Human-Induced Contaminant Impacts on Migratory Birds: Lessons from the North American Eared Grebe (Podiceps nigricollis)
American Association of Zoo Veterinarians Conference 2003

Kurt K. Sladky1,2, MS, DVM; Charlotte Quist3, DVM, PhD, DACVP; Pedro Ramirez4, MS, David Hill5, BS; F. Joshua Dein1,6, VMD, MS

1Conservation Health Consortium, School of Veterinary Medicine, University of Wisconsin, Madison, WI, USA; 2Department of Surgical Sciences, School of Veterinary Medicine University of Wisconsin, Madison, WI, USA; 3Wildlife Health Associates, Inc., Dillon, MT, USA; 4United States Fish and Wildlife Service, Cheyenne, WY, USA; 5North Carolina Zoological Park, Hanes Veterinary Medical Center, Asheboro, NC, USA; 6United States Geological Survey-National Wildlife Health Center, Madison, WI, USA


The effects of aquatic contaminants generated by soda ash mining processes on the North American eared grebe (Podiceps nigricollis) population illustrates important issues associated with human­induced habitat degradation on the health of a migratory species. Eared grebes have been extensively studied in their staging and breeding habitats, but little is known about their 2- to 3-day migratory periods.2,3,5 During migration, few bodies of water are available to the birds for refuge between freshwater breeding areas in Canada and hypersaline lakes (e.g., Great Salt Lake in Utah or Mono Lake in California) to which they migrate. One geographic refuge area includes a series of "tailings" ponds associated with soda ash mining operations in southwestern Wyoming. The ponds range from 100–1,200 acres, with water containing high concentrations of sodium decahydrate (Na2CO3 • 10H2O). At cool temperatures (generally <40°F) sodium decahydrate precipitates out of the water and crystallizes on solid objects in the ponds or on the water surface. Bird mortality on these ponds has been recognized since the early 1970s, and the mining companies have developed hazing strategies and rehabilitation programs in order to minimize mortality. In order to determine causes of grebe mortality and devise strategies to reduce mortality, a field epidemiologic investigation was developed with the following objectives: 1) to determine whether eared grebes have quantifiable physiologic abnormalities associated with exposure to soda ash mine pond water; 2) to evaluate physical effects of sodium decahydrate crystallization on grebe survival; 3) to establish cause of death based on necropsy of deceased grebes; 4) to determine long-term survivability of eared grebes after exposure to the pond water; and 5) to evaluate water quality and determine whether aquatic invertebrates are present in the ponds as a possible food source.

This is an ongoing study, with objectives still in progress. Preliminary data presented here include serum biochemical parameters from 27 adult, eared grebes of both sexes, and brain biochemical concentrations from five adult and juvenile, eared grebes collected in August 2001 and August 2002. Grebe "controls" refer to grebe data collected from birds captured or shot on Mono Lake (n=8 for serum biochemical data; n=2 for brain biochemical data). Blood samples were opportunistically collected from grebes caught on the ponds, and our sampling protocol was incorporated into the standard rehabilitation procedures employed by the mines. We requested that all birds captured be maintained on freshwater for 24 hr in order to collect a pre- and post-capture blood sample for comparison of serum biochemical changes over time. Water samples from several different ponds were analyzed in August 2000.

Eared grebes captured on soda ash mine pond water had significantly higher mean (±SD) serum sodium concentrations pre-rehabilitation (155.8±5.2 mg/dl) compared to both post-rehabilitation (149.2±3.6 mg/dl) and control grebes (152.3±2.9 mg/dl). Recently deceased eared grebes removed from the surface of the pond water had elevated brain sodium concentrations (4041.6±1895.3 ppm) compared to control grebes from Mono Lake (1429.8±142.3 ppm). All grebes collected at the soda ash ponds had brain sodium concentrations greater than 2,000 ppm, while controls had brain sodium concentrations below 2,000 ppm (Table 1). Gross and histopathologic lesions from the deceased Wyoming grebes were consistent with drowning. Samples of pond water contained sodium carbonate decahydrate (Na2CO3 • 10H2O), with hydrated sodium sulfate (Na2SO4 • 10H2O) and NaCl. The water was highly alkaline (pH>10.2; range 10.2–510.66), salinities were in excess of 174 ppt (range 174–240 ppt), and total dissolved solids ranged between 80,000–150,000 ppm. There was a complete absence of zooplankton in the ponds; however, phytoplankton was relatively dense, with flagellated chlorophytes being most prevalent.

