Mercury effects in San Francisco Bay-Delta birds


Caspian TernThe Bay-Delta watershed has a history of mercury contamination resulting from mercury mining in the Coast Range and the use of this mercury in gold mining activities in the Sierra Nevada Mercury can cause a serious health threat to birds, in particular those that eat fish. A survey of mercury in bird eggs from both San Francisco Bay and the Sacramento River/San Joaquin River Delta done in 2000 and 2001 found dangerously high levels of mercury in some bird eggs. As a result we have been funded to do a detailed study to determine the effects these high levels of mercury may be having on the birds.

Project Goal

Our goal is to use a combined field and laboratory approach to estimate the risks of mercury to birds in the San Francisco Bay-Delta. This information is necessary to develop and prioritize ways to restore wetlands that will not increase the amount of toxic mercury available to birds.

We will study three different kinds of birds—terns (Caspian terns and Forster’s terns), shorebirds (American avocets and black-necked stilts), and diving ducks (surf scoters). Each of these birds feed in different areas on different kinds of food.The terns eat only fish, the shorebirds eat bugs living in the mud and water, and diving ducks eat mussels.


In the field biologists will place radio transmitters on adult birds and track them to see where, what, and how much they are eating. Then samples of the food items from those areas are collected to see how much mercury is in the bird’s diet.By collecting eggs, blood, feathers, and tissue from some of the birds we can tell how much of the mercury stays inside the birds and how much goes into the eggs. Careful observations of the bird nests and chicks will tell us how the mercury might have harmed the birds. Observations we make include failure of eggs to hatch, deformed embryos in the eggs, tissue damage, odd behavior of chicks, and survival of chicks.

In the laboratory mercury will be injected into bird eggs to see how sensitive the developing embryo is to mercury.We can then compare the field results to those in the laboratory to confirm the effects we see in the field and determine how much risk the birds are at from mercury.

This work is done in cooperation with other studies on mercury sources, movement of mercury in the environment, and availability of mercury to fish and wildlife funded by the California Bay-Delta Authority (CALFED).

For more information, contact Tom Maurer, USFWS, at (916) 414-6590