Article
Elemental contaminants in the livers and ingesta of four subpopulations of the American coot (Fulica americana): an herbivorous winter migrant in San Francisco Bay

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Abstract

Water birds with diets high in animal foods in the San Francisco Bay area are exposed to trace elements that are potentially health impairing. Water birds with herbivorous diets have been less thoroughly examined. The concentrations of trace elements in the livers and the esophageal contents of an herbivorous water bird, the American coot (Fulica americana) were measured to compare levels of contaminant exposure among different locations in the Bay system and with other water birds. A total of 39 coots were collected from four sites: Napa River and Mare Island Strait in the north, Berkeley in the middle, and Coyote Creek in the south. Livers of Berkeley samples differed significantly from those of Napa River and Mare Island Strait by their greater concentrations of As and B and lower concentrations of Cu, but they seemed to be within normal ranges for birds. Otherwise the concentrations of trace elements in the livers did not differ among sites. Ingesta samples from Berkeley differed from the other sites because they tended to be higher in Al, V, and Zn. In contrast to waterfowl, livers from the herbivorous coots in San Francisco Bay showed little exposure to Cd, Hg, Pb, or Se. Coot ingesta showed few samples with measurable levels of Cd, Hg, or Se and had low levels of Pb. The herbivorous diet of coots may shield them from exposure to such elements. However, high levels of V were present in coot livers and ingesta from all four sites, suggesting adaptation to this toxic element.

Introduction

The greater San Francisco Bay (Bay) is a major resource used by migratory birds in western North America. The Bay system has been designated one of the four sites in the Pacific Flyway that are of hemispheric importance (Harvey et al., 1992). The Bay receives water from the San Joaquin and Sacramento Rivers which drain California's Central Valley and from local drainages such as the Napa River and Coyote Creek (Fig. 1). The heavy agricultural use of the Bay's watershed, the mining history along rivers in northern California, modern industry, and dense urbanization around the Bay, subject San Francisco Bay and its freshwater sources to many substances of anthropogenic origin.

Studies of the contaminant burdens of avian migrants show distinct spatial patterns of elemental contaminants in the Bay (Ohlendorf et al., 1991, Miles and Ohlendorf, 1993). These studies have been of birds with a large component of animal matter in their diets. Birds that have a largely herbivorous diet may be subject to different contaminant exposures. In Suisun Bay (Fig. 1) direct comparisons of elemental analytes in the livers of coots (Fulica americana) and surfscoters (Melanitta perspicillata) showed distinctly different burdens of some elemental analytes, due partially, if not primarily, to dietary differences there (Möller, 1996). Information about herbivorous birds in different parts of the Bay will help balance our understanding of the spatial distributions and potential effects of elemental contaminants on the avifauna in the San Francisco Bay system.

I present here a study of elemental contaminants in the ingesta and livers of American coots collected at four sites in the San Francisco Bay system. These sites are far enough apart to ensure they are not subject to the same (if any) contaminant source (except the two northern sites which enables different interpretation). Coots are primarily herbivorous throughout the annual cycle with their low consumption of animal foods increasing during the breeding season (Alisauskas and Arnold, 1994). In the western states, they breed primarily in the Dakotas, Montana, and southern Canada (Alisauskas and Arnold, 1994). In the Bay area, the coot populations consist of resident and winter migrants (Gullion, 1953).

Section snippets

Sample collection

From 10 February to 11 April 1995, 39 coots were shot using steel shot at four locations where coots are known to occur in the Bay area. From north to south, the collection sites were the Napa River north of the Highway 37 bridge (122°13′W×38°09′N) (n=10), Mare Island Strait on the Napa River south of the Highway 37 bridge (122°11′W×38°06′N) (n=9), Berkeley (122°18′W ×37°52′N) (n=10), and Coyote Creek (122°57′W ×37°28′N) (n=10) (Fig. 1). The tarsal colors were noted and the specimens packed in

Specimens

The collection consisted of only two females at each of three sites and four at the Napa River site, so the numbers of females are inadequate for separate statistical treatment. Sex was ignored in the statistical testing.

The Bursa of Fabricius was less than 3 mm in diameter in 31 specimens, was greater than 3 mm in diameter in four specimens, and was not found in four specimens. The Bursa of Fabricius indicates a coot is a juvenile if its diameter exceeds 3 mm during January–May (Eddleman and

Specimens and samples

The sampled birds were all adults. Whether winter migrants or year-round residents, they very likely had several seasons of ingesting the local forage with its contaminants. Although the resident birds may be carrying different burdens than the migrants, it is not possible to separate the two groups.

The Napa River and Mare Island Strait sites were adjacent sites along the Napa River (Fig. 1). Site differences might be expected due to the industrialization at Mare Island but the MANOVA indicated

Conclusion

Migratory birds can be exposed to elemental contaminants on the wintering grounds or on their breeding sites or along the migratory route. For the herbivorous American coots on their San Francisco Bay area wintering sites, there were no indications that they carry burdens similar to those of birds with large components of animal material in their diets which also winter in the Bay. These diet-related differences support Möller's (1996) findings in his study of coots on Suisun Bay. However,

Acknowledgements

I thank A.K. Miles and S.E. Wainwright for assistance with the field collection; J. Cheung and H. Wei for assistance with dissections; and D. Roster of the Don Edwards San Francisco Bay National Wildlife Refuge for his cooperation and support for collections in Coyote Creek. I also thank R. Hothem, A.K. Miles, G. Heinz, T. Custer, and two anonymous reviewers for comments on an earlier draft of this manuscript.

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