Tidal Flats – Shorebirds
What is Tidal Flat?
Updated: 20250808
Tidal Flats are alternately exposed and submerged by the daily tides. This dynamic habitat is made of fine-grained sediments that build up in sheltered areas where wave energy is low. Tidal flats host important biodiversity both under the surface (rich invertebrates), on the surface (biofilm and other algae) and above the surface (shorebirds, fish when it’s underwater). This habitat is critical resource for migratory shorebirds who fuel up in SF Bay before heading to their summer breeding grounds farther north and again during their return south.
Wintering shorebirds have declined but there are signs of a rebound in recent years.
Large Shorebirds: Willet (Tringa semipalmata), American Avocet (Recurvirostra americana), and Marbled Godwit (