USGS



BIOLOGICAL AND ECOTOXICOLOGICAL CHARACTERISTICS OF TERRESTRIAL VERTEBRATE SPECIES RESIDING IN ESTUARIES

Snowy Egret Snowy Egret photo by B. Truitt
(Photo by B. Truitt)
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Biological Characteristics

Species

Egretta thula is 56-66 cm in length, with an approximate mass of 371 grams (Dunning, 1993). This small white heron is distinguished by its black bill, black legs, and yellow feet (Bull and Farrand, 1977). No sexual dimorphism has been reported for this species.

Status in Estuaries

This species typically breeds in mixed-species colonies in salt marshes, freshwater marshes, ponds, and shallow coastal bays (Bull and Farrand, 1977; Spendelow and Patton, 1988). Compared to other egrets, snowy egrets tend to nest in more open areas, usually about 1.0 to 1.5 meters above the ground (Jenni, 1969). A typical clutch consists of 3 or 4 pale bluish-green eggs (Bull and Farrand, 1977). Young are altricial (Ehrlich et al., 1988).

Abundance and Range

Breeds along the Atlantic Coast, and along inland water bodies such as the Mississippi River. This species may winter in Florida, the Caribbean or South America (Hancock and Kushlan, 1984). The species was nearly brought to extinction as a result of plume-hunting during the early 20th century.  Presently, more than 40,000 individuals are estimated nationally (NACWCP, 2001).

Site Fidelity

Egrets nesting earliest in the breeding season tend to occupy nest sites from previous seasons (Jenni, 1969).

Ease of Census

Simple.

Feeding Habits

Generalist. The snowy egret forages by walking slowly or standing motionless in water and striking at prey. It has been found feeding at oyster bars, tidal creeks, freshwater ponds, and saltflats (Custer and Osborn, 1978). This heron typically forages within 2 km of the breeding colony, though feeding has been observed up to 18 km away (Custer and Osborn, 1978). Prey includes aquatic organisms and insects, specifically shrimp, small fish, mollusks, frogs, and terrestrial and aquatic insects (Hancock and Kushlan, 1984).


Snowy Egret Contaminant Exposure Data

I.

Organochlorine Contaminants

A.

Concentrations in Adults and Nestlings

1.

From 1969 to 1971, and in 1977, snowy egret carcasses were collected from Maryland, Alabama, Louisiana, and New Mexico (Ohlendorf et al., 1981). Six carcasses (minus head, skin, feet, wing tips, liver, kidney, and GI tract) were collected from Maryland (Long Marsh Island) in 1969, one was collected from Mobile, Alabama in 1970, one was collected from Sabine NWR in Louisiana in 1971, and one was collected from Albuquerque New Mexico in 1977. For these dates and locations, all organochlorine contaminants analyzed were either not detected or found at levels <1 mg/g wet weight.

2.

Octachlorostyrene in carcasses of nestling snowy egrets (5-10 days of age) collected from Goat Island in Galveston Bay, Texas ranged 0.01 to 0.8 mg/g wet weight and heptachlorostyrene ranged from 0.02 to 2.6 mg/g (Rice and Custer, 1991).

3.

Brains were collected from nestlings found dead at three sites in Florida between 1987-1991, and pooled samples were analyzed for several pesticides (Spalding et al., 1997).  Mean concentrations of oxychlordane, heptachlor epoxide, and trans-nonachlor were 0.04, 0.04, and 0.03 µg/g wet weight, respectively at Lake Okeechobee, and below detection limits in estuarine sites.  The mean DDE concentration was 0.95 µg/g at Lake Okeechobee, and ranged from 0.05-0.06 µg/g at other sites. HCB was not detected. 

B.

Concentrations in Eggs

1.

In 1970, ten snowy egret eggs were collected from various locations in Texas (King et al., 1978). Mean contaminant concentrations were 3.26 mg/g wet weight sigma.gif (58 bytes)DDT (sum of DDD, DDE and DDT), 1.06 mg/g dieldrin , and 2.03 mg/g PCB.

2.

A snowy egret egg collected in 1972 along the west coast of Florida contained concentrations of 20.9 mg/g dry weight DDE, 161 mg/g PCB, and 0.9 mg/g dieldrin (Lincer and Salkind, 1973).

3.

