Egretta thula

Geographic Range

Egretta thula is found throughout North, Central, and South America as well as the Caribbean. It breeds in coastal and inland wetlands, but its range limits have changed over time due to the effects of hunting and habitat loss. Small breeding populations are located in Nova Scotia, Canada, and more heavily populated locations are found across the United States. Egretta thula is common among northern Nevada, Utah, and southeastern states, especially Florida and states bordering the Gulf of Mexico. This egret is most prevalent throughout Mexico, Central America, and South America. Egretta thula is a partially migratory species, as it relocates from its northern habitats of the United States and Canada to its winter ranges located in Mexico, Central America, South America, the West Indies, and Bermuda. Snowy Egrets begin their northward migration in early March and depart in September to migrate to their wintering areas.

• Biogeographic Regions
• nearctic nearctic
  • native native
• neotropical neotropical
  • native native
• oceanic islands oceanic islands
  • native native

Habitat

Egretta thula generally prefers an environment of shallow water inlets for feeding purposes. Salt-marsh pools, tidal channels, shallow bays, and mangroves are among the most preferred habitats in North America. Habitats are most common among coastal areas and islands due to the availability of stable and abundant food sources. During the winter months, egrets migrate to the Caribbean to nest and roost in the mangroves. The Caribbean is home to other favorable egret habitats including salt-water lagoons, freshwater swamps, grassy ponds, beaches, shallow reef areas, flooded rice fields, and wet grassy meadows. Throughout Central America, E. thula prefers mainly lowland areas near freshwater swamps, lakes, and large river mouths. South American species also prefer coastal mangroves, mudflats, and swamps rather than highland areas.

• Habitat Regions
• temperate temperate
• tropical tropical
• terrestrial terrestrial

• Wetlands
• marsh marsh
• swamp swamp

• Other Habitat Features
• riparian riparian
• estuarine estuarine

Physical Description

Egretta thula is a medium-sized heron with a delicate build. Adult egrets generally measure between 56 to 66 cm and have a wingspan of approximately 100 cm. Egrets average 370 g in weight and the males tend to be slightly larger than the females. Egretta thula has entirely white plumage, a long, slender black bill, bright yellow lores, and long, slender black legs with bright yellow feet. Eyes are yellow. Breeding adults develop long, delicate plumes off their breast and are also characterized by their change in foot color, from yellow to orange. There are no overall differences in appearance between breeding populations, however, populations studied in North America and Central America are found to have a larger bill than egrets of South America.

• Other Physical Features
• endothermic endothermic
• bilateral symmetry bilateral symmetry

• Sexual Dimorphism
• male larger

Development

Female egrets generally lay 3-6 eggs and both parents incubate the eggs for approximately 22-25 days. Upon hatching, the young nestling is a grayish color. It has a dark blue area around the eyes and the bill is a pale, pinkish gray. Once the eggs are fully hatched, the adults remove the eggshells from the nest. The hatchlings are covered in white down except for their wings. Pinfeathers appear by the first week. Juvenal feathers emerge on the body and wings by 2 to 3 weeks of age. Leg color varies from yellow to black. The hatchlings have a yellow colored bill tipped with black until five weeks of age, when the entire bill changes to black. Both parents brood their semialtricial young for the first 10 days. After 10 days, only one parent remains in the nest for 50% of the time. This generally lasts until the nestlings become 14 days old. The nestlings leave the nest after two weeks, but some may leave the nest as early as 10 days (Howell 1995; Parsons 2000).

Reproduction

Breeding begins in late March or early April when the male egrets perform flight displays and sound vocalizations to attract female mates. The most common courtship display is the "Stretch" display, in which the male pumps his body up and down with his bill pointed towards the sky. The male then produces a call to attract females. The changing foot color from yellow to reddish orange indicates the beginning of breeding behaviors. Breeding adults are also characterized by the distinctive display of long, delicate plumes off their breasts. Once a male finds a mate, the pair performs sexual displays and eventually builds a nest for their offspring.

