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Species Name:    Egretta thula
Common Name:        (Snowy Egret)

 

I.  TAXONOMY

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


The Snowy Egret, Egretta thula, in a tidal mudflat.  Photo courtesy of C. Sewell, used with permission.

Species Name:
Egretta thula (Molina, 1782)

Common Name:
Snowy egret

Potentially Misidentified Species:
Can be confused with  immature little blue herons (Egretta caerulea) which are slightly thinner, but have greenish legs and feet.

 

 


Species Description/Taxonomy:
Egretta thula is a member of the Order Ciconiiformes (Herons and storks) which encompasses the long-necked wading birds. The Family Ardea includes the herons, egrets, bitterns, etc. E. thula is a medium sized egret whose body color is entirely white. It has a thin black bill, and black legs which have a yellow stripe posteriorly. A major distinguishing feature of this species is its bright yellow feet, which are used to startle prey while feeding. When in breeding plumage, thin, wispy plumes grow along the neck, breast and back. Those growing on the back turn outward at the tail, while plumes on the breast and neck remain straight.


II.  HABITAT AND DISTRIBUTION

Regional Occurrence:
In the United States, the range of the snowy egret extends throughout the continental United States from northern California to Maine, and south to Florida, the Gulf of Mexico, and much of South America.

IRL Distribution:
Occurs throughout the IRL.


III.  LIFE HISTORY AND POPULATION BIOLOGY

Age, Size, Lifespan:
Egretta thula is a medium sized egret which grows to approximately 20 - 24 inches, with a wingspan of 38 inches.

Abundance:
Snowy egrets are among the most common wading birds in the southern United States.

Reproduction:
Snowy egrets seldom initiate monospecific colonies, preferring to nest in mixed colonies beginning in mid-March in the southern United States. Like the tricolored heron and several other species, snowy egret nest numbers increase where water levels are low or declining (Smith and Collopy 1995). Breeding at Lake Okeechobee, Florida begins in mid-March and April (Smith 1997). This finding is in agreement with a study of mixed nesting colonies in the Indian River Lagoon by Maxwell and Kale (1977). In this study, snowy egrets which nested along the IRL typically chose nesting areas somewhat upland of the lagoon edge, with 78% of nests being built in black mangrove trees, and 19% of nests being built in white mangroves.

Rodgers (1980) reported a mean clutch size of 3.15 for snowy egrets in Hillsborough Bay on Florida's west coast; however, up to 5 eggs per clutch have been reported (Ehrlich et al 1988). One brood is raised per year; but replacement clutches can be laid if the first clutch is destroyed or abandoned.

Embryology:
Eggs are incubated for 20 - 24 days prior to hatching. Rodgers (1980) reported a hatching rate for snowy egrets in the IRL of 91.2%, and a 70.8% survival rate to 2 weeks of age. Eggs hatch asynchronously over several days (Custer and Peterson 1991).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 growing the fastest. Data from Custer and Peterson (1991) illustrate this point:


Mass:
(g/day)

Forearm growth (mm/day)

Tarsus growth (mm/day)

Chick 1

         14.1 
   +/-   1.9

             4.4 
      +/-   1.3

              2.8 
      +/-   0.1

Chick 2

        11.4 
  +/-   1.5

              3.8 
      +/-   1.6

              2.6 
      +/-   0.3

Chick 3

          9.7 
  +/-   2.9

              2.9 
      +/-   1.7

              2.0 
      +/-   0.5


Young fledge after approximately 30 days (Smith 1997).


IV.  PHYSICAL TOLERANCES

Temperature:
Snowy egrets are eurythermal. Snowy egrets on the west coast of the U.S. overwinter from California southward, while those on the east coast spend the winter from Virginia south to the gulf of Mexico. However, they can be cold sensitive, especially while nesting. Early spring cold events are linked to nest abandonments and decreased foraging success in parental feeding of nestlings (Smith and Collopy 1995).


V.  COMMUNITY ECOLOGY

Trophic Mode:
E. thula is a highly gregarious species, which breeds and feeds in mixed colonies. Strong et al. (1997) reported that over 90% of snowy egrets in their study fed in mixed aggregations, with over half of these groups consisting of 40 or more individuals. Miranda and Collazo (1997) reported that snowy egrets in Puerto Rican lagoons primarily ate shrimp, followed by guppies. In Florida studies, E. thula has been reported to primarily eat fish (Kent 1987; Smith 1997). Kent (1986) postulated that snowy egrets employ one method of resource partitioning while feeding in mixed groups by preying upon the smallest size classes of shrimps and fishes.

In comparing foraging efficiencies among several species of wading birds, Kent (1987) found the foraging behavior of the snowy egret to be highly plastic and correlated with prey type. Walking slowly in tidal flats was correlated with foraging for Polychaete worms; active pursuits such as disturb-and-chase and foot-stirring behavior were associated with shrimp and other crustaceans; and standing was correlated to fish capture. When preferred prey become scarce due to some environmental or other change, snowy egrets respond by altering their foraging habitat and feeding tactics in order to continue to encounter preferred prey items. This differs from some other species of wading birds such as the great egret (Egretta alba) and the little blue heron (Egretta caerulea) which expand their diets to include more species of prey (Smith 1997).

In foraging efficiency, the snowy egret falls between the little blue heron (Egretta caerulea) and the tricolored heron in terms of striking efficiency, the percentage of strikes that result in capture and ingestion of prey. However, in terms of food intake per minute (g/min), the snowy egret falls behind both the little blue heron and the tricolored heron. Kent's (1987) study estimated that the snowy egret needed to forage as much as 1.4 times longer than the little blue heron, and 2 times longer than the tricolored heron to take in the same amount of food. Snowy egrets achieve the greatest striking efficiency while standing still (Kent 1986); however, this highly efficient behavior is used less than 10% of the time in favor of foot stirring, and wading slowly (Willard 1977; Kent 1987). As evidenced by their foraging behaviors, snowy egrets may not have the goal of achieving maximum feeding efficiency, but rather try to maximize energy intake through the infrequent capture and ingestion of high quality prey (Kent 1987).

E. thula nestlings are offered a wide variety of foods, with as many as 31 species being included in their diets. Mosquitofish are the primary nestling food, followed by sailfin mollies, killifishes, and grass shrimp (Smith 1997).

Competitors:
Other medium sized wading birds such as the tricolored heron, little blue heron, and white ibis utilize similar habitats and food resources, but probable resource partitioning among species minimizes direct competition.

Habitats:
Though sympatric with other species of herons and egrets, the snowy egret is somewhat more variable in its habitat preferences, preferring shallow bays, coastal marshes and mangrove habitats over inland marshes and sloughs (Chavez-Ramirez and Slack 1995; Strong et al. 1997). Willard (1977) and Kent (1986) found significant habitat overlap of great egrets with snowy egrets, tricolored herons and little blue herons. However, it appears there is some level of habitat partitioning that occurs, with great egrets and great blue herons often utilizing somewhat deeper waters, while medium sized wading birds utilize shallower areas (Chavez-Ramirez and Slack 1995).

In Florida, snowy egrets prefer habitats with high water levels in winter, followed by receding water levels in early spring (Smith and Collopy 1997). Recession of water levels in spring concentrates prey in a smaller area, thus 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 implications 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.


VI.  SPECIAL STATUS

Special Status:
In Florida, listed as a species of species of special concern (SSC). 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