Snowy egret eggs were collected from National Wildlife Refuges located in the eastern half of the United States in 1972 and 1973 (Ohlendorf et al., 1978; 1979). Grouping samples geographically, DDE and PCBs were highest in the eggs collected along the North Atlantic. On average, DDE was detected in snowy egret eggs at a concentration of 2.66 mg/g wet weight and PCBs were detected at 5.00 mg/g. Overall, the highest concentrations of DDE, 11.0 mg/g, and DDD, 2.0 mg/g, were detected in eggs from Gardiner's Island, New York. The highest concentration of PCBs, 7.9 mg/g, was found in eggs from Long Island, New York.

4.

In 1979, organochlorine residues were measured in snowy egret eggs collected from the Blackfoot Reservoir, the Minidoka National Wildlife Refuge, and Mud Lake Wildlife Management Area located in Idaho (Findholt, 1984). DDE residues were detected in all eggs. Mean (range) DDE concentrations were 4.8 (0.42-33) mg/g fresh wet weight at the Blackfoot Reservoir and 4.2 (0.62-26) mg/g at Minidoka. Other organochlorine contaminants and PCBs were detected at mean concentrations of <1 mg/g.

5.

Between 1981 and 1983, 41 of 42 snowy egret eggs collected from Stillwater, Nevada were found to have detectable concentrations of DDE (Henny et al., 1985). The geometric mean concentrations of DDE was 1.43 mg/g wet weight in 1981, 1.91 mg/g in 1982, and 0.62 mg/g in 1983. The number of the eggs tested with a DDE concentration greater than 5 mg/g declined from 33% in 1981, 27% in 1982, to 7% in 1983.

6.

Ten snowy egret eggs were collected in 1982 from Bair Island, located in San Francisco Bay (Ohlendorf et al., 1988). DDE was detected at a geometric mean concentration of 2.04 mg/g wet weight (range 0.88-7.1), and PCBs at 3.29 (range 0.8-13) mg/g. Mean concentrations of DDD, DDT, dieldrin, oxychlordane, cis-chlordane, trans-nonachlor, cis-nonachlor, and toxaphene were <1 mg/g.

7.

Between 1989 and 1991, snowy egret eggs were collected from various locations in San Francisco Bay (Hothem et al., 1995). The geometric mean concentration of PCBs ranged from 0.185 mg/g wet weight in the comparison site located outside of San Francisco Bay (South Wilbur Flood Area) up to 5.38 mg/g from Bair Island. DDE ranged from 0.824 mg/g at West Marin Island to 3.01 mg/g at the comparison site. Concentrations of BHC, oxychlordane, heptachlor epoxide, trans-nonachlor, dieldrin, cis-nonachlor, DDD, and DDT were detected at <1 mg/g.

8.

In 1993, eggs were collected from the National Audubon Sanctuary Island of Lower Laguna Madre (Mora, 1996a). Chemical analysis of 4 snowy egret eggs revealed an average total PCB concentration of 0.074 mg/g wet weight. The most common PCB congeners detected were 82, 92, 110, 138, and 153.

9.

In 1993-94, four snowy egret eggs were collected from the National Audubon Sanctuary Islands of the Lower Laguna Madre, Texas (Mora, 1996b). Median (range) contaminant concentrations were 0.249 (0.026-9.65) mg/g wet weight DDE and 0.078 (0.033-0.107) mg/g PCBs.

10.

Chlorostyrene contamination was detected in snowy egret eggs and carcasses located on Goat Island in Galveston Bay, Texas (Rice and Custer, 1991). Concentrations of octochlorostyrene and heptachlorostyrene were analyzed in one egg and found to be low (0.01 mg/g wet weight).

II.

Cholinesterase-Inhibiting Pesticides

 

No residue data available.

III.

Trace Elements, Metals, and Metalloids

A.

Concentrations in Adults, Juvenile, and Nestlings

1.

 From 1974 to 1976, three snowy egrets were collected from Brunswick, Georgia (Gardner et al., 1978). Mean Hg concentrations of 6.3 mg/g dry weight in the muscle, of which 79% was MeHg, and 24 mg/g in the liver, of which 40% was MeHg, were found.

2.

In 1977, 3 snowy egrets were collected from the Brunswick Estuary in Georgia (Odom, 1978). Mercury residues ranged from 1.65 to 34.27 mg/g in the liver and from 0.81 to 2.80 mg/g in the breast tissue.