• Mating System
• monogamous monogamous

The male and female pair-bond is maintained through a series of sexual displays. Breeding begins in March or early April. Female egrets usually build nests in the territories defended by the males. Nests are often built in isolated, estuarine habitats and can be located either on the ground or as high as 30 feet in the trees. The nests are composed of woven twigs and small sticks that female egrets collect from the ground or steal from other nests. Egretta thula may also reuse old nests. These egrets are highly social nesters and build nests close to other egrets or herons. No preliminary rituals are performed prior to copulation, which takes place in the nest. Males stand on the backs of females and cloacal cavities come into contact during copulation to fertilize the eggs. The average duration of contact is 10 seconds. Females lay 3-6 eggs at a time (on average); eggs have a pale, greenish blue color. Incubation lasts 24 days on average and the chicks usually fledge 14 days after hatching. Young reach reproductive maturity after 1 to 2 years.

• Key Reproductive Features
• iteroparous iteroparous
• seasonal breeding seasonal breeding
• gonochoric/gonochoristic/dioecious (sexes separate)
• sexual sexual
• fertilization fertilization
  • internal internal
• oviparous oviparous

Both parents incubate the eggs and feed the nestlings by dropping food into the nest. Once the eggs hatch, parents remove the eggshells from the nest. Both parents brood their altricial young continuously until the hatchlings are 10 days old. From 10 to 14 days, only one parent is present in the nest to brood the young. After 10 days, parents are only in the nest 50% of the time. However, when storms occur, the young are brooded continuously. During the first five days after hatching, parents feed their young by regurgitating food onto the nest floor for the hatchlings to eat. Sometimes the parents' bill is placed directly into the hatchlings' mouth and food is regurgitated. The younger nestlings are fed before the older hatchlings. Adults keep the nest clean by dumping waste over the sides of the nest.

• Parental Investment
• no parental involvement
• altricial altricial
• pre-fertilization
• pre-hatching/birth
  • protecting
    • male
    • female
• pre-weaning/fledging
  • provisioning
    • male
    • female
  • protecting
    • male
    • female

Lifespan/Longevity

Egretta thula has a 71.6% mortality rate during its first year and a 31.4% mortality rate during years 2 to 17. The oldest egret was recorded in Utah and lived 22 years, 10 months. Snowy egrets generally live between 2 and 17 years. Egretta thula has been subject to nematode parasitism, which causes death. Starvation and inclement weather are likely causes of death for young nestlings.

Behavior

Egretta thula walks upright with its wings held close to its body. This upright posture is ideal for foraging, because it allows the egret to make quick directional changes to catch its prey. They are most active at dawn and dusk (crepuscular); they have flying capabilities but do not swim. Snowy egrets engage in various self-maintenance behaviors that include grooming their wings, head-scratching to remove insects on their body, and bathing. Egretta thula spends most of the day resting. Males protect the nesting territories. Egretta thula is a highly social bird that engages in group foraging with other aquatic bird species. Group interaction with other bird species is also common in nesting territories. Snowy egrets avoid predators such as owls, hawks, poisonous snakes, and raccoons.

• Key Behaviors
• flies
• crepuscular crepuscular
• motile motile
• migratory migratory
• social social

Home Range

We do not have information on home range for this species at this time.

Communication and Perception

Egretta thula communicates through sound vocalizations and posture. Young birds produce soft, buzzing calls and mature birds produce high and low-pitched calls. High-pitched calls signify plentiful foraging sites and low-pitched calls signify aggressive situations. Greeting calls are common among egrets. Only males tend to use high sound vocalizations, especially to attract a female mate. Communication sounds are also used to defend the territory surrounding the nest. An egret's upright posture with fully erect feathers marks the onset of an attack on another bird.

• Communication Channels
• visual visual
• acoustic acoustic

• Perception Channels
• visual visual
• tactile tactile
• acoustic acoustic
• chemical chemical

Food Habits

Egretta thula prefers foraging habitats near bodies of shallow water, which are ideal for food sources. Its broad diet consists of earthworms, annelid worms, aquatic and terrestrial insects, crabs, shrimp, crayfish, snails, freshwater and marine fish, frogs, toads, lizards and snakes. The egret's diet is generally composed of 75% fish and 25% crustaceans. This egret has the widest range of foraging behaviors when compared to other herons. Food capturing is performed by pecking, walking slowly or quickly, running, hopping, hovering, and "disturb and chase" behaviors. Snowy egrets primarily feed during the early morning and evening hours. Egrets occasionally engage in group flights to fly to far-away foraging environments. Otherwise, egrets independently fly approximately 3 km from their colonies to foraging sites. However, foraging in larger groups allows for greater success in finding substantial food sources and helps provide protection from predators.