Our preliminary data indicate that eared grebes landing on soda ash mine tailings ponds may be experiencing mortality due to the effects high brain sodium concentrations, with drowning being the ultimate cause of death.1,5 The toxic range for sodium in domestic species is considered to be greater than 2,000 ppm in the brain, and the diagnosis of salt toxicosis in wildlife species has been extrapolated from this information.1,5 However, we currently have limited control data, and cannot attribute cause of death with confidence. It is possible that the normal brain sodium concentrations of eared grebes is high compared to other bird or mammal species, and that this species has evolved physiologic compensatory mechanisms. Alternative explanations for causes of mortality include weight of sodium decahydrate crystal encrustation directly leading to drowning, and hypothermia. Due to the natural history and behavior of the eared grebe, susceptibility to water-borne contaminants may be particularly important. Eared grebes are diving birds, which, when hazed, will retreat underneath the water surface. This defensive behavior keeps them immersed in the water, allowing for more rapid accumulation of sodium decahydrate crystals. In addition, when preening and/or attempting to remove encrusted sodium decahydrate from feathers, eared grebes will ingest feathers and sodium decahydrate crystals, thereby increasing the likelihood of systemic absorption. Unfortunately, grebes are not the only species affected on these ponds, with total mortality of all bird species approximating 1,000–1,500 annually. The mining companies are attempting to develop methods for reducing pond size by either recycling the tailings water or promoting water evaporation. Ultimately, these techniques may successfully reduce bird mortality, but as long as the pond water is present, aquatic birds will continue be impacted.

Table 1. Individual brain sodium concentrations in parts-per-million (ppm) on five eared grebes found dead on soda ash mine ponds in southwestern Wyoming, and two “control” eared grebes collected at Mono Lake, California.

Bird number


Brain sodium concentration

1 (4782-1)

Adult male


2 (4782-2)

Adult female


3 (4782-3)

Juvenile male


4 (4782-4)

Juvenile female


5 (4782-5)

Juvenile female


Control grebe 1



Control grebe 2




This project was funded by the United States Fish and Wildlife Service Ecological Services, through agreements with the USGS National Wildlife Health Center, and the USGS Wisconsin Cooperative Wildlife Research Unit. J. Jehl Jr. has been instrumental in contributing to knowledge of grebe biology and natural history. We gratefully acknowledge S. Hurlbert for his technical assistance, J. C. Franson for access to serum biochemical data from Mono Lake grebes, and the environmental departmental personnel affiliated with the soda ash mining industry in Wyoming (FMC Corp.: C. Demshar, J. Lutz, M. Wendorf, B. Kroll, S. Lee & R. McNally; OCI Wyoming, LP: T. Johnson & L. Cherny; and Solvay Minerals: D. Potter & T. Brown).

Literature Cited

1.  Gordus, A.G., H.L. Shivaprasad and P.K. Swift. 2002. Salt toxicosis in ruddy ducks that winter on an agricultural evaporation basin in California. J. Wildl. Dis. 38: 124–131.

2.  Jehl, J.R. Jr. and C. Johansson. 2002. The autumnal migration of eared grebes through southwestern Wyoming: a key to assessing the size of the North American population. W. N. Amer. Nat. 62: 335–340.

3.  Jehl, J.R. Jr. 1993. Observations on the fall migration of eared grebes, based on evidence from a mass downing in Utah. Condor 95: 470–473.

4.  Jehl, J.R. Jr, and R. McKernan. 2002. Biology and migration of eared grebes at Salton Sea. Hydrobiologia 473: 245–253.

5.  Windingstad, R.M., F.X. Kartch, R.K. Stroud and M.R. Smith. 1987. Salt toxicosis in waterfowl in North Dakota. J. Wildl. Dis. 23: 443–446.


Speaker Information
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Kurt K. Sladky, MS, DVM
Conservation Health Consortium
Department of Surgical Sciences
School of Veterinary Medicine,
University of Wisconsin
Madison, WI, USA

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