3.

In 1980, a snowy egret was collected from Brunswick, Georgia and one from Savannah, Georgia (Odom, 1981). Mercury concentrations were highest in Brunswick where levels reached 24.69 mg/g wet weight in the liver and 6.27 mg/g in the breast muscle. Mercury levels from Savannah were <1 mg/g in both breast and liver tissue.

4.

In 1984, immature snowy egrets were collected from the Drum Island heronry in South Carolina (White and Geitner, 1996). In the liver, Pb was not detected and Hg levels were <0.25 mg/g wet weight. In the kidney, Cd was not detected and Se levels ranged from 0.91 to 1.5 mg/g.

5.

A snowy egret nestling found dead between 1987 and 1990 contained Hg concentrations of 0.42 mg/g wet weight in the liver and 2.4 mg/g dry weight in feather (Beyer et al., 1997).

6.

Nestlings, fledglings, and adults found dead were collected from four areas in south Florida between 1987-1991 and analyzed for hepatic Hg (Sundlof et al., 1994). Mean Hg concentrations were 0.37, 0.38, and 5.38 µg/g wet weight at Lake Okeechobee, the Everglades, and eastern Florida Bay, respectively, and below detection at Big Cypress National Preserve.  Mean Hg concentrations ranged from 0.38-5.38 µg/g in nestlings and were below detection in fledglings.

7.

Mercury was determined in breast feathers of snowy egret nestlings and eggs from the New York Bight (Burger and Gochfeld, 1997). Mean concentrations in nestling feathers in 1990 were approximately 1.0 mg/g dry weight in samples collected from Canarsie Pol, New York, and, in 1995, 3.0 mg/g from Lavallette, New Jersey and 3.5 from Goosebar, New Jersey.

8.

Livers were collected from nestlings found dead at a mangrove estuary Florida between 1987-1991 (Spalding et al., 1997).  Mean concentrations of Pb, Cu, and Cd were 0.26, 21, and 0.08 µg/g wet weight, respectively.

B.

Concentrations in Eggs

1.

Ten snowy egret eggs collected in 1982 from Bair Island, San Francisco Bay, had a mean Hg concentration of 0.21 mg/g wet weight (Ohlendorf et al., 1988).

2.

From 1989 to 1991, snowy egret eggs collected at San Francisco Bay had geometric mean Hg concentrations ranging from 0.965-1.50 mg/g dry weight on Bair Island, and Se from 3.02 mg/g at West Marin Island to 5.26 mg/g at the South Wilbur Flood Area (Hothem et al., 1995).

3.

Mercury was determined in breast feathers of snowy egret nestlings and eggs from the New York Bight (Burger and Gochfeld, 1997).  Eggs from Lavallette, New Jersey had an approximate mean concentration of 0.9 mg/g.

4.

In 1993-94, four snowy egret eggs were collected from the National Audubon Sanctuary Islands of the Lower Laguna Madre, Texas (Mora, 1996b). Median (range) metal concentrations were as follows (mg/g wet weight): 0.07 (0.05-0.17) Hg, 0.30 (0.09-0.41) Se, 0.74 (0.40-1.22) B, 0.11 (0.08-0.14) Cr, 0.93 (0.89-1.15) Cu, 14.6 (12.6-18.6) Fe, 89 (77-95) Mg, 0.46 (0.32-0.49) Mn, 1.25 (0.75-2.17) Sr, and 9.5 (8-9.8) Zn.

IV.

Petroleum

 

No residue data available 

 


Snowy Egret Contaminant Response Data

I.

Organochlorine Contaminants

1.

To determine average shell thickness, 79 snowy egret eggs were collected from Texas in 1970 (King et al., 1978). There was a significant 9% decrease in shell thickness between eggs collected in 1970 and the thickness of 38 eggs collected prior to 1943.

2.

No statistically significant difference in eggshell thickness was found between snowy egret eggs collected from various National Wildlife Refuges in 1972 and 1973 and those collected prior to 1947 (Ohlendorf et al., 1978; 1979).

3.

