by , Daniel Fulford a student of Biogeography in the Fall of 1999
San Francisco State University
Geography 316: Biogeography
The Biogeography of The
Great Blue Heron (Ardea herodias)
by , Daniel Fulford a student of Biogeography in the Fall of 1999
Kingdom:Animalia
Phylum:Chordate
Class:Aves
Order:Ciconiiformes
Family:Ardeide
Genus:Ardea
Species: (Ardea herodias)
Few other animals better symbolize a vision of conservation for ecosystems than the Great Blue Heron. It lives year-round and migratorily on seashores, wades on its beaches and in its streams, rivers, and marshes, hunts in grasslands and from kelp forests, nests in old-growth forests and penetrates the urban landscape. As sentinels, the heron’s eggs provide a means to monitor contaminants in the rivers, estuaries and oceans, and its reproductive success might just provide clues to the overall health of water ecosystems. Conserving the Great Blues and their environment would go a long way toward ensuring the conservation of much of the quality of riparian and marine ecosystems throughout the America’s. Robert Bateman
THE GREAT BLUE HERON
Description of Species
The Great Blue Heron is
a large, graceful, dark gray bird having a white crown, cheeks and throat. It has a length
between 97-137cm and a wingspan that can span 5 feet in length. They have a beautiful
black stripe on the side of their crown that merges into a long occipital crest. The neck
is gray with a violaceous tinge in the back and sides, and is striped black and white
underneath. The back is blue-gray, the sides blackish, and the belly gray and white
striped. The thigh feathers are often described as a distinctive chestnut. The irises are
yellow, the lories dull green, and the legs greenish brown. Juveniles are somewhat darker
than the adults and have an entirely dark crown, no crest, and more ventral striping. The
bill is slaty with a yellowish lower mandible. The iris and lores are yellow as well. They
have adapted a fascinating feature in that they have feathers that crumble and make a
powder used to clear off slime from fish. By rubbing it’s head and neck feathers
through the powder making feathers, the slime clumps can be extracted with one swipe of
their claws. This most likely became an adaptation due to combating disease and infection
form slime and other side products in the estuaries.
Behavior
Flying: When taking off and flying short distances, the
Great Blue Heron often keeps its neck extended. Its wingbeats are slow, 2.3-3.2
beats per second, but extremely powerful. (McAllister & Maxwell 1971).
Feeding: This particular heron feeds by day in most situations, but nocturnal
foraging is very common, especially in tidal habitats. It typically feeds Standing
and by Walking Slowly, usually in water. These behaviors account for over 90% of its
foraging time. Its eyes can shift focus to prey in front or below without moving its
head, its long neck can be unleashed to strike forward and its bill works like finely
tuned tweezers to snatch wriggling fish from the water. Its long legs allow access
to deep water but it often uses such behaviors as Hovering, Plunging, Jumping, and
Swimming Feeding to forage in even deeper water than its leg length would allow. On
about two-thirds of all attempts, it captures fish. Prey items include a wide range
of insects, fishes, amphibians, reptiles, and mammals. The Great Blue Heron being a
large bird can catch and use both small and large prey (Horn 1983): remarkably large play,
such as stilts Himantopus, have been eaten, and even larger prey have been attacked before
proving to big to swallow (Bayer 1979). Western herons of this species are often
feed on dry land, and on mammals. Individuals, especially on islands such as Florida
Keys and the Galapagos, frequent human habitations for scraps of food provided for
them. This can in some cases increase nesting success.
Breeding
The displaying male usually occupies an old nest
sites. Mock (1976) described the displays used. An Advertising call is seldom
given, the bird instead advertising with a highly stereotyped Stretch display, which
includes a lateral swaying in the head descent. The Snap display on the other hand
is highly variable but includes an audible mandible ‘clack’. Other
displays used include Wing Preen, Circle Flight, Twig Shake and Fluffed Neck. Crest
Raising is performed by both sexes throughout the breeding season. Aggressive
displays include the pairing, Contact and Non-contact Bill Clappering are common. A
gathering-ground dance has been described, but Bayer (in prep.) has shown that these are
aggressive displays away from the colony site. Great Blues nest either solitarily or
in colonies at locations characterized by the presence of suitable trees, isolation from
disturbance and available food supplies. The number of individuals in a colony can
vary from Year to year.
