Wood Storks (Mycteria americana) are large
wading birds that typically feed on fish in shallow wetlands. In
the U.S. they occur mostly in Florida, Georgia, and South Carolina.
The North American breeding population of Wood Storks decreased
from about 20,000 pairs in the 1930s to less than 4,000 pairs
in 1983. Since then the population has increased to more than 6,000
pairs. The Wood Stork was listed as a federally endangered species
in 1984 due to population declines resulting from loss of foraging
habitat. They have been feeding and roosting in aquatic habitats
on the Savannah River Site (SRS) since the 1950s, including
the Steel Creek Delta of the Savannah River swamp system. When the
decision was made in the early 1980s to restart the Sites
L-Reactor, concerns were raised regarding the Wood Stork because
restarting the reactor would raise water levels too high for the
Steel Creek Delta to be used as foraging habitat. After consultation
with the U.S. Fish and Wildlife Service (USFWS), a research program
focusing on Wood Storks was created at the Savannah River Ecology
Laboratory (SREL) to monitor stork use of the SRS and to study the
breeding biology and ecology of the Wood Stork in this unstudied
(northern) portion of its range. Using research findings from the
initial years of this program, impoundments stocked with fish were
built to "replace" the potentially impacted Steel Creek
Delta. This habitat, the Kathwood Foraging Ponds, was constructed
in cooperation with the National Audubon Society (NAS) on its nearby
Silverbluff Sanctuary. Research on this endangered species also
focuses on issues such as the effects of contaminants.
SREL stork research overview
SREL stork studies initially focused on the Birdsville
breeding colony in Jenkins County, GA, which was thought to be the
source of the storks commonly observed on the SRS. Wood Stork breeding
success has been studied at this colony since 1984. These studies
have shown that rainfall patterns and their effects on colony water
levels and foraging site hydrology generally determine how many
nestlings are produced and survive until fledging age. Analyses
of foraging flights from the Birdsville colony suggested that storks
rarely traveled as far as the SRS to feed during the breeding season,
and that late-summer/early fall use of SRS wetlands was more common
after the birds dispersed from their breeding colonies. This conclusion
is supported by nearly 1,000 aerial surveys for storks in the Savannah
River swamp system of the SRS since 1983. To feed efficiently, Wood
Storks were shown to require shallow wetlands with high densities
of prey. Environmental disturbances that affect these habitats can
limit the success of these storks.
Kathwood foraging ponds
Twenty-five acres of managed ponds were constructed
at Kathwood Lake in 1985-86. These impoundments are jointly managed
by NAS and SREL, which study the wading birds using the ponds.Wood
Storks have fed in the ponds every year since their construction,
with 305 storks observed in one pond on one day in August, 1998.
Storks that were color-banded as nestlings in Birdsville and a coastal
Georgia breeding colony have been observed at these ponds. These
impoundments may be particularly beneficial as foraging habitat
for juvenile storks, which typically make up 60-75% of the storks
using the ponds, enhancing survival at a critical time of their
The impoundments also function as an "outdoor"
laboratory for studying the behavior of storks, including feeding
rates and competition with other wading birds. Data collected from
SREL studies at Kathwood have indicated that storks are very active
nocturnally, feeding at least as much at night as during the day.
Collection of behavioral information, such as feeding rates and
length of time storks actively feed, is necessary to determine potential
risks from contaminants to storks that may forage on the SRS.
Current research topics
SREL continues to monitor the use of SRS wetlands
by storks to ensure compliance with USFWS recovery objectives for
the Wood Stork. With the cessation of reactor operations, SRELs
stork research has focused more on contaminant concerns, primarily
on determination of possible risks associated with mercury in SRS
wetlands. SREL monitors mercury concentrations in prey-sized fish
in historical, active, and potential stork foraging sites and also
studies the processes that affect those concentrations. Many feeding
sites are Carolina bays and other temporary wetlands that do not have known sources
of contamination. However, mercury has been found in stork prey
from these sites; this mercury presumably results from atmospheric
deposition and its availability to potential prey is enhanced by
seasonal water fluctuations characteristic of temporary wetlands.
Prey-sized fish from SRS reservoirs also are monitored for mercury.
Although storks do not typically feed in lake systems, they did
feed in one SRS reservoir when it was drawn down for repair in 1991.
SREL, in a project partially funded by USFWS, is analyzing mercury
levels in stork nestlings and the prey they receive as food. Monitoring
the use of wetlands by storks and the levels of contaminants in
potential prey, actual prey, and nestlings will lead to more accurate
estimates of effects, if any, associated with Wood Storks feeding
on the SRS.
Other SREL Wood Stork studies include coastal foraging
ecology, large-scale movement patterns monitored via satellite telemetry,
and determination of stork foraging strategies using stable isotopes.