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The Great Egret and
Habitat Selection
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The role of adaptability to a
species' survival
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This is a pictorial essay on
the role of habitat in the life history of the Great Egret.
The egret is classified as a waterbird but is also found in
grasslands,
wetlands,
agricultural fields, desert,
wooded
swamps, and even the
ocean.
This series of pictures demonstrates the adaptability of the
egret by showing the different types of environment that it
can successfully exist in.
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Perhaps no other taxonomic
group has, and presumably exercises, the potential for
habitat selection that birds do. Birds are extremely mobile
and wide ranging, and of the range of habitats they pass
through or over, only specified ones are used for breeding
or foraging or wintering. The uniqueness of birds with
respect to habitat choice was discussed by Hilden (1965),
who distinguished between and summarized the ultimate and
proximate factors involved in the choice. The evolution of
habitat preferences is determined by, and determines, the
bird's morphological structure and behavioral functions, its
ability to obtain food and shelter successfully in the
habitat. The proximate stimuli for the choice of habitat
might be structural features of the landscape, foraging or
nesting opportunities, or the presence of other species.
Such factors might operate independently, hierarchically as
a system of sequential decisions or overrides, or
synergistically in a complex fashion or
"gestalt".
Habitat Selection in
Birds, Martin L. Cody
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A habitat is used for
breeding, foraging, and wintering. When a species does all
three activities in the same area then it is a permanent
resident. The Great Egret in most areas is not a permanent
resident. It breeds as a colony in suitable trees near water
sources, and then disperses.
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The Great Egret nests with
many other Great Egrets, generally in an evergreen forest
near a convenient source of water. This site is the Audubon
Canyon Ranch in Bolinas California, a sanctuary set aside
for nesting egrets. There are about thirteen birds visible
just left of center in this picture. The lagoon and ocean
can be seen in the background.
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By nesting in the
trees the egrets are protected from many of their enemies
but they are still vulnerable to Gt.
Horned Owls from the
air and racoons coming up the tree to take the eggs from the
nest.
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Here there are at least four
pair of egrets at nest sites and up at the upper right is a
Great Blue Heron that is sitting on its eggs.
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Two birds are mating at
their nest site. The egrets will gather together to breed
and will feed near the colony but after the young have
fledged they will disperse from the breeding area.
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The Great Egret is
classified as a water bird. It has long legs that allow it
to wade in the water to obtain food. But its long legs and
long beak make it possible for this predatory bird to also
feed in a variety of other habitats. Its ability to feed in
different habitats makes it more successful as a species.
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There are many types
of grasslands such as this grassland in Los Banos, in the
Central Valley of California. There is water in the
background but the birds are feeding in the grass.
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In addition to the Great
Egret there are White-faced
Ibis,
Snowy
Egrets and
Cattle
Egrets. There is
very little competition between the four species. The
White-faced Ibis has a slightly decurved
beak which requires that it feed on different prey items
than the Great Egret. The Snowy Egret and Cattle Egret are
smaller than the Great Egret, and faster and feed on
different prey.
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Seeing a Great Egret in the
California desert is a strange site. This bird was
photographed in Salton Sea State Park in California.
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Great Egrets have a wide
range of food that they can eat. It includes all the
vertebrates (fish, amphibian, reptile, bird, and mammals)
and many invertebrates such as insects. In this environment
it is probably looking for insects and other
invertebrates.
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Habitat provides the
opportunity to find appropriate food and the opportunity to
feed in a secure manner. Basically, each species wants to
feed in a manner that allows it to not compete, for a
prolonged period of time, with other species. Habitats can
successfully facilitate species being able to feed in a
non-competitive way.
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Many species are specific to
one or two types of habitat, or even one or two parts of a
single habitat. Other species are very adaptable to a
variety of habitats. The White-tailed
Kite in California
is easily found in most wetlands, sometimes sitting near a
Great Egret. But it feeds almost exclusively on Meadow Voles
(Microtus microtus). This is why it is classified as
a Microtene Obligate.
Its choice of habitat is determined by the presence of the
vole. (For a discussion on the White-tailed Kite go to the
White-tailed
Kite Curriculum).
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The Great Egret, as with
other long-legged waders, has a variety of hunting styles.
This bird is feeding on land and utilizes its long legs,
long neck, and sharp beak.
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Generally, egrets feed by
themselves. On this day some kind of signal went out that
there was a special source of food. Perhaps the egrets
noticed that the White Pelicans had gathered in this area to
feed.
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There are about 30 Great
Egrets, and almost the same number of Snowy Egrets all
competing for food with a group of White
Pelicans (in the
middle). This waterway was part of a much larger wetland
(Baylands) in Palo Alto, California.
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This egret is flying over a
wetlands found in California, that is made up mostly of
pickle weed. This is a tidal area so the egret will
fluctuate its feeding in response to the tides. A high tide
will eliminate much of the real-estate that the mammals
depend on and make them more accessible to the egret during
that time.
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A water site has three areas
that an egret can feed in. It can feed on the shore of the
area, as this bird is doing, looking for small vertebrates
and invertebrates. It can then move into the water and feed
within the vegetation close to shore. Eventually it can move
into the deeper water to feed, as the next picture
demonstrates.
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One of the feeding
techniques of a Great Egret is to stir things up to find
food. It does this by using its feet to disturb the bottom
of the wetlands. It drags its foot along the bottom and this
will dislodge anything that has sought refuge on the bottom.
If the animal swims up to the surface, then the egret can
grab it with its sharp beak.
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The egret is capable of
feeding at many different levels in a particular habitat.
This egret is as deep in the water as it can go, as the
water level is almost up to its feathers. The feathers are
not waterproof, so it needs to avoid getting them wet, as
they would soak up the water and make it heavier and thus
harder to fly.
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Egrets are generally
solitary feeders, but here a Great Egret is feeding with a
Snowy Egret in the middle and a Great Blue Heron on the
right.
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The food must be abundant
here. If you look at the rocks behind the birds you can see
droppings from these birds or others as proof that this is a
frequent feeding area for large birds.
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This bird is feeding in an
area that is protected by a pier so it is not vulnerable to
waves. From afar it looks as if the egret is standing on the
water, but actually is stranding on a kelp bed which can
support the bird and its hunting. A kelp bed attracts a wide
variety of vertebrates. It can hunt for food from this
position. Despite its size the egret does not weigh very
much and can easily balance its weight on a bed of kelp.
This raises the question of whether or not the egret has to
compensate after it feeds in the ocean instead of freshwater
that it usually feeds in.
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In the Corkscrew Swamp
National Park in Florida the egret is surrounded by mostly
land. It can use its long beak to grab small vertebrates and
invertebrates that it finds on the ground or along a tree
limb. The water that is available is in the form of small
waterways that move through the swamp.
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One way to appreciate the
role of habitat in a species' success is to look at
distribution maps. In Sibley's Birds of North
America, multicolored maps give a fair report of each
species distribution.
When we look at the
Reddish
Egret we realize
that it is only found along the coastal waters of the
southeast; mainly between and including Texas and Florida.
The Great Egret, which in many ways is morphologically
similar to the Reddish Egret, is found in 45 of the 48
contiguous states. The Great Egret is not found consistently
in Montana, Wyoming, and Colorado. The relative of the Great
Egret, the Great
Blue Heron, is found
in all 48 contiguous states.
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Why is the Great Blue Heron
found in more states than the Great Egret? Can we learn
something about the role of habitat in the ecology of
waterbirds by comparing the distribution of the Great Blue
Heron and the Great Egret?
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Appendix A
Top
Food Items of Gt.
Blue Heron, Great
Egret, Reddish
Egret , Snowy
Egret and White-faced
Ibis (From Bent's Life
Histories of North American Birds
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Great Blue
Heron Food Items (From Bent's Life Histories of North
American Birds)
Food: The principal food of
the great blue heron is fishes of various kinds and it seems
to be willing to accept whatever kind of fish is most easily
available. Ora W. Knight (1908) says:
Frogs, eels, horn-pouts,
pickerel occasionally, suckers, shiners, chubs, black bass,
herrings, water puppies, salamanders, and tadpoles, are the
items I have discovered among their rations. They do not
frequent as feeding grounds the spots where trout usually
congregate, and I have very strong doubts that they eat
trout, except very rarely, let alone consuming them in the
vast quantities certain persons have affirmed.
It fishes by night as well
as by day and employs two very different methods, still
hunting and stalking. The former is the best known and
probably the commonest method. Standing as still as a graven
image in shallow water, where fish are moving about, it
waits patiently until one comes within reach, when a swift
and unerring stroke of its well trained bill either kills or
secures the fish. Usually the fish is seized crosswise
between the mandibles; if it is a small one, it is tossed in
the air and swallowed head first, so that it will slip down
easily; but if the fish is a large one, the heron may walk
ashore with it and beat it on the ground to kill it or may
kill it by striking it in the water. I have never had the
patience to watch a heron long enough to learn how long it
would stand and wait for a fish to come to it. I have found
it more interesting to watch it stalking its prey, a more
active operation. Slowly and carefully, with stately tread,
it walks along in water knee deep, its long neck stretched
upward and forward; its keen eyes are scanning the surface
and an occasional quick turn of the head indicates a glimpse
of a fish; suddenly it stops, as if it had seen a fish, but
it moves on again; at last comes its chance, as in a
crouching attitude the long neck darts downward, quick as a
flash; the stroke is not always successful, but sooner or
later the heron secures a meal. Sometimes, in its eagerness,
the heron may step beyond its depth and lose its balance,
but a few flaps of its wings restores its equilibrium and
its dignity.
Audubon (1840)
says:
The principal food of the
great blue heron is fish of all kinds; but it also devours
frogs, lizards, snakes, and birds, as well as small
quadrupeds, such as shrews, meadow mice, and young rats, all
of which I have found in its stomach. Aquatic insects are
equally welcome to it, and it is an expert flycatcher,
striking at moths, butterflies, and libellulae,
whether on the wing or when alighted. It destroys a great
number of young marsh-hens, rails, and other birds; but I
never saw one catch a fiddler or a crab; and the only seeds
that I have found in its stomach were those of the great
water lily of the Southern States. It always strikes its
prey through the body, and as near the head as possible. Now
and then it strikes at a fish so large and strong as to
endanger its own life; and I once saw one on the Florida
coast, that, after striking a fish, when standing in the
water to the full length of its legs, was dragged along for
several yards, now on the surface and again beneath. When,
after a severe struggle, the heron disengaged itself, it
appeared quite overcome, and stood still near the shore, his
head turned from the sea, as if afraid to try another such
experiment.
Wilson (1832) includes in
its food grasshoppers, dragon-flies and the seeds of
splatter docks. Mr. Hastings says that it eats great
quantities of insects and mice. When the grasshoppers have
been thick he has seen it feeding in the open meadow on
these insects entirely, often for two hours at a time; it
does not chase them but stands very still, allowing the
insects to come within reach of its quick beak. Arthur H.
Howell (1911) adds crustaceans to the list. Bartlett E.
Bassett wrote me that a bird he shot for me was carrying a
large black snake in its bill. Altogether the food habits of
this species are decidedly beneficial. It may occasionally
take a few trout, but it does not ordinarily frequent the
streams where trout are found.
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Great Egret
Food Items (From Bent's Life Histories of North American
Birds)
Food: Egrets obtain their
food in the marshes and rice fields and around the marshy
shores of lakes and ponds where their tall, graceful figures
tower above the low vegetation or are reflected in the
smooth waters as beautiful silhouettes in white. Their
movements are stately and the strokes of their rapierlike
bills are quick and sure. Their food consists only partially
of small fishes and it includes frogs, lizards, small
snakes, mice, moles, fiddlers, snails, grasshoppers, and
other insects, as well as some vegetable matter. Oscar E.
Baynard (1912) says:
Food of 50 young egrets that
was disgorged by them at the nest immediately after being
fed, running over a period of four weeks. The total of the
50 meals follows: 297 small frogs, 49 small snakes, mostly
the water moccasin, 61 young fish, suckers, not edible, 176
crayfish.
Dr. Alexander Wetmore (1916)
says of a bird taken in Porto Rico:
The single stomach available
for examination contained 4 per cent of vegetable rubbish
taken as extraneous matter with the animal food. Remains of
one mole cricket (Scapteriscus didactylus) and seven
entire grasshoppers, with fragments of many more, were
found, as well as a moth and three large dragon flies. A
small goby and seven entire frogs (Leptodactylus
albilabris) with fragments of others, made up 69 per
cent of the contents. Orthoptera amounted to 15 per cent, a
surprising fact and one that should be given due weight in
considering the status of this species.
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Reddish Egret
Food Items (From Bent's Life Histories of North American
Birds)
Food: Being a bird of the
seacoast the reddish egret probably obtains most, if not
all, of its food in salt water. Large numbers of these birds
maybe seen at times standing in the shallow waters around
their breeding grounds, or way off on the mud banks or sand
shoals in the lagoons, where they stand motionless watching
for their prey or walk about slowly in search of it, until
the rising tide forces them to leave. Mr. Cahn (1923),
however, writes:
Just where the old birds
went for food is a question. On a quiet evening hundreds of
them would be seen standing in the shallow water that
surrounds their island, but the birds remained almost
motionless in the red glow of the setting sun, and there was
little evidence that they caught their food so near home. On
the contrary, with the approach of evening and the lessening
of the intensity of the sun, the birds usually took wing and
disappeared in small groups to the southwest, in which
direction undoubtedly lay their feeding ground. The food
consists of a small fish and frogs, tadpoles, and an
occasional crustacean, which are probably caught in the
marshes of the mainland coast. Before dark the birds were
all back and at the nest, and there was relatively little
night activity. With the daylight the birds would fly away
once more to the feeding grounds, returning again before the
heat of the sun was sufficiently intense to endanger their
precious eggs or babies. Then followed another period of
inactivity during which the birds remained close to the
nest, preening their wonderful feathers or playing at
repelling intruders.
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Snowy Egret
Food Items (From Bent's Life Histories of North American
Birds)
Food: Audubon (1840) has
described the feeding habits of the snowy egret so well that
I can not do better than to quote his words, as
follows:
The snowy heron, while in
the Carolinas, in the month of April, resorts to the borders
of the salt-water marshes and feeds principally on shrimps.
Many individuals which I opened there contained nothing else
in their stomachs. On the Mississippi, at the time when the
shrimps are ascending the stream, these birds are frequently
seen standing on floating logs, busily engaged in picking
them up; and on such occasions their pure white color
renders them conspicuous and highly pleasing to the eye. At
a later period, they feed on small fry, fiddlers, snails,
aquatic insects, occasionally small lizards, and young
frogs. Their motions are generally quick and elegant, and,
while pursuing small fishes, they run swiftly through the
shallows, throwing up their wings. Twenty or 30 seen at once
along the margins of a marsh or a river, while engaged in
procuring their food, form a most agreeable sight. In autumn
and early spring they are fond of resorting to the ditches
of the rice fields, not unfrequently in company with the
blue herons.
Wilson (1832) adds: "It also
feeds on the seeds of some species of nymphae, and of
several other aquatic plants." Oscar E. Baynard (1912) found
that 50 meals of young snowy egrets consisted of 120 small
suckers, 762 grasshoppers, 91 cut-worms, 2 small lizards, 29
small crayfish, and 7 small mocasins, a most interesting
collection, which proves that this species is decidedly
beneficial. Dr. Alexander Wetmore (1916) says of its feeding
habits in Porto Rico:
Frequently the snowy egret
feeds in lowland cane fields, especially when these are wet
or partly flooded. Often in flocks of three or four they
feed in the dry upland pastures. Two stomachs were available
for examination, both of birds which had been feeding in
mangrove swamps. The main content of these is animal matter,
vegetable remains occurring only as rubbish secured with
other food and amounting to but 1 per cent. One bird taken
near Rio Piedras had eaten two dragon-fly nymphs, a small
crab, a lizard, and a small frog. The stomach of the other,
secured, near Mameycs, was nearly filled with bones of small
gobies, the remainder of the animal food consisting of
fragments of flies of the family Dolichopodidae and
bits of a grasshopper. In their excursions to drier fields
the birds must secure other insects. They feed to a large
extent upon fish, but the fishes taken are of no great
importance and the birds are not abundant enough to become
noxious.
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White-faced
Ibis Food Items (From Bent's Life Histories of North
American Birds)
Food: Like
the white ibis, this species often makes long flights to its
favorite feeding grounds, along the banks of rivers and on
the shallow margins of muddy pools, ponds, and marshes. On
the hog-wallow prairies of the coastal plains of Texas are
many such pools, where we often saw this species feeding,
walking about gracefully and probing in the mud; the crops
of birds we shot here were crammed full of ordinary
earthworms. Its food also consists largely of crawfish,
various small mollusks, insects and their larvae, small fish
and frogs, newts, leeches, and various other forms of low
animal life. Probably a certain amount of aquatic vegetation
is also eaten.
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Appendix B
Comparative body sizes of certain
Ciconiformes
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Species
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Length
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Wing
span
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Weight
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Great Blue
Heron
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46"
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72"
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5.3
lbs.
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Great
Egret
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39"
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51"
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1.9 lbs
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Reddish
Egret
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30"
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46"
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1 lb.
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Snowy
Egret
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24"
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41"
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.75 lbs
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Bibliography