Nest,eggs and young
The nest is a large platform
of sticks over one meter across. Sites are variable, often tall trees in the north
of the range and mangrove bushes in the south, and if necessary artificial structures
(Henny & Kurtz 1978). Nest construction is quite fascinating in that the male
will retrieve the sticks and present them to the female who is constructing the
nest. Often times it is just additions to existing nests from the previous
year. The eggs are pale blue, having a size range of 61.3-65.6 by 41.9-46.5mm (
Schonwetter 1976). The clutch size varies from 3 to 7 eggs increasing form south to
north. The incubation period is about twenty-eight days. The chicks fledge at
about two months. Nesting success depends on food supplies (Powell 1983) and can be
two to three young per successful nest (Kelsall & Simpson 1979). Most nesting
loss is due to starvation due to competition within the nest, although predators such as
eagles, raccoons, and bears attack nestlings. Mortality rates are 69% in the first
year, decreasing thereafter and with regional differences.
Habitat
The Great Blue Heron is a bird of river and lake edges, as
well as marshes and swamps. It can feed on dry land but nests in trees, usually
surrounded by water. It has been recorded up to 1500m (4900ft) in Panama. It
also occurs in salt water, feeding along shores, in mangroves and on tidal mudflats.
The white form is found almost entirely I marine habitats, particularly tidal grass
flats. Great Blue’s not infrequently feed in relatively deep water, in the
surf, or along blong neck and keen eyesight.eaches. The bird has evolved specific
features that enable it to be an efficient hunter in the water.
Natural History
There are 60 species of herons recognized. Most have been
distinguished for many decades, but species limits of some herons are still being
defined(Hancock'84). The heron family, the Ardeide, is one of six families
generally included in the next higher taxonomic category, the large-wading-bird order
Ciconiiformes . The families in relation include; the stork family- Ciciniidae. The
ibis family- Threskiornithidae; the two aberrant stork-like species- the Shoebill
placed in the Balaenicipitidae and the Hammerkop placed in the Scopidae. These all
differ in many ways from Herons.
It is by no means certain that the order Ciconiiformes is a natural, monophyletic
grouping of birds (Sibley & Ahlquest1972). Some of these diverse families may
actually be more closely related to the waterfowl order, to the shorebird order, or to the
pelican order (JH, JK 84). Alternatively, it is possible that what is presently
recognized as the Ciconiiformes may be part of a larger group that includes
representatives of currently identified Ciconiiformes. The herons appear to be the most
morphologically distinctive group, an on that, it has the greatest likelihood of
eventually being found to be unrelated to the rest.
There are four distinctive groups of herons: Tiger Herons, the
Night Herons, the Bitterns, the Day Herons. Classifying these groups has been a
different story from each reference book I’have looked look at. According to
“The Herons Handbook” (Hancock '84), the bitterns are separated into one
subfamily and all other herons relegated to another, reflecting the formers divergence
from the rest. Payne & Risly (1976) showed that there are considerable
differences of opinion existing as to the appropriate sequence of listing.
Ideally, groups are sorted according to their evolutionary relationships i.e. the
Morphological species concept. Payne & Risly (1976) showed that the bitterns
were highly derived herons, very different from from their presumed ancestral
herons. They are probably the extant group that split earliest from the other
groups. The relict distributions of the large bitterns suggest that they are a
relatively old group of species (Hancock '84). Early authorities placed herons in
numerous single -species genera. This was done on the basis that herons, being of
very ancient origin, have evolved to a point where differences between the species are
substantial. Within genera some species are considered to be more closely related,
and therefore descendents from a common ancestor.
The heron family, the Ardeide, is one of
six families generally included in the next higher taxonomic category, the
large-wading-bird order Ciconiiformes . The families in relation include; the stork
family- Ciciniidae. The ibis family- Threskiornithidae; the two aberrant stork-like
species- the Shoebill placed in the Balaenicipitidae and the Hammerkop placed in the
Scopidae. These all differ in many ways from Herons.