Eggs collected from Blackfoot Reservoir and Minidoka National Wildlife Refuge, Idaho, were examined for eggshell thickness (Findholt, 1984). No significant difference in thickness existed between pre-1947 eggshells (0.222 mm) and the eggs collected for this study (0.215 mm). There was a significant and inverse correlation between shell thickness and DDE, DDD, and DDT concentrations. Eggshell thickness decreased by 9.5% in the group which had residual DDE values >5 mg/g fresh wet weight, and by 4.3% in the group that had residual values <5 mg/g. In the group with "high" DDE, 45% of the eggs disappeared or were broken and in the "low" group, 15% of the eggs disappeared or were broken.

4.

As DDE concentration increased, mean shell thickness was found to decrease in snowy egrets nesting in Oregon and Nevada (Henny et al., 1985). The number of young per successful nest reared also decreased with increasing DDE concentration (>5 mg/g wet weight)

5.

The analysis of eggs collected from Bair Island, California, found that there was a nearly significant negative correlation between the DDE concentration and eggshell thickness (Ohlendorf et al., 1988).

II.

Cholinesterase-Inhibiting Pesticides

1.

In 1979, fenthion applied to a field within its recommended limits resulted in the death of 7 snowy egrets and 2 great egrets near Vallejo, California (Zinkl et al., 1981). Brain cholinesterase activity in 6 of the dead snowy egrets ranged from 0.3-3.4 mU/mg protein in brain, which is believed to be more than 50% below the norm.

2.

Plasma (serum) samples were collected between 1991 and 1996 from nestling snowy egrets (8-31 days old) from Boston Harbor, MA (Sarah Island, Gallops Island, and Middle Brewster Island); New York Harbor, NY (Prall’s Island, and Isle of Meadows); Nantucket Sound, MA (Sampson’s Island, Monomoy Island, and Coatue Island); Delaware Bay, DE (Pea Patch Island); and Rehoboth Bay, DE (Middle Island) (Parsons et al., 2000).  Immune status (total white-blood-cell counts), nutritional status (size: mass ratio), and exposure to anti-ChE compounds (ChE reactivation assay and land use pattern around the estuary) were measured.  For all sites, mean  total ChE activity was 0.284 U/ml (n=189), mean AChE activity was 0.271 U/ml (n=104), and mean BChE activity was 0.337 U/ml (n=104).   Total ChE activity was found to vary by estuary and age for this species. 

III.

Trace Elements, Metals, and Metalloids

 

No response data available

IV.

Petroleum

1.

In 1990, after 5.7 million liters of oil were spilled in New York Harbor, the productivity of snowy egrets dropped (Parsons, 1996). Studies found that by 1993, productivity had increased to levels prior to the spill. 


References for Snowy Egret

Beyer, W.N., M. Spalding and D. Morrison. 1997. Mercury concentrations in feathers of wading birds from Florida. Ambio 26:97-100.

Bull, J. and J. Farrand, Jr. 1977. The Audubon Society Field Guide to North American Birds. Alfred A. Knopf, New York. 784 pp.

Burger, J. and M. Gochfeld. 1997. Risk, mercury levels, and birds: Relating adverse laboratory effects to field biomonitoring. Environ. Res. 75:160-172.

Custer, T.W., and R.G. Osborn. 1978. Feeding habitat use by colonially-breeding herons, egrets, and ibises in North Carolina. Auk 95:733-43.

Dunning, Jr., J.B., ed. 1993. CRC Handbook of Avian Body Masses. CRC Press, Ann Arbor. 371 pp.

Ehrlich, P.R., D.S. Dobkin, and D. Wheye. 1988. The Birder's Handbook Simon & Schuster, New York. 785 pp.

Findholt, S.L. 1984. Organochlorine residues, eggshell thickness, and reproductive success of snowy egrets nesting in Idaho. Condor 86:163-169.

Gardner, W.S., D.R. Kendall, R.R. Odom, H.L. Windom, and J.A. Stephens. 1978. The distribution of methyl mercury in a contaminated salt marsh ecosystem. Environ. Pollut. 15:243-251.

Hancock, J. and J. Kushlan. 1984. The Herons Handbook. Harper and Row Publishers, New York. 288 pp.

Henny, C. J., L. J. Blus., and C.S. Hulse. 1985. Trends and effects of organochlorine residues on Oregon and Nevada wading birds, 1979-83. Colon. Waterbirds 8:117-128.

Hothem, R.L., D.L. Roster, K.A. King, T.J. Keldsen, K.C. Marios and S.E. Wainwright. 1995. Spatial and temporal trends of contaminants in eggs of wading birds from San Francisco Bay, California. Environ. Toxicol. Chem. 14:1319-1331.

