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Stanislaus River Report
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Stanislaus River Basin and Calaveras River Water Use Program|
Threatened and Endangered Species Report - March 1995
Bay Delta and Special Water Projects Division, CA Dept. of Fish and Game
Western GrebeAechmophorous occidentalis
California Department of Fish and Game "Special Animal"
Life HistoryThe western grebe is a black and white bird measuring about 25 inches in length with a long, swan-like neck and long, thin bill. The black coloration extends from the forehead, along the back of the neck, and over the entire back. In the dark phase, this black cap includes the eyes and the bill is yellow-green in color; the light phase does not have such an extensive black cap and the bill is yellowish-orange in color. The dark phase is the predominate color in the north and east while the light phase is predominate in the south. Calls of the two phases are distinctive of one another. The dark phase has a two-note courting call while the light phase has a longer single note.
Courtship occurs from April to May and nests are occupied from May to August. Courtship consists of dramatic displays that involve sprinting across the water with their necks' kinked. These gregarious birds require large, open water areas for courtship, feeding, and flocking, and frequent extensive beds of tall, emergent vegetation such as tules or cattails for nesting. These monogamous, colonial nesters construct a nest platform that is built up from the bottom and floats in water up to ten feet deep. The nest is usually constructed near open water. The clutch size is usually three to four eggs and replacement clutches are not uncommon. They are single brooders but both parents care for the precocial young. Incubation is about 23 days long. Upon hatching, they are tended to for about four to five weeks at which time they are almost full grown.
All food is obtained by diving and pursuing prey at depths of at least four feet but it is not uncommon for them to use shallower water during the summer months. The diet consists primarily of fish but also includes insects and other invertebrates, but rarely includes amphibians and plants. As with other grebes, feathers are eaten by adults and young and accumulate in the stomach.
Findings and ConclusionsThe destruction of wetlands and the introduction of pesticides into watersheds are major causes of a continuing decline in numbers (Feerer and Garrett 1977). Other human disturbances such as lakeshore development near nesting colonies and disturbance by boaters and fisherman are also detrimental (Gould 1974, Lederer 1976).
Any project affecting flows of the Stanislaus River or the wetlands associated with the river could have adverse affects on the western grebe and would warrant further studies.