Evolution
Dromaeosaurs were
thought to be the closest relatives of birds and share the most recent common
ancestry. The early fossil record of dremaeosaurs was obscure thus far, known
dromaiosaurs appeared fairly later during the Cretaceous, when birds were already well
established. Dromaeosaurs did not continue to become more birdlike. Instead,
in their evolutionary course they specialized in killing mechanisms and became
considerably larger than birds. The common ancestor of birds and dromaeosaurs has
yet to be found until recently. According to the latest National Geographic magazine
the missing link has been discovered. Three of the fossils were found recently in
Liaoning Province, China, the area that produced fossils of flightless feathered
dinosaurs, 1996-1997. All four a therapods (meat-eaters), a dromaesaur, one of
family of small- to medium sized predators that includes the raptors displayed in
"Jurassic Park"; an oviraptorasaur from Mongolia with a bird-like tail; a seven
foot long therizinosaur; and a creature that has the arms of a bird and a tail of a
dinosaur. The animal is named Archaeoraptor liaoningensis. It has a long stiff
tail which under magnification represents dinosaur and long arms and small body which
screams bird. The animal discovered is a animal discovered is a dromaesaur-like
primitive bird, but what fascinated scientists were the remains of feathers surrounding
the bones. Some are similar to the hairkike protofeathers of the flightless
Sinasauroptryx found in 1996. But others liik ling and broad, seductively suggesting
flight feathers. According to Stephen Czerkas "It's a missing
link!". They can't prove that it flew but aside from its feathers, anatomy- arms,
birdlike shoulders, hands and wrists doesn’t make sense unless it
did. For years, Archaeopteryx was considered to be the oldest bird
known, but its position was usurped by Protoavis texensis from the Late Triassic Dockum
Group of Texas, predating Archaeopteryx by 75 million years (Chatterjee 1987a, 1991, 1994,
1995, in press; Kurochkin 1995; Peters 1994). Resembling a small
monavian theropod in the rear, Protoavis reveals its avian identity in the front portions
of the skeleton. It is an excellent example of mosaic evolution, in which some
conservative ancestral characters of contemporary nonavian theropods occur with the
advanced characters typical of later birds. This mingling of primitive and advanced
characteristics seems to have been a common evolutionary pattern in the origination of
higher groups of vertebrates.
Evolutionary history within the herons has been interpreted
differently by various systematics. The "day herons" (Ardia) were
considered the most primitive by Peters (1931) in his linear sequence of taxa. But
in later lists of heron classification many list the bitterns first, implying
primitiveness. Herons have been revised and relisted several times but usually
without discussion of the reasoning used in the decisions. In the only published
rationale (Brock, 1956) of the classification of herons, the characters used to
distinguish the major groups, genera, and super species were derived mainly from those of
doubtful phylogenetic significance and of probable multiple evolutionary history among
different specie. The characters included the dispersion of nesting pairs and the
structure of feathers used in courtship display. These very likely change rapidly
due to selective pressures of local deeding ecology and species recognition, and so they
carry little information about the differentiation of genera and subfamilies. In
contrast, the skeletons of the herons vary in such a manner that many corresponding
character stateskeletons of the herons vary in such a manner that many corresponding
character states can be found among the most closely related groups outside the herons.
The other ciconiiform birds, are usually more similar among closely related species than
are the plumes of the species, indicating that differences reflect phylogenetic
relationships. Referring to figure 12.8 the common ancestor was the
Neocathertes. These where long legged, big winged, soaring scavengers. Their
explosive evolution occurred after the KT extinction at the beginning of the Paleocene
Era.
Distribution
Subspecies of Great Blue Heron: Ardea herodias
occidentalis: Keys near Key West Florida. Ardea herodias wardi: Estero Bay, Florida. Ardea
herodias fannini: Skidegate, Grahm Island, in Queen Charlotte Islands. Ardea herodias
cognata: Indefatigable Island, Galapagos.