• Primary Diet
• carnivore carnivore
  • piscivore piscivore

• Animal Foods
• amphibians
• reptiles
• fish
• insects
• mollusks
• terrestrial worms

Predation

Egretta thula has shown an increased preference for island nest sites in urbanized, coastal estuaries. Egrets choose urbanized locations over isolated locations, because isolated locations have more predators. Egrets use flight to escape predation from terrestrial animals and they are known to have innate recognition and avoidance of poisonous snakes.

Known predators include: Procyon lotor (racoon), Bubo virginianus (great-horned owl), Strix varia (barred owl), Corvus brachyrhynchos (American crow), Corvus ossifragus (fish crow), Alligator mississippiensis (American alligator), Elaphe obsoleta (rat snake) and Buteogallus anthracinus (common black-hawk).

Ecosystem Roles

Egretta thula serves as a biological indicator of ecosystem health and habitat quality. In marshes, bays, and swamp habitats, the absence of egrets may reflect disturbances in the ecosystem, such as pollution, contamination of water, habitat loss, or human disturbance. In some habitats, researchers have sampled eggs and feathers to test levels of environmental contamination. Egrets are positioned at the top of the food chain, thus their decline may also infer a decline of other species, such as fish or insects. Egretta thula is a highly social bird and will not attack humans or disturb other bird species in its habitat.

Economic Importance for Humans: Positive

In the United States from 1880 to 1910, adult egrets were shot by plume hunters. Egretta thula was hunted for its delicate back plumes that were used to decorate women's hats and clothing. In 1886, plumes were valued at $32 per ounce, which was twice the price of gold at the time. In 1910, most hunting ceased due to citizens' requests to stop the slaughter of egrets. However, hunting still continued in Central and South America due to the European demand for plumes.

• Positive Impacts
• body parts are source of valuable material

Economic Importance for Humans: Negative

There are no known adverse affects of snowy egrets on humans.

Conservation Status

Populations appear to be declining along the Atlantic coast due to pollution and competition with other bird species. Egretta thula is at risk because of chemical contamination and the decline of wetland environments. Snowy egrets depend on wetland areas for food. Eggs in agricultural areas are contaminated by pesticides, which cause death. Egrets have also died from consumption of styrofoam, plastics, and lead found in the environment. Oil spills have also caused mortality. Egretta thula has been protected in North America since 1916 under the Migratory Bird Treaty Act. The Migratory Bird Treaty Act prohibited the hunting of egrets for their plumes, thus allowing them to return to their previous levels of abundance.

Other Comments

Similar species to Egretta thula include Egretta caerulea , little blue heron, and Egretta garzetta , little egret. Little blue herons have completely white plumage, but can be distinguished from E. thula by their gray lores. Little egrets are larger and stockier than E. thula and also have longer legs, bill, and neck.

Additional Links

Encyclopedia of Life

Contributors

Alaine Camfield (editor), Animal Diversity Web.

Andrea Weslosky (author), University of Michigan-Ann Arbor, Phil Myers (editor), Museum of Zoology, University of Michigan-Ann Arbor.

References

Bowles, M. 1991. "Snowy Egret" (On-line). Accessed 12/09/03 at http://www.inhs.uiuc.edu/chf/pub/ifwis/birds/snowy-egret.html .

Chandler, R. 1997. Snowy Egret. Journal of Field Ornithology , 68: 287-295.

Howell, S., S. Webb. 1995. A Guide to the Birds of Mexico and Northern Central America . Oxford: Oxford University Press.

Parsons, K., T. Master. 2000. Snowy Egret. The Birds of North America , 489 489: 1-23.

Robbins, C. 1966. A Guide to Field Identification Birds of North America . New York: Western Publishing Company.