Jenni, D.A. 1969. A study of the ecology of four species in herons during the breeding season at Lake Alice Alachua County, Florida. Ecol. Monographs 39:245-270.

King, K.A., E.L. Flickinger., and H.H. Hildebrand. 1978. Shell thinning and pesticide residues in Texas aquatic bird eggs, 1970. Pestic. Monitor. J. 12:16-21.

Lincer, J.L. and D. Salkind. 1973. A preliminary note on organochlorine residues in the eggs of fish-eating birds of the west coast of Florida. Florida Field-Nat. 1: 19-22.

Mora, M.A. 1996a. Congener-specific polychlorinated biphenyl patterns in eggs of aquatic birds from the lower Laguna Madre, Texas. Environ. Toxicol. Chem. 15:1003-1010.

Mora, M.A. 1996b. Organochlorines and trace elements in four colonial waterbird species nesting in the lower Laguna Madre, Texas. Arch. Environ. Contam. Toxicol. 31:533-537.

NACWCP.  2001.  Review Draft II—North American Waterbird Conservation Plan. Volume One: Seabirds and Colonial Waterbirds, 23 October 2001, Waterbird Conservation Steering Committee, Washington DC (www.nacwcp.org/).

Odom, R.R. 1978. Statewide wildlife investigations: Mercury contamination studies. Georgia Game and Fish Div. 15 pp.

Odom, R.R. 1981. Statewide wildlife investigations: Mercury contamination studies. Georgia Game and Fish Div. 17pp.

Ohlendorf, H.M., E.E. Klaas, and T.E. Kaiser. 1978. Organochlorine residues and eggshell thinning in anhingas and waders. Proc. Conf. Colon. Waterbird Group 3:185-195.

Ohlendorf, H.M., D.M. Swineford and L.N. Locke. 1979. Organochlorine poisoning of herons. Proc. Conf. Colon. Waterbird Group 3:176-185.

Ohlendorf, H.M., D.M. Swineford, and L.N. Locke. 1981. Organochlorine residues and mortality of herons. Pest. Monitor. J. 14:125-135.

Ohlendorf, H.M., T.W. Custer, R.W. Lowe, M. Rigney and E. Cromartie. 1988. Organochlorines and mercury in eggs of coastal terns and herons in California, USA. Colon. Waterbirds 11:85-94.

Parsons, K.C. 1996. Recovering from oil spills: the role of proactive science in mitigating adverse effects. Colon. Waterbirds 19:149-153.

Parsons, K.C., A.C. Matz, M.J. Hooper, and M.A. Pokras.  2000.  Monitoring wading bird exposure to agricultural chemicals using serum cholinesterase activity.  Environ. Toxicol. Chem.  19:1317-1323.

Rice, C.P. and T.W. Custer. 1991. The occurrence of chlorostyrenes in egrets and herons collected in Galveston Bay. Proceedings: Galveston Bay Characterization Workshop, February 21-23, 1991. Galveston Bay Natl. Estuary Program Publ. GBNEP-6. p. 80-82.

Spalding, M.G., C.K. Steible, S.F. Sundlof, and D.J. Forrester.  1997. Metal and organochlorine contaminants in tissues of nestling wading birds (Ciconiiformes) from southern Florida. Fla. Field Nat. 25:42-50.

Spendelow, J.A. and S.R. Patton. 1988. National atlas of coastal waterbird colonies in the contiguous United States: 1976-82. Biological Report 88(5). U.S. Fish and Wildlife Service, Washington, D.C.

Sundlof, S.F., M.G. Spalding, J.D. Wentworth, and C.K. Steible.  1994.  Mercury in livers of wading birds (Ciconiiformes) in Southern Florida. Arch. Environ. Contam. Toxicol. 27:299-305.

White, D.H. and J.G.H. Geitner. 1996. Environmental contaminants and productivity in an extinct heronry at Charleston harbor, South Carolina, U.S.A. 1984. Environ. Monitor. Assess. 40:137-141

Zinkl, J.G., D.A. Jessup, A.I. Bischoff, T.E. Lew and E.B.Wheeldon. 1981. Fenthion poisoning of wading birds. J. Wildl. Dis. 17:117-119.

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