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Definition and Characterization of
the Sources of DDT Contamination Currently Impacting White-faced Ibis
Populations
The white-faced ibis (Plegadis
chihi) is a long-legged wading bird that feeds primarily on invertebrates in
wetlands and irrigated croplands. It
is a highly social colonial nester and often forages in large aggregations.
Due to restricted nesting habitat and potential vulnerability to
pesticides, the species is listed by the U.S. Fish and Wildlife Service as a
migratory non-game bird of management concern.
Dr. Charles Henny (U.S. Geological
Survey; USGS) has documented continued troubling levels of DDE in a large
segment of the white-faced ibis population nesting at Carson Lake in western
Nevada. Of twenty eggs collected in
1996, 45% contained >4 ppm DDE, and eggshells averaged 18.3% thinner than
normal. This represented no
improvement from DDE levels documented in 1985-86, which is contrary to patterns
shown for most other avian species in the USA.
Analyses of upper digestive tract contents in 18 Carson Lake ibis
revealed no organochlorine insecticides; the conclusion is that DDE/DDT
contamination is not occurring on the Nevada breeding grounds.
Identification of contaminant sources is highly desirable because it is
likely that different regional cohorts of ibis and other wildlife species are
frequenting those same areas.
Accordingly the Raptor Research
Center, along with Earthspan, the Center for Conservation Research and
Technology, USGS, U.S. Fish and Wildlife Service, and Nevada Division of
Wildlife conducted a pilot study in 1997 to determine whether white-faced ibis
could be tracked via satellite-received telemetry. This was part of another study funded by the Department of
Defense (DoD) Legacy Resource Management Program. Two ibis were successfully
tracked. This proof-of-concept
convinced Legacy to fund an investigation into source(s) of DDE contamination in
ibis, which are known to frequent various DoD lands throughout the west.
In
early May 2000, with our cooperators, we captured 15 white-faced ibis from the
vicinity of a nesting colony at Carson Lake, NV.
Ten birds were marked with ~20g satellite-received Platform Transmitter
Terminals (PTTs). Blood samples
(~1ml) were taken from the brachiocephalic vein.
PTT transmissions were received by Argos satellites and location
estimates relayed to researchers. To
conserve battery energy to obtain most locations after the birds left the
breeding area, we programmed the units in an unconventional and untested manner.
During the breeding season, the PTTs transmitted only six hours every two
weeks, while we hoped to maintain battery integrity with minimal loss of
capacity. The PTTs were programmed
to begin transmitting eight hours every three days in October, near the time our
subjects should be departing the breeding ground.
The plan was a success, and seven PTTs produced high quality locations
for the detailed information we needed to identify potential sources of DDT. An
eighth PTT provided good data, but the ibis apparently died in western Colorado
while still on migration.
Seven of our ten subjects had detectable blood DDE levels;
three had high levels (>0.25 ppm), and four others low levels (<0.15 ppm).
Winter fieldwork began in January 2001. Using the most accurate Argos location
data to guide us, we visited the wintering sites of our sentinel ibis from
Carson Lake and collected samples of invertebrates where ibis were feeding.
Such sampling may reveal significant sources of contaminants posing a
threat to numerous species. Information
revealed by the technology applied on this project prompted, in another study,
mitigation of organophosphate pesticide poisoning of thousands of Swainson’s
hawks in Argentina. We made
necessary collections from four such sites in California’s Central Valley
between the Bakersfield and Yuba City areas, and documented associated birdlife,
habitat characteristics, and farming practice in those areas. We had excellent cooperation from landowners and managers.
A colleague collected samples and similar information where another
individual apparently died on its wintering area in Jalisco State, Mexico. Two
other individuals wintering in Mexico (Colorado River Delta area) had high DDE
blood levels, but collection permits were not forthcoming in time to visit the
area during the wintering period. Analyses of invertebrate samples revealed
elevated DDE levels in some earthworm samples, so further collections at
wintering sites focused on earthworms.
In late May 2003, with Legacy and Earthspan’s support, we
blood sampled and outfitted another ten ibis at Carson Lake with PTTs. All ten
had detectable blood DDE levels, and they were generally higher than those
recorded for 2000
subjects. Five of the
ten had high levels (>0.25 ppm), and three of those were higher (range 0.744
- 0.99 ppm) than any recorded in 2000. Two
had medium levels (0.15 – 0.24 ppm), and three had low levels (<0.15 ppm).
Satellite locations defined the
wintering areas of five subjects, three in Mexico and two in California’s
Central Valley.
During
February 2004 earthworm collections were made in Mexico at the three current
wintering sites and at the two 2001 wintering sites that could not be visited
previously. Three of those sampling sites were in the Colorado River Delta area,
and the blood DDE levels of the ibis associated with those sites were among our
highest recorded during the study. We also made earthworm collections at the two
current California wintering sites and at three 2001 sites where we had
previously sampled different invertebrates. Analysis found high DDE levels
only among earthworms collected at the three Mexicali Valley wintering sites,
identifying that locale as a significant source of organochlorine contaminants.
We wish to acknowledge the
participation and contributions of:
Questions and comments regarding the Raptor Research Center: mark_fuller@usgs.gov
Questions and comments regarding this website:
kbledsoe@boisestate.edu
This page was last reviewed on 20 December 2006.