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DNR News

September 17, 2012

1,827 wood stork nests counted in SC this year

The federally endangered wood stork (Mycteria americana) nested successfully in South Carolina during 2012 despite unfavorable drought conditions. The United States breeding population of wood storks was listed as endangered after nesting pairs declined from between 15,000 and 20,000 in the 1930's to 2,500 pairs by 1978. Historically, wood storks used South Carolina as a feeding area during the summer and fall after dispersing from nesting colonies in Florida and Georgia. In 1981, the first successful wood stork nests were documented in South Carolina (11 nests). Since 1995, wood storks have built between 800–2,060 nests in South Carolina each year.

The S.C. Department of Natural Resources (DNR) counted 1,827 wood stork nests in South Carolina this year. The number of stork nests was down slightly from the 2,031 nests that were counted during 2011. Wood stork nests were observed in 19 colonies in South Carolina this year, including 2 colonies where storks were not known to nest in the past. Aerial surveys were used to locate the nesting colonies. Stork nests were counted during ground surveys or, when ground surveys were not possible, from photographs taken during aerial surveys. During 2012, storks nested in the following counties: Beaufort (5 colonies), Charleston (4 colonies), Colleton (3 colonies), Georgetown (2 colonies), Horry (3 colonies), and Jasper (2 colonies). Four of the colonies where storks nested during 2011 but not 2012 were found to be dry during the surveys.

Drought conditions leading up to the breeding season may be responsible for the decrease in stork nests. Stork colonies (also called rookeries) in Georgia and northern Florida also appear to have been adversely affected by weather conditions this year. Favorable weather conditions for storks include a wet season (fall and winter) prior to the onset of nesting followed by dryer conditions during the nesting season. Storks typically nest in trees growing in water or on small islands. Alligators living below the stork nests deter raccoons and other animals from preying upon the stork eggs and chicks. In addition to requiring water in the colonies for protection, rain during the months leading up to the nesting season increases the amount of fish and other food that is available to nesting storks.

Unlike herons and egrets, which hunt visually, wood storks are tactile feeders, which means that they hunt by feeling for fish, crustaceans, and other prey. This feeding strategy requires high concentrations of prey in water that is shallow enough for storks to wade. When natural and impounded wetlands with abundant fish slowly dry out, excellent feeding conditions are created for the storks. In order for the storks to nest successfully, prey must be abundant and available throughout their nesting season. When adequate food is not available, adult wood storks will abandon their chicks and leave the area to find food.

During 2011, DNR began to monitor individual stork nests in a few index colonies to determine how successful the storks are at raising young in South Carolina. This year, DNR staff and two extremely dedicated volunteers monitored nests at seven colonies located between Hilton Head and Charleston. Two of the colonies are on land managed by DNR (Donnelley Wildlife Management Area and Dungannon Plantation Heritage Preserve), and the other five colonies are on private land. Nests were mapped when the storks started incubating eggs and monitored from a distance until the chicks reached fledging age (mature enough to fly, which is about 8 weeks after hatching). The average number of chicks survived to fledging age per nest was determined for each colony.

In addition to initiating fewer nests compared to 2011, when an average of 1.6 chicks fledged per nest (89 nests, 2 colonies), storks raised fewer chicks per nests during 2012. This year, a total of 312 stork nests were monitored in seven colonies, and an average of 1.1 chicks fledged per nest. The federal recovery goal for wood storks is an average of 1.5 chicks per nest. Fluctuations in the number and success of nests among years are normal.

An aerial survey was conducted in late June to determine if storks reproduced successfully at the colonies where nests were not monitored. The storks appeared to have had a successful season at 15 colonies, but failed to fledge a significant number of chicks at 4 colonies. Potential causes of colony failure for wood storks include predation, inadequate food during the chick rearing period, and disturbance. Storks nested successfully in all seven of the index colonies that were monitored this year, but an average of 40% of the monitored nesting attempts failed to produce any fledglings. Low prey availability is suspected to have resulted in nest failures this year, particularly for colonies farther from the coast where storks feed in tidal creeks at low tides.


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