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Species Name:    Ardea alba
Common Name:      (Great Egret)

 

I.  TAXONOMY

Kingdom Phylum/Division: Class: Order: Family: Genus:
Animalia Chordata Aves Ciconiiformes Ardeidae Ardea

The Great Egret, Ardea alba.  Photo courtesy of C. Sewell, used with permission.  


Great egret alighting in a red mangrove tree. Photo courtesy of C. Sewell, used with permission.

 


Species name:
Ardea alba Linnaeus, 1758)

Common name:
Great Egret

Synonymy:
Casmerodius albus

Potentially Misidentified Species:
Can be confused with the white morph of the great blue heron (Ardea herodias), the "great white heron", which occurs only in southern Florida. The great egret's black legs and feet distinguish it from the great white heron, which has yellow legs.

Species Description/Taxonomy:
Ardea alba is a member of the Order Ciconiiformes (herons and storks) which encompasses the long-necked wading birds. The Family Ardeidae includes the herons, egrets, bitterns, etc. A. alba is a large heron whose body color is entirely white. The bill is thick and yellow, while the legs and feet of the animal are black. When in breeding plumage, the great egret can appear somewhat shaggy due to the fine, threadlike feathers which grow downwards along the neck and back.


II.  HABITAT AND DISTRIBUTION 

Regional Occurrence:
In the continental United States, A. alba ranges from Oregon, Wisconsin and Massachusetts south to Florida, the Gulf of Mexico and most of South America.

IRL Distribution:
Occurs lagoon-wide


III.  LIFE HISTORY AND POPULATION BIOLOGY

Age, Size, Lifespan:
A. alba is one of the largest heron species. It grows to a height of 32 inches, and has a wingspan of 55 inches.

Abundance:
Great egrets are common in the IRL and throughout most of Florida.

Reproduction:
Nesting in Florida's Lake Okeechobee region occurs from mid-December through January (Ehrlich et al 1988), and is preceded by courting behavior from as early as late July or August. Smith and Collopy (1995) found a strong positive correlation between receding water levels in Lake Okeechobee and nesting sites chosen by great egrets. It is believed that these areas are preferred because receding water has the effect of concentrating prey in a smaller area, thus making it simpler to feed nestlings (Smith and Collopy 1995). Between 3 - 5 eggs are laid per clutch. One brood is raised each year; however, should the first nest be destroyed, a replacement clutch can be laid.

A study which followed a mixed nesting colony in Vero Beach, Florida reported that great egrets nesting along the Indian River Lagoon preferred to nest somewhat upland of the water's edge, constructing 77% of their nests in black mangrove trees; and 23% of nests in white mangrove trees (Maxwell and Kale 1977). In this study, most egg laying had been completed by the end of the first week of April.

Embryology:
The incubation period for great egret eggs is 23 - 26 days. Eggs hatch asynchronously over a period of several days. The first chick to hatch thus becomes the most experienced at food handling, and quickly becomes the most experienced aggressor toward its siblings. As a result, nestmates have varied growth rates, with the first chick generally growing the fastest. Data from Custer and Peterson (1991) show that first and second chicks have somewhat even growth rates, with third chicks showing slower growth.

 

Mass:
(g/day)

Forearm growth (mm/day)

Tarsus growth (mm/day)

Chick 1

          27.6 
    +/-   3.3

                4.0 
        +/-   0.4

                3.2 
        +/-   0.3

Chick 2

          30.3 
    +/-   3.5

               4.2 
        +/-   1.3

               3.4 
        +/-   0.3

Chick 3

          18.8
    +/-   3.6

                3.2 
        +/-   1.4

                2.9 
        +/-   0.3

 


IV.  PHYSICAL TOLERANCES

Temperature:
Great egrets are eurythermal. On the west coast of the U.S., they are known to winter from northern California southward; on the east coast, they overwinter from New Jersey south to the Gulf of Mexico (Farrand 1988). However, they can be cold sensitive during the breeding season, with spring cold events linked to nest failures at some sites (Smith and Collopy 1995).


V.  COMMUNITY ECOLOGY

Trophic Mode:
Great egrets are heterotrophs. They are opportunistic predators with a wide variety of prey species including fish, crustaceans, amphibians, small mammals, insects and reptiles. Miranda and Collazo (1997) found that great egrets feeding in South American lagoons ate primarily shrimp, followed by guppies, crabs and insects. In the Florida Everglades, great egrets prey primarily on fish (Smith 1997). Great egrets eat more large fish of more different species than do other wading birds. They thus have low dietary overlap with other heron and egret species. This can be partially attributed to their large size, which contributes to their ability to forage in deeper waters where they encounter more large fish than other species do. Great egrets can thus exploit a niche not open to smaller birds (Willard 1977). Another reason for lower dietary overlap with other wading birds is that great egrets are capable of adjusting their diets based on changing environmental conditions. When the amount of preferred prey in a habitat is decreased, great egrets alter their diets to include other prey species. This is different from patterns observed in other heron and egret species such as the snowy egret and tricolored heron, which alter their foraging habitat and feeding tactics in order to continue to encounter preferred prey (Smith 1997).

Striking efficiency (the percentage of strikes which result in the capture and ingestion of prey) in great egrets is maximized when they feed while wading slowly through the water. However, Willard (1977) showed that great egrets capture and ingest more intermediate sized prey if they stand still while foraging. This finding suggests that the goal of most wading birds may not be to maximize feeding efficiency, but rather, to capture and ingest more high quality prey items.

Smith (1997), in studying a nesting colony at Lake Okeechobee, Florida, found that the diets of great egret nestlings are among the most varied for wading birds, and includes as many as 40 different species. Mosquitofish were the primary prey items offered to nestlings, followed by crayfish, sailfin mollies, bluegills, and shad.

Competitors:
Other large wading birds such as the great blue heron utilize similar habitats and food resources, but probable resource partitioning among species minimizes direct competition.

Habitats:
Great egrets are sympatric with other species of wading birds. Willard (1977) and Kent (1986) found significant habitat overlap of great egrets with snowy egrets, tricolored herons and little blue herons. However, it appears that there is some level of habitat partitioning that occurs, with great egrets preferring to inhabit freshwater pools and lakes (Chavez-Ramirez and Slack 1995), while other species utilize estuarine habitats.

In Florida, great egrets prefer habitats with high water levels in winter, followed by receding water levels in early spring (Smith and Collopy 1997). Recession of water in spring concentrates prey in a smaller area, helping to increase nest success, and the later foraging success of fledglings. Collopy and Smith (1995) have suggested this choice of breeding habitat has direct applications to wetland management around Lake Okeechobee. With the large numbers of wading birds nesting in this area, the authors suggest it could be most beneficial to bird populations for wetland managers to consider keeping lake levels low in the late winter or early spring months of the nesting season.

Associated Species:
Great egrets are often seen in association with other wading birds in wetland habitat areas.


VI.  SPECIAL STATUS

Special Status:
In Florida, listed as a species of Species of Special Concern (SSC), but is not federally listed as threatened or endangered.

Benefit in IRL:
The environmental sensitivity of wading birds, coupled with the relative ease of assessing their numbers, makes them attractive as biological indicators of ecosystem health and habitat quality (Custer and Osborn 1977; Powell and Powell 1986; Powell et al. 1989).

Economic Importance:
None.

 

Report by:  K. Hill, Smithsonian Marine Station
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Page last updated: July 25,  2001