 1994. At the Water's Edge: Wading Birds of North America. Voyageur Press, Stillwater, Minn.