San Francisco State University
Department of Geography

Geography 316:  Biogeography

In progress 12/13/2001

The Biogeography of the Great Egret (Ardea alba egretta)

by  Anne McTavish,  student in Geography 316, Fall 2001

Kingdom: Animalia

Phylum: Chordate

Class: Aves

Order: Ciconiiformes

Family: Ardeidae

Genus: Ardea

Species: (Ardea alba)

New World Subspecies: (Ardea alba egretta)

Great Egret (Ardea alba egretta) (copyright, A. McTavish, 2001)

The scientific and common names for the Great Egret have changed over time.  The genus, especially,  has changed and the bird, which was known as Casmerodius alba or Egretta alba, is now known as Ardea alba. In addition to multiple scientific names, there have been many common names, including "American Egret," "Common Egret," "Large Egret," "White Egret," and "Great White Egret."  It is sometimes called "Great White Heron," but this is incorrect because the Great White Heron is actually a white morph of the larger Great Blue Heron.  While this bird's official common name in North America is now Great Egret, be prepared for anything!

Morphological Description of the Great Egret:
 

The species of the Ardeidae (heron)  family are mainly tropical birds, but they have spread out all over the world and occupy all but extremely high latitudes and elevations (Elphick, 2001).  There are four well-defined races of Great Egret in the world, with the New World race, Ardea alba egretta being the largest (Hancock, 1999). The Great Egret is a large white bird with long-legs, a heavy yellow bill, and blackish legs and feet. The neck is long with a characteristic kinked S-curve; the sixth cervical vertebra is elongated and modified so that the upper part of the neck can easily be retracted or extended (Pratt, 1993).  The neck is specialized this way so the bird can use the bill as a harpoon for capturing prey (Martinez-Vilalta, 1992).  The tail is short and square-ish.  The length is 99 cm and the wingspan is 130 cm.  There is no pronounced sexual dimorphism in terms of size and appearance males and females look alike.  After the fall molt (losing and replacing their feathers) both male and female egrets grow long, flowing plumes, known as “aigrettes,” from their shoulder areas that trail from the back, extending beyond the tail.  When the birds are feeding the plumes are flattened and compressed, so they show as needle-like extensions at the end of the tail.  During the post-breeding molt the display plumes are lost.  Flight is not very agile, but strong.  Speeds have been measured at about 28-51 km/h in the Great Egret (Martinez-Vilalta, 1992). While the mortality rate is high for the young, wild birds have been known to live 10 to 20 years.

Great Egret neck (copyright, Elphic, 2001)

This drawing by David Sibley of the egret neck shows how the esophagus and trachea (in red) actually cross over and run behind the vertebrae (shown in blue) on the lower part of the neck, protecting them from damage in case the fore-neck bumps anything (Elphick, 2001). 

Habitat:
 
The Great Egret can be found in all kinds of wetlands, both inland and along the coast, including marshes, floodplains, river margins, lake shores, salt pans, estuaries, coastal swamps, mangroves and mudflats.  They can also be found in more terrestrial habitats, including open fields, agricultural land, rice fields and drainage ditches.

Behavior:
 

Snowy Egrets, Great Blue Heron, and Great Egret (center)  feeding together (copyright, A. McTavish, 2001)


Stately and slow moving, the birds hunting strategies involve a variety of techniques, which give them the flexibility needed to adapt to varying types of prey, enabling them to exploit whatever may be abundant at the moment.  They search for prey with their necks extended at an angle to the body, peering into the water or at the ground in front of them. They can rotate their eyes slightly outward in the sockets to improve depth perception through binocular vision.  When nesting, they prefer to feed locally, but will travel far from the nesting colony if necessary, going distances up to 6-20 km.  The Great Egret is a sociable bird, especially during nesting seasons when they will often breed in colonies mixed with Great Blue Heron and Snowy Egret and other heron species.  Different theories have been proposed to explain why they are sociable; perhaps it is so they can readily share information about where food is abundant, or perhaps it is to reduce the impact of predation.  It has been observed that when mixed species of birds feed together they combine surveillance capacities to better detect predators (Martinez-Vilalta, 1992).

