eWestern Birds

Abundance and Distribution of Shorebirds in the San Francisco Bay Area

Lynne E. Stenzel, Catherine M. Hickey, Janet E. Kjelmyr, and Gary W. Page

ABSTRACT: On 13 comprehensive censuses of the San Francisco-San Pablo Bay estuary and associated wetlands we counted 325,000-396,000 shorebirds (Charadrii) from mid-August to mid-September (fall) and in November (early winter), 225,000 from late January to February (late winter); and 589,000-932,000 in late April (spring). Twenty-three of the 38 species occurred on all fall, early winter, and spring counts. Median counts in one or more seasons exceeded 10,000 for 10 of the 23 species, were 1,000-10,000 for 4 of the species, and were less than 1,000 for 9 of the species. On rising tides, while tidal fiats were exposed, those fiats held the majority of individuals of 12 species groups (encompassing 19 species); salt ponds usually held the majority of 5 species groups (encompassing 7 species); 1 species was primarily on tidal fiats and in other wetland types. Most species groups tended to concentrate in greater proportion, relative to the extent of tidal fiat, either in the geographic center of the estuary or in the southern regions of the bay. Shorebirds' densities varied among 14 divisions of the unvegetated tidal fiats. Most species groups occurred consistently in higher densities in some areas than in others; however, most tidal fiats held relatively high densities for at least one species group in at least one season. Areas supporting the highest total shorebird densities were also the ones supporting highest total shorebird biomass, another measure of overall shorebird use. Tidal fiats distinguished most frequenfiy by high densities or biomass were on the east side of central San Francisco Bay and adjacent to the active salt ponds on the east and south shores of south San Francisco Bay and along the Napa River, which flows into San Pablo Bay. The bay is critical to large numbers of wintering, migrating, and breeding shorebirds, despite extensive loss of natural wetlands. Geographic limita- tions of species' distributions in the bay should be considered when wetland restora- tion is planned.

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