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
1930’s 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 1950’s, including the Steel Creek Delta of the
Savannah River swamp system. When the decision was made
in the early 1980’s to restart the Site’s 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 life cycle.
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, SREL’s 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.
Wood Stork Research
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