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Foraging Ecology and Environmental Contaminants in Migratory Birds in the San Francisco Bay Ecosystem

Surf scoter
San Francisco Bay is one of the most important wintering and stopover sites for migratory birds on the Pacific Flyway.  Several species, including sea (tribe Mergini) and bay ducks (tribe Aythyini, genus Aythya), dependent on this estuary for wintering habitat, are currently declining.  Among suggested contributing factors are: alterations in wintering ground conditions, including increased contamination; introductions of non-native invasive species (NIS); and increased disturbance (Savard et al. 1998, N. American Waterfowl Management Plan - NAWMP 2000, Austin et al. 2000). The San Francisco Bay estuary (SFBE) is a highly urbanized area where large numbers of Pacific Flyway diving duck species are exposed to all of these potentially negative influences (Nichols et al. 1986, Carlton et al. 1990, Cohen & Carlton 1995); thus, this system is a key place to test hypotheses on the contribution of wintering ground factors to population declines.

Project Details

Of the several issues facing SFBE wintering diving ducks, accumulation of contaminants is perhaps the best documented (Ohlendorf et al. 1986, White et al. 1989, Ohlendorf et al. 1991, Miles & Ohlendorf 1993, Hothem et al. 1998). Studies conducted in the 1980s indicate that surf scoter (Melanitta perspicillata) and greater scaup (Aythya marila) accumulate higher concentrations of mercury, selenium, and cadmium while wintering in the SFBE than any other waterfowl species (Hoffman et al. 1998).  Such concentrations exceed those that negatively affect reproduction in several other waterbird species, including dabbling ducks (Anas spp.) (Skorupa & Ohlendorf 1991, Heinz & Hoffman 1996).

            Diving ducks accumulate contaminants mainly from their invertebrate prey; thus, knowledge of a species’ foraging ecology is imperative for understanding contaminant uptake.  Benthic invertebrates are known to have elevated contaminant concentrations in SFBE (Luoma & Cain 1979, Luoma et al. 1985, Linville et al. 2002).  In addition, introductions of NIS (Cohen & Carlton 1995) such as the over-bite clam (Corbula amurensis) have drastically altered food resources for avian benthivores.  Recent studies indicate that the over-bite clam can concentrate contaminants like Se as much as three times more than other clam species in the estuary (Linville et al. 2002).

            Reduced habitat quality and outright habitat loss on wintering grounds can influence population sizes of migratory birds (Sutherland & Goss-Custard 1991).  Carrying capacity is frequently used to measure a wintering area’s value to waterfowl.  In complex systems like the SFBE, interactions of such factors as predation, disturbance, contaminants, NIS, and abiotic conditions have the potential to affect prey quality, quantity and accessibility, and thus may influence carrying capacity. Furthermore, even if carrying capacity of an area has not been reached, poor habitat conditions or disturbance may prevent some individuals from meeting their energy needs (Goss-Custard et al. 2002). 

            We designed this study to build upon our previous work with SFBE diving ducks and address major issues identified as possible causes of declines.  First, we will study surf scoter winter foraging ecology and determine relationships between foraging patterns, body condition, and the high concentrations of contaminants they are known to accumulate in SFBE.  Second, we will quantify benthic prey densities and waterbird disturbance with the ultimate goal of modeling energy-based carrying capacity for benthic-foraging waterfowl species.  Lastly, we will pioneer cross-seasonal research to determine whether or not waterbird populations incur reproductive harm due to the elevated contaminants concentrations they accumulate in the estuary. We will use satellite and conventional telemetry to identify migration routes and breeding sites of SFBE scoter and scaup populations, and collect eggs for methyl Hg and other contaminant analyses.

USGS Contact For This Project
Susan De La Cruz
(415) 859-1511
San Francisco Bay Estuary Field Station
505 Azuar Drive
Vallejo, CA 94592
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