The Great Blue’s Breed throughout North
and Central America, the Caribbean and Greater Antilles, and in the Galapagos. Its
distribution is a representation of the range of coastal habitat where they migrate and
breed. Food sources are much higher in estuaries where oceanic fish have massive numbers
and density that breed, spawn and feed in these intertidal zones. Great Blues do not like
to live and have a hard time surviving freezing temperatures, which is why they have a
migratory. Many of the most abundant food sources are in the north where it freezes every
year. In Puget Sound on the coast of Canada there are populations that are sedentary. This
may be due The race wardi occurs from
Kansas and Oklahoma across the Mississippi river basin to Florida, while herodias occurs
over most of the remainder of the North and Central American range. The race fannini is
found along the Pacific coast of North America from southeastern Alaska to coastal
Washington. The race occidentalis occurs in extreme south Florida, ranging north to Tampa
Bay(Bancroft 1969). Its range limits south of Florida are uncertain but many believe that
they inhabit many Caribbean islands as well. Breeding has been reported in Cuba, St
Thomas, and on islands off Venezuela (de Schauensee & Phelps 1978). It has occurred in
Yucatan, Mexico, but has not been confirmed to be breeding there.
Population sizes have been difficult to
determine, because nesting sites are often highly dispersed. Over 10,000 breeding adults,
however, have been counted along the upper Gulf of Mexico coast (Portney 1981). The
Florida Bay population of occidentalis has been estimated to include 2,500 adults.
(Robertson & Kushlan 1974). The sized of northern populations may depend on the
severity of the winter (Blus & Henry 1981).
MIGRATION
Great Blue Herons form northern North America are highly
migratory, although some remain sedentary in the rather far north. Along the Hudson
Bay there are many non- migratory populations (Butler 93). Southern populations
appear to be more sedentary. The winter range is in Florida, Cuba, Jamaica, the
Antilles and South America, whereas mid western individuals follow the Mississippi River
drainage to winter in Cuba and Central America. Great Blue Herons disperse widely
after nesting. Northern individuals have wandered to Newfoundland, Greenland, and
near the Azores (King & Curber 1972).
Map of Distribution
Other interesting issues
Herons have no natural predators but great damage is done however
to their eggs and nestlings by the crow, ravens, bald eagles, gulls, and raccoons.
Bald Eagles are especially a problem given the desire to return their population
status. Great Blue Herons have been known to simply leave the nest and young if bald
Eagles are roaming the territory. The eagles will attack and kill juveniles who are
naive to their presence, whereas adults recognize the threat of an approaching eagle and
begin crooking loudly. No heron species in North America is seriously
threatened with extinction. However in 1978 there was a major decrease of about 71%
of the Great Blue’s population that was caused by the shootings of adults and young
along with the destruction of their habitats. Windstorms and cold weather are also
major factors in this. Although the drop in population, the number of Great Blue
Herons is stable and thriving due to their versatility. But habitat loss and
pollution will be the most vital and disturbing problems in the future. In some
areas of the world, the Great Blue Heron is considered rare or extinct, as in
Jamaica. PCB’s, mercury and selenium in heron eggs have been at levels that
impair hatchling viability. Because they are at the top of the food chain the
concentration of toxins increase by each tropic level to the point where the
concentrations are harmful or deadly. A problem is that eggshells become too fragile
to survive the incubation period. These contaminants have declined in most parts of
America since the eighties, which makes conservation attempts a more successful
science. In the Upper Mississippi River Herons exhibited adverse affects that were
correlated with pollution levels. A positive correlation was found between several
measures of oxidative stress and the levels of polychlorinated
dibenzodioxins(PCDD's). These are a common-byproduct of Kraft pulp produced by
various mills along the Mississippi river.
Bibliography
Amadon, Dean, 1966. Birds Around the
World
Butler, Robert, 1997.
The Great Blue Heron "A Natural History and Ecology of a Seashore
Sentinal"
Hancock, James, 1984. The
Herons Handbook
Horton, Tom, April
1999, "Great Blues are Going Great Guns".
Smithsonian
p131-137
Mock, Douglas, 1991. Behavior
& Evolution of Birds
Sloan, Christopher, Nov.
1999, "Feathers for T-Rex?". National
Geographic.
p99-105
www.inhs.uiuc.edu/chf/pub/ifwis/birds/great-blue-heron.html
www.denison.edu/~shriner/bio355/heron.html
send comments to bholzman@sfsu.edu
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