Egretta thulasnowy egret

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. (Par­sons and Mas­ter, 2000)

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. (How­ell and Webb, 1995; Par­sons and Mas­ter, 2000)

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. (Chan­dler, 1997; How­ell and Webb, 1995; Par­sons and Mas­ter, 2000)

  • Sexual Dimorphism
  • male larger
  • Average mass
    370 g
    13.04 oz
  • Average mass
    314 g
    11.07 oz
    AnAge
  • Average length
    56-66 cm
    in
  • Average wingspan
    100 cm
    39.37 in

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. (Par­sons and Mas­ter, 2000; Rob­bins, 1966; Par­sons and Mas­ter, 2000; Rob­bins, 1966)

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. (Bowles, 1991; Par­sons and Mas­ter, 2000)

  • Breeding season
    The breeding season begins in March or early April.
  • Range eggs per season
    2 to 8
  • Average eggs per season
    3-6
  • Average eggs per season
    4
    AnAge
  • Range time to hatching
    22 to 29 days
  • Average time to hatching
    24 days
  • Range fledging age
    10 to 25 days
  • Average fledging age
    14 days
  • Range age at sexual or reproductive maturity (female)
    1 (low) years
  • Average age at sexual or reproductive maturity (female)
    2 years
  • Range age at sexual or reproductive maturity (male)
    1 (low) years
  • Average age at sexual or reproductive maturity (male)
    2 years

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. (Par­sons and Mas­ter, 2000)

  • Parental Investment
  • no parental involvement
  • 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. (Chan­dler, 1997; Par­sons and Mas­ter, 2000)

  • Range lifespan
    Status: wild
    22 (high) years
  • Typical lifespan
    Status: wild
    2 to 17 years

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. (Par­sons and Mas­ter, 2000; Rob­bins, 1966)

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. (How­ell and Webb, 1995; Par­sons and Mas­ter, 2000)

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. (How­ell and Webb, 1995; Par­sons and Mas­ter, 2000)

  • 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), Pan­therophis ob­so­le­tus (rat snake) and Bu­teogal­lus an­thrac­i­nus (com­mon black-hawk). (Par­sons and Mas­ter, 2000)

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. (Rob­bins, 1966)

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. (Par­sons and Mas­ter, 2000)

  • 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. (Par­sons and Mas­ter, 2000)

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. (Par­sons and Mas­ter, 2000)

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.

Glossary

Nearctic

living in the Nearctic biogeographic province, the northern part of the New World. This includes Greenland, the Canadian Arctic islands, and all of the North American as far south as the highlands of central Mexico.

World Map

Neotropical

living in the southern part of the New World. In other words, Central and South America.

World Map

acoustic

uses sound to communicate

altricial

young are born in a relatively underdeveloped state; they are unable to feed or care for themselves or locomote independently for a period of time after birth/hatching. In birds, naked and helpless after hatching.

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.

carnivore

an animal that mainly eats meat

chemical

uses smells or other chemicals to communicate

crepuscular

active at dawn and dusk

endothermic

animals that use metabolically generated heat to regulate body temperature independently of ambient temperature. Endothermy is a synapomorphy of the Mammalia, although it may have arisen in a (now extinct) synapsid ancestor; the fossil record does not distinguish these possibilities. Convergent in birds.

estuarine

an area where a freshwater river meets the ocean and tidal influences result in fluctuations in salinity.

fertilization

union of egg and spermatozoan

internal fertilization

fertilization takes place within the female's body

iteroparous

offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).

marsh

marshes are wetland areas often dominated by grasses and reeds.

migratory

makes seasonal movements between breeding and wintering grounds

monogamous

Having one mate at a time.

motile

having the capacity to move from one place to another.

native range

the area in which the animal is naturally found, the region in which it is endemic.

oceanic islands

islands that are not part of continental shelf areas, they are not, and have never been, connected to a continental land mass, most typically these are volcanic islands.

oviparous

reproduction in which eggs are released by the female; development of offspring occurs outside the mother's body.

piscivore

an animal that mainly eats fish

riparian

Referring to something living or located adjacent to a waterbody (usually, but not always, a river or stream).

seasonal breeding

breeding is confined to a particular season

sexual

reproduction that includes combining the genetic contribution of two individuals, a male and a female

social

associates with others of its species; forms social groups.

swamp

a wetland area that may be permanently or intermittently covered in water, often dominated by woody vegetation.

tactile

uses touch to communicate

temperate

that region of the Earth between 23.5 degrees North and 60 degrees North (between the Tropic of Cancer and the Arctic Circle) and between 23.5 degrees South and 60 degrees South (between the Tropic of Capricorn and the Antarctic Circle).

terrestrial

Living on the ground.

tropical

the region of the earth that surrounds the equator, from 23.5 degrees north to 23.5 degrees south.

visual

uses sight to communicate

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: 1-23.

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