Species-Specific Management (SSM)

Black-Crowned Night Heron

Habitat

The majority of breeding and non-breeding black-crowned night herons (Nycticorax nycticorax) can be found in wet, marshy areas. They can be found all year round breeding and feeding in the coastal marshes of Virginia. Birds found further inland will migrate south to similar aquatic environments. This kind of habitat is critical in order to maintain a viable population of black-crowned night heron colonies. Herons roost high in the tops of tall trees and they must have access to trees that will provide a sufficient platform for their nests, and protect them from predators. These nests are usually of poor construction and can be destroyed by various factors such as large storms. Since nesting success is necessary for population growth, a habitat with tall trees is invaluable for their continued survival. For heronries with few trees, vegetative cover and tree species can be introduced to increase protection from avian predation. All flora species considered should be highly tolerant to guano deposition. Perhaps the most serious problem of black-crowned night herons is their limited and decreasing habitat due to urbanization in their coastal area habitat.

Social Grouping

Black-crowned night herons nest in single-species colonies or in heronries. They can be found in North America throughout the United States. In heronries, they can be associated with great blue herons (Ardea herodias), snowy egrets (Egretta thula), little blue herons (Florida caerulea), cattle egrets (Bulbulcus ibis), herring gulls (Larus argentatus) and other colonial water species. When it is time to breed (early spring to mid summer), the herons form breeding colonies.

Nesting Success

The nesting success is affected by the types of habitat present. Tree top nesting likely evolved in response to mortality resulting from predation and flooding.

There must also be an abundance of food and water for the newly fledged young. The right interspersion of these necessities is important to successful nesting. Also, materials for constructing nests must be available in the habitat. Nests are generally constructed of roots, twigs, or grasses. These can easily be found in their typical wet, marshy habitats.

Food

Food availability is not thought to be a limiting factor in black-crowned night heron habitat. Some experts believe that the access to food is most important. Some prey is easier to subdue than others. For instance, the diet of a colony in South Carolina consisted of 83.3% salt water fish, 14.3% salt water crustaceans, and 2.7% terrestrial vertebrates. Black-crowned night herons are primarily night feeders. Frogs, snakes, lizards, crustaceans, mollusks, insects, spiders, small mammals, birds, and some vegetable matter also form the basis of their diets. Herons have also been observed raiding the nests of other birds. Abundance of these foods is necessary for maintaining the birds' energy requirements.

Predation and Urbanization

As black-crowned night herons become increasingly exposed to humans, they also become more exposed to predators. Those who have studied the heron have noted that when humans visit the colonies, predators tend to follow them. This predator behavior has been learned and developed over time by watching the actions of humans. Birds unaccustomed to human visitors will, as a result of predator avoidance, abandon their nests when humans are present. This allows predators such as the ring-billed gull (Larus delawarensis), mink (Mustela vison), and opossum (Didelphis virginiana) to raid the unprotected nests. As urbanization increases, so does the attraction of other animals associated with urban-type environments. House cats (Felis cattus), and raccoons (Procyon lotor)are likely to be found in areas of urbanization. These animals will prey upon black-crowned night heron eggs and on the birds themselves.

Management

There are some basic guidelines which can be followed when managing the black-crowned night heron.

1. Keep human disturbance to an absolute minimum. When necessary, colonies should only be observed through a blind, late in the brooding season. This will limit possible nest abandonment and predation while monitoring the colony.
2. Encourage management which will maintain healthy populations of salt-water fish and other preferred food items. Reducing pollution is a basic strategy.
3. Manage for critical vegetation. Tall trees enable the herons to protect their nest from predators. In addition, cover that is resistant to guano deposition provides protection from predators.

A contribution by Chris Anderson and Terri Surrells (1992), Department of Fisheries and Wildlife Sciences, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061-0321

This Web site is maintained by R. H. Giles, Jr.
Last revision January 17, 2000.