Food and Feeding:

Great Egrets feed both alone and in groups, they are carnivorous, and they have a broad and variable diet related to the available prey in their area.  The diet consists chiefly of aquatic animals, such as fish, amphibians, reptiles, birds, small mammals, and insects, crustaceans, and mollusks (Hancock, 1984).  Their behavior has been observed closely and described extensively, with special names given to their many "ritualized" movements. To hunt they will "Walk Slowly" or "Quickly,"  "Hop,"  "Jump,"  "Foot Stir,"  "Wing Flick,"  "Foot Paddle,"  "Head Tilt," and "Peer Over.It will even "Hover,"  "Dip," or "Plunge" (Hancock, 1999). Their most common feeding technique is to stand motionless at the water’s edge or in shallow water, waiting until the prey comes close enough to catch.  Standing upright, they have a better field of vision; crouching permits the bird to put the bill closer to the intended victim.  Since they are frequently hunting in water, they need to compensate for refraction.  This is why they move their head from side to side and backward and forward.  The Great Egret tilts its head towards the shaded side of the body and strikes into its shadow to avoid the glare of the sun on the surface of the water (Payne, 1976). They capture their prey by seizing it with their bill.  They generally eat their prey whole.

Breeding:

The Great Egret is seasonally monogamous, reuniting with the same mate if available; to study this more closely would require observing banded birds.  Great Egrets do not breed until they are two years old.  Most Great Egrets breed in large colonies, often mixed with other species such as Great Blue Heron or Snowy Egret.  Great Egrets produce a single brood per year (Elphick, 2001).  In temperate zones they breed in spring or summer, choosing the time based on when food is abundant; in the tropics, they can breed at any time of the year (Martinez-Vilalta, 1992). Courtship and pair formation are complex and color changes of the bare parts of the bird and the nuptial plumes are important.  As the egrets come into breeding condition, the color of the featherless parts of their bodies changes.  The lores (skin by the eye) turn from yellow to lime green, and the bill becomes dark along the top.  The lores change back to yellow after the egrets finish laying their eggs, but the bills retain some of the dark color for most of the breeding season (Pratt, 1993).

Mixed breeding colony of egrets and herons
 
(copyright, A. McTavish, 2001)

The male arrives at the breeding colony first and selects a nest site.  Site selection is age relative, with the older birds making the first selections, usually toward the center of the colony.  Great Egrets do switch nest locations and even switch colonies, so when they get to the nesting colony the males assert their claim to a site and display to attract females.  The male dances, using a variable succession of stylized movements that include: the "Stretch" display,  "Wing Preen,"  "Snap" display, and "Twig Shake."  Interested females gather on branches and watch.  An attracted female may perform the "Wing Preen" while she watches the male.  Sometimes the female will fly using the ritualized "Circle Flight."  A female may chase other females away with the "Supplanting Attack".  Male and female egrets proceed with caution in establishing their pair bonds, sometimes even chasing each other away.  Soon after the pair bond is established, nest building begins in earnest (Pratt, 1993).

Nest, eggs and young:

An egret platform nest is less a nest and more a heap of sticks laid down in one place, sometimes over a period of years, although colony sites may be abandoned or different trees within a colony may be favored one year and ignored the next.  Egrets will get sticks and material for their nests wherever they find it, including stealing material from other nests.  After the females arrive and a pair bond, the male will generally bring back nest material and the female will arrange it in the nest (Pratt, 1993).

The Great Egret genus has changed, having been reclassified from Casmerodius or Egretta to Ardea, currently recognizing the species close evolutionary relationship to the Great Blue Heron.  This chart, showing a comparison of nesting and feeding characteristics, illustrates the two species similarities and differences.

