Great Blue Heron (ardea herodias)
Life History
Great blue herons breed across the United States
and southern Canada, and more than half of the Atlantic coast's breeding
population nest in Chesapeake Bay—predominantly in wetlands. Although
many herons migrate through the Bay region, some great blues remain in
the Bay area year-round. These waterbirds use a variety of different feeding
methods to procure their primary diet of small fish, which they swallow
head first. They also eat frogs, salamanders, lizards, snakes, crawfish,
small birds, rodents and insects. Great blue herons are one of the top
predators of the Bay food chain.
Nesting
Herons form pair-bonds, usually in March and April, after
a series of courtship rituals performed by both sexes. The occipital crests
are raised in display during courtship and both sexes change body coloration,
although the male becomes more brightly adorned.
Although great blue herons occasionally nest singly, most
breed in localized colonies of up to hundreds of nesting pairs. Heron
colonies are often termed rookeries after a European member of the crow
family, the colonial rook, that also nests in colonies. In Chesapeake
Bay, nesting sites are primarily living, but occasionally dead, trees
and bushes. The location of the colony depends upon an available food
supply for raising the young in close proximity to nesting trees; however,
the birds show a preference for stands of loblolly pine, beach, oak and
large, old sycamore trees. Initially, the inner section of a stand of
trees is utilized, but continual colony use may eventually kill that area.
When the center of the tree stand dies, the colony moves circularly outward
in succeeding years. This strategy creates a "donut" or "bullseye" effect
when the tree stand is aerially viewed. Nests are constructed of sticks
and, if not collapsed by winter weather, may be repaired and used year
after year. The nest is lined with reeds, mosses and grasses to help cushion
three to seven eggs that are laid during March and April. Eggs hatch after
about 28 days, and both parents care for the chicks. The young are initially
fed a diet of regurgitated food, but eventually eat whole fish dropped
into the nest. Juveniles leave the nest after about 60 days and, if they
survive their first winter, may live for another fifteen years.
Abundance
Recently, the Maryland Department of Natural Resources
and the Virginia Commission of Game and Fisheries have sponsored surveys
to monitor populations and annual nesting success of great blue herons.
They also monitor colonies of other species of herons and egrets. In early
spring before the trees have leaves, aerial surveys are conducted to locate
colony sites and count nests. At larger colonies, ground counts are made
of active nests.
Chesapeake Bay, the nation's largest estuary, and surrounding
areas provide both the ideal food and habitat necessary for great blue
heron survival. In 1990, surveys found 4600 great blue heron pairs in
more than 38 colonies in Maryland. The Nanjemoy Creek colony in Maryland
may be the largest along the entire Atlantic coast, with 1100-1200 nests
counted in recent surveys. Fortunately, that land area is protected by
the Nature Conservancy.
In the past, herons and egrets were shot for their feathers,
which were used as cooking utensils and to adorn hats and garments, and
they also provided large, accessible targets. The slaughter of these birds
went relatively unchecked until 1900 when the federal government passed
the Lacey Act, which prohibits the foreign and interstate commercial trade
of feathers. Greater protection was afforded in 1918 with the Migratory
Bird Treaty Act, which empowered the federal government to set seasons
and bag limits on the hunting of waterfowl and waterbirds. With this protection,
herons and other birds have made dramatic comebacks.
In recent years, great blue herons have had to face new
challenges. Loss of nesting sites, and deterioration of water quality
and wetland habitat are issues of concern for heron survival. Natural
generation of new nesting islands, created when old islands and headlands
erode, has decreased due to artificial hardening of shorelines with bulkheads.
Poor water quality reduces the amount of large fish and invertebrate species
available in wetland areas. If suitable feeding and nesting areas are
not maintained, populations of great blue herons will eventually decline.
Toxic chemicals that enter the Bay from runoff and industrial discharges
pose yet another threat. Although great blue herons currently appear to
tolerate low levels of pollutants, these chemicals can move through the
food chain, accumulate in the tissues of prey and may eventually cause
reproductive failure in the herons.
Care must be taken to preserve nesting sites, as well
as feeding areas. Erosion of island nesting areas due to artificial structural
development, as well as sea level rise, needs to be carefully monitored.
Nesting sites should be observed from a distance of at least 200 meters
to minimize disruption of the colony. If herons are disturbed frequently,
they may abandon their nests or neglect their young. Deterioration of
submerged aquatic vegetation limits foraging area
potential. Wetland foraging sites within 15-20 kilometers of heron colonies
need special protection to ensure prey availability.
Currently, Chesapeake Bay is home to relatively stable,
substantial populations of great blue herons, but we cannot become complacent
in our efforts to preserve them. Preservation of shallow water habitat,
feeding areas and rookeries must remain a priority if we want to continue
to enjoy this majestic symbol of Chesapeake Bay.
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