II. HABITAT AND
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.
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.
Great egrets are common in the IRL and
throughout most of Florida.
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
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.
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.
Forearm growth (mm/day)
Tarsus growth (mm/day)
IV. PHYSICAL TOLERANCES
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
V. COMMUNITY ECOLOGY
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.
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.
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
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.
Great egrets are often seen in association
with other wading birds in wetland habitat areas.
VI. 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).
Report by: K. Hill,
Smithsonian Marine Station
Submit additional information, photos or comments to:
Page last updated: July 25, 2001