  Great Blue Herons Great Egret
Nest Large, well-made platform of interwoven sticks lined with finer material. Flimsy, frail platform of sticks, may be unlined or lined with finer material. 
Habitat Lakes, marshes, bays, estuaries, swamps, rivers, mangroves, occasionally on cliffs. Lakes, marshes, bays, estuaries, swamps, irrigation ditches.
Eggs 3-6, blue green; 2.5” (64 mm), semialtricial, 1 brood. 3-6 (usually 3), blue green, 2.2” (57 mm), semialtricial, 1 brood.
Incubation 28 days, by both parents 23-26 days, by both parents
Fledging 56-60 days.  Can fly at 8 weeks, but may not fledge until 12 if food is abundant and parents are still feeding. 42-49 days  Can take extended flights by 7 weeks, but continue to depend on parents until 10-11 weeks.
US Breeding Range In appropriate habitat throughout most of the United States. Common in the southeastern United States, the West and in the Atlantic states.
Diet Mostly fish, but opportunistic, including human food scraps, nestlings, small mammals. Insects, lower vertebrates, small birds, frogs, crayfish, fish.
Young fed Regurgitant delivered into nest. Regurgitant delivered into nest at first; later, directly into nestlings’ mouths.

 

The Great Egret reproductive strategy for survival is brood reduction – they lay a large clutch and let sibling competition and food abundance play a large role in how many survive to fledge.  Eggs are laid at two to three day intervals.  Incubation is usually performed by both sexes and begins after the first or second egg has been laid.  As a result the eggs hatch days apart and there are significant size differences between chicks of the same brood.
The newly hatched chicks are semialtricial, meaning when they hatch they are helpless, practically naked, and have to be fed (Martinez-Vilalta, 1992).  Competition for food is fierce.  At feeding time the older chicks may peck at their smaller nestlings and try to monopolize the food.  The larger chicks will be more successful at competing for enough food for themselves; if food is scarce the smallest chick starves.  In an unusually good year, with abundant food, all the chicks may fledge.  Often the egrets successfully raise two young; some years only one survives; less often three and rarely four live long enough to fledge (Pratt, 1993).  The youngest chick may be considered an “insurance” chick that may replace an older sibling that dies or an egg that doesn’t hatch (Elphick, 2001).
Great Egret with three chicks (copyright, A. McTavish, 2001)

Mortality:

Mortality of Great Egrets is high in the nest and in the first few months after they fledge.  After the young leave the nest they do not stay near their parents.  They must find good foraging areas and learn to capture prey on their own.  Estimates for Great Egret mortality is 76% in the first year, then 26% (Pratt, 1993).  The longest life span recorded a bird in the wild is 22 years, 10 months.  The average life span for egrets after the beginning of their second year is 3.3 years.  Said another way, an egret that reaches the reproductive age of two years will typically live to breed for three more years.  The average total life span of a Great Egret is about five years (Pratt, 1993).

Movements:

Once breeding is over, and before any migration towards the winter quarters takes place, there are dispersive movements, especially of the young birds, soon after they have become independent. Distances of 400 km are recorded for Great Egret dispersals (Martinez-Vilalta, 1992).  In the winter, birds from more northerly colonies begin a southward migration. California birds breeding near San Francisco may be found wintering in the Central Valley, southern California and Mexico, or they may stay nearby.

Distribution:

The Great Egret is a cosmopolitan species, with world-wide distribution.  Primarily found in tropical areas, their range extends into temperate regions as well.  The factors limiting distribution are not well studied, but range correlates with warmer temperature and lower altitude. There are four subspecies of the Great Egret. Ardea alba alba, the nominative species, was named by Linnaeus in 1758.  It is found from Central Europe to Central Asia, South to Iran; it winters in North and Central Africa and the Persian Gulf to South China and South KoreaArdea alba egretta was named by Gmelin in 1789.  It is found in  North, Central, and South America, from northern USA to Central Argentina.  Ardea alba melanorhynchos was named by Wagler in 1827.  It is found in Africa south of the Sahara and on Madagascar. Ardea alba modesta was named by J. E. Gray in 1831.  It is found in India, Southeast Asia, Japan and Korea and south through Indonesia to Australia and New Zealand (Martinez-Vilalta, 1992).

Map of Distribution:
 

Blue

Zone occupied by species when it doesn’t normally breed, or breeding has not been documented.

Yellow

Zone occupied for breeding but species not normally present outside breeding time.

Green

Species present all year round.

(copyright, Martinez-Vilalta, 1992) 

 Evolution:
 

Birds are in the kingdom Animalia, phylum Chordata, which includes all  vertebrates and certain marine animals that have a notochord (primitive backbone).  Shown here are representatives of the Chordata phylum, including the bird class, Aves.

I, a tongue worm, Hemicordata
II, a sea squirt, Urochordata
III amphioxus, Cephalochordata
IV a lamprey, Agnitha
V, a shark, Elasmobranchii
VI, a fish, Osteichteyes
VII, a frog, Amphibia
VIII a lizard, Reptilia
IX a bird, Aves
X, a mammal, Mammalia

(copyright, Wallace, 1955)

In the search for the ancestors of today's birds, some ornithologists ask how could a large, heavy, two-footed reptile with a long, heavy balancing tail and greatly foreshortened forelimbs, fighting gravity all the way, give rise to flight? Are the dinosaurs the true ancestors of birds?

Theories of avian evolution are going through revolutionary changes in both discovery and theory, sometimes putting opposing sides in the midst of bitter controversy.  One argument is between paleontologists who advocate that the origin of birds was from terrestrial dinosaurs and ornithologists who favor the theory that  evolution of flight came from a tree-dwelling ancestor.

We know that convergent evolution is a recurring event in species evolution.  Convergent evolution means that creatures that are unrelated have developed similar survival strategies in response to similar environmental pressures and may act or look alike; examples include fish and dolphins, bats and birds.  Could the similarities between birds and reptiles be the result of convergent evolution?

This much we do know: approximately 150 million years ago a feathered creature the size of a crow died and its body was covered by limestone sediments, preserving not only the shape of the bones but also the delicate impression of feathers.  This creature was both bird-like and reptile-like.

Possible evolution of birds from dinosaurs, showing the ancestors and descendants of Archaeopteryx (copyright, Feduccia, 1999).
The Archaeopteryx fossil is a superb example of a creature considered intermediate between two groups, a missing link in the chain of evolutionary history.  What came before Archaeopteryx?  Do all birds follow from this ancestor?  The questions are still being explored today.
The fossil record shows that the Cretaceous-Tertiary (K-T) boundary event was marked by mass extinction; surely it was as significant for birds as for all the other life forms that became extinct at that time.  The evolution of modern orders of birds has been post-Mesozoic and the radiation of modern species was explosive, probably occurring some 5 to 10 million years ago.

Morphological classification systems compare and contrast various anatomic features of birds to develop the taxonomic order and insight into the ancestral relations of birds.  DNA data continues to yield more details to this picture.

Extinction of species at the Cretaceous-Tertiary boundary, with subsequent radiation of modern orders of birds (copyright, Feduccia, 1999)

Systematics is based on the premise that biological classification should reflect evolutionary history. While avian systematists agree this is important, they disagree on the final taxonomic arrangement of all the species.  Different authors may consider one characteristic more important than another as an accurate reflection of evolutionary relationships. The fossil record for birds is scant because of their fragile bone and feather construction.  Using anatomy and DNA, a phylogeny of birds can be constructed even without fossil record; however, developing the correct morphological system for birds is especially challenging because they are very much alike anatomically.  The demands of flight placed tight restrictions on evolution, limiting structure and anatomy that will work for flight.  As a result, birds are finely tuned machines with high metabolic rates and constrained aerodynamic morphologies.  They are the most structurally uniform of all vertebrate orders (Feduccia, 1999).

The Sibley and Monroe classification system based on DNA-DNA hybridization lets us trace the evolution of the Great Egret to the  Ciconiiformes order (Sibley, 1991).

Class, Aves
Subclass Neonithes
Superorder Neognathae
Order Ciconiiformes

The order, Ciconiiformes, is a heterogeneous group of long-legged waders living in marshlands or shallow waters..  These birds include the storks (Ciconiidae); herons, egrets and bitterns (Ardeidae); ibises and spoonbills (Threskiornithidae or Plataleidae); hamerkop (Scopidae); and shoebill (Balaenicipitidae). 

(Copyright, Sibley, 1991)

 

 The origins of the Ardeidae family are ancient, dating back to the lower Eocene, about 55 million years ago.  Some of the present day genera are very old – Ardea is known from the late Miocene, over 7 million years ago.  Over the last 100 years the recognized number of heron, egret, and bittern species has varied from 60 to 93 and the number of genera from 15 to 35 (Sheldon, 1987), with the  current trend being to lump rather than split groups. Today the Ardeidae family is divided into four subfamilies: day herons (Ardeinae); night herons (Nycticoracinae); tiger-herons (Tigrisomatinae); and bitterns (Botaurinae) (Elphick, 2001).
The Ardeidae family of the Ciconiiformes order ((Martinez-Vilalta, 1992

Two heron genera Ardea and Egretta comprise most of the day heron subfamily.  Generally, small to medium-sized species are placed in Egretta and larger ones in Ardea.  The Great Egret has been difficult to place because the North American sub-species is large and more Ardea-like than the other three sub-species of the world.  Early classifications placed the Great Egret in its own genus, Casmerodius; later it was moved to Egretta because of the similarities with others in that genus – consider how similar it looks to the Snowy Egret (Hancock, 1984).  Currently it is classified as part of the Ardea genus – look at the similarities to the Great Blue Heron.

Human Interactions, Conservation:
 

Birds of the heron family have been noticed throughout history.  They are mentioned in the old testament of the bible, were used in the Middle Ages as quarry for falconry.  Since ancient times the beauty of their feathers led to their use by different peoples; the Maoris of New Zealand used the Great Egret’s nuptial plumes as decorations in the chief’s headdresses. 
Egg collectors, hobbyists, hunters, and the fashion industry exploited birds unmercifully in the 1800’s.  In the mid nineteenth century and the beginning of the 20th century Europe and North America used bird feathers to adorn ladies hats.  London, Paris, New York, Berlin and Vienna became the principal trading centers for feathers from all types of birds.  Feather collecting affected practically all the continents and all types of birds, including gulls, albatrosses, condors, owls, parrots, and hummingbirds, but especially egrets because of their long nuptial feathers (known as aigrettes).  One Great Egret has 40-50 aigrettes; so about 150 birds were needed to collect 1 kg. of aigrettes. 

This well-written and beautifully illustrated book tells the story of some of the people who worked to save the Great Egret and other birds from feather traders (Lasky, 1997).

It was reported that at a single London auction almost 50,000 ounces of feathers were offered for sale.  In the area around Paris over 10,000 people were employed in this business.  In the first quarter of 1885, 750,000 bird skins were sold on the London market alone.  In 1892 a merchant in Florida shipped 130,000 skins to New York.  In 1898 one and a half million birds were exported from Venezuela.  The London Commercial Sales Room sold 48,240 ounces of feathers in 1902.  If you calculate that four birds were killed for each ounce of feathers, that means a minimum of 192,960 Great Egrets were slaughtered to supply feathers for one firm  (Martinez-Vilalta, 1992).  Hunting Great Egrets happened during breeding season, so for each pair of adult birds killed, three or four eggs or young were lost.  Imagine similar auctions repeated in many places in North America and Europe, compound these figures over a half-century, and you can readily see how the egrets came close to disaster.  Fortunately, early environmentalists were horrified by this slaughter and began writing articles and proposing legislation to save the birds, resulting not only in the egrets protection but in the formation of the American Ornithological Union (1883), the Audubon Society (1886) and, in Great Britain, the Royal Society for the Protection of Birds (1898) (Dunning, 1994).

The Great Egret has made a remarkable recovery all over the world since laws have been enacted for their protection and demand for plumage has declined.  The large colonies of Great Egret that were once found in the United States have been replaced by small, scattered nesting groups.  This change in breeding colony size is partially due to the effects of hunting and partially due to the destruction of their breeding and feeding grounds.  Advances in technology in this century have accelerated development and the transformation of the aquatic habitats on which these birds are dependent.  Water pollution and pesticides affect the birds through their diets.  High levels of mercury, selenium, chromium, and chlorine compounds have been found in their tissues, eggs, and droppings around the world.  Since the Great Egret is high on the tropic level (food chain) they are vulnerable to biological magnification of pesticides.  Great Egrets, and wading birds in general, are good indicators for monitoring the quality of habitats. Because nesting and fledging success varies with food availability they are a good reflection of the productivity of the wetland ecosystem.  Alteration of their habitat remains the greatest threat to the species today.

Acknowledgements:

John Kelly, Audubon Canyon Ranch, for reading the paper and clarifying several points.

Annie Malley, California Academy of Sciences, for help with the research.

Bibliography:

Chavez-Ramirez, F. and R. D. Slack. 1995. “Differential Use of Coastal Marsh Habitats by Nonbreeding Wading Birds.” Colonial Waterbirds 18 (2): 166-171.

Cogswell, Howard L.  1977. Water Birds of California. California. University of California Press.

Dickinson, Mary B. 1999. National Geographic Field Guide to the Birds of North America. Washington, DC. National Geographic Society.

Dunning, Joan. 1994. Secrets of the Nest. New York. Houghton Mifflin Company.

Ehrlich, Paul, David Dobkin, and Darryl Wheye. 1988. The Birders Handbook. New York. Simon and Schuster.

Elphick, Chris, John Dunning, and David Allen Sibley. 2001. The Sibley Guide to Bird Life and Behavior. New York. Alfred A. Knopf.

Feduccia, Alan. 1999. The Origin and Evolution of Birds.  New Haven and London.  Yale University Press.

Fisher, Chris and Joseph Morlan. 1997. Birds of San Francisco and the Bay Area. Washington. Lone Pine Publishing.

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Guster, T. W. and D. W. Peterson, Jr. “Growth Rates of Great Egret, Snowy Egret, and Black-crowned Night-heron Chicks.” Colonial Waterbirds 14 (1): 46-50.

Hancock, James. 1999. Herons and Egrets of the World, A Photographic Journey. California. Academic Press.

Hancock, James and James Kushlan. 1984. The Herons Handbook. New York. Harper and Row.

Kelly, J. P., H. M. Pratt, and P. L. Greene. 1993. “The Distribution, Reproductive Success, and Habitat Characteristics of Heron and Egret Breeding Colonies in the San Francisco Bay Area.” Colonial Waterbirds 16 (1): 19-27.

Lasky, Kathryn and David Catrow. 1997. She’s wearing a Dead Bird on Her Head. New York. Hyperion.

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Martinez-Vilalta, Albert and Anna Motis.  1992. “Family Ardeidae (Herons).” Pp. 376-429. In Handbook of the Birds of the World, Volume 1.  Barcelona, Spain.  Lynx Editions.

McCrimmon, D. A. 1978. “Nest Site Characteristics Among Five Species of Heron on the North Carolina Coast.” The Auk 95 (2): 267-280.

Mock, D. W. 1984. “Siblicidal Aggression and Resource Monopolization in Birds.” Science 225 (4663): 731-733.

Mock, D. W. 1985. “Siblicidal brood reduction: The prey-size hypothesis.” American Naturalist 125 (3): 327-343.

Payne, Robert B. and Christopher J. Risley.  1976. Systematics and Evolutionary Relationships Among the Herons (Ardeidae). Michigan. Museum of Zoology, University of Michegan.

Pratt Helen. 1993. Herons and Egrets of Audubon Canyon Ranch. California. Audubon Canyon Ranch.

Sheldon, Fredrick H. 1987. “Phylogeny of Herons Estimated from DNA-DNA Hybridization Data.” The Auk 104 (1): 97-108.

Sibley, Charles G. and Jon E. Ahlquist. 1990. Phylogeny and Classification of Birds: A Study in Molecular Evolution. New Haven and London. Yale University Press.

Sibley, Charles G. and Burt Monroe. 1991. Distribution and Taxonomy of Birds of the World. New Haven and London. Yale University Press.

Sibley, David Allen. 2000. The Sibley Guide to Birds. New York. Alfred A. Knopf.

Soffer, Ruth. 1999. Sea and Shore Birds Coloring Book. New York. Dover Publications, Inc.

Wallace, George. 1955. An introduction to Ornithology. New York. MacMillan Company.
 

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