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River-Lab 5 Guide Manual
BLACK-CROWNED NIGHT HERON (Nycticorax nycticorax)
This bird’s appetite is an
important contribution that helps keep a
basin system healthy. Night heron
predation on medium to large fish,
which make up fifty to eighty percent of
its diet, has valuable toning effects for
those fish populations. Night herons
help tone crustacean populations in
both fresh and salt water. They also
help control some fish, insect, and
rodent populations. Adult night herons
are not known to have any predators, but
their young are eaten by a few animals,
including some people.
The night heron helps keep at least one potentially over-populating fish species in
balance with the rest of the life in a basin system. They regularly follow the spring runs
of large coastal fish called alewives as these fish move up into rivers to spawn. Because
alewives tend to be overly-reproductive, predation is needed to keep their numbers in
balance with other species of fish. A night heron is ideally suited to eat alewives. Adult
alewives are a very bony meal. Strong chemicals in the heron’s digestive system can
dissolve both the scales and numerous bones of this fish. Because of their boniness,
alewives are not a preferred meal for other predators, including people.
Night herons also prey on and tone the populations of freshwater crayfish, and
saltwater crabs and shrimps. Night heron consumption of aquatic insects and, to a lesser
extent, of mice and rats, is a valuable contribution to the control of these organisms
whose high rates of reproduction can cause imbalance and upset the basin system of life.
Night herons fish by making a lunging dive from a standing position on a bank or
muddy edge into shallow water. Their awkward-looking grab almost always results in a
catch. The night heron will then toss its prize into the air, so that when recaught in the
heron’s bill, it will slide head-first down through the heron’s open throat.
Night herons are not usually in direct competition with kingfishers, since herons
eat larger fish (up to 14 inches). Nor do night herons seem to mind sharing fishing areas
with gulls, which catch female alewives and rip open their bellies to eat the roe (eggs).
Night herons will, however, grab fish out of another heron’s mouth. This often occurs
when they are following the first runs of alewives upstream in April.
5GM – 31
© 1999 Mill River Wetland Committee, Inc.
River-Lab 5 Guide Manual
BLACK-CROWNED NIGHT HERON, cont’d.
The black-crowned night heron is a medium-size (23–28 inches tall) chunky bird
with short yellow legs and a thick, black bill. It has a black back and cap that contrasts
with its gray wings (44-inch span) and white or pale gray underparts. Three eight-inch
long white plumes stretch from the back of its cap down its back. During breeding
season its legs turn pinkish.
The night heron’s call, a loud, barking “quok,” is most often heard at dusk when it
begins to fish. Large groups, or colonies, of these birds roost in the tall trees in or near
wetlands, ponds, and rivers. They usually sleep by day and fly up and down inland and
coastal waterways at night, wherever fish and other kinds of food are available.
Night heron nests (sometimes eight in one tree) are platforms of sticks. These are
scantily lined with finer material. The female lays three to six pale dull blue eggs
(1 2/5 x 2 inches), which are incubated by both parents for 24 to 26 days. The young are
practically helpless for the first three weeks. They anxiously squawk, squeal, scream,
grunt, croak, or bark as they wait to be fed. Since these birds live in colonies of twelve to
fifty nesting birds, their combined night noise can be very loud.
Young night herons will be on their own when they are half-grown. At this stage
they are streaked brown and white and have lighter bills. Those that do not become a
meal for a crow, raccoon, or other predator will learn from observing adults in the colony.
They will develop into fully participating members of this somewhat comical but no less
valuable segment of the basin system of life.
© 1999 Mill River Wetland Committee, Inc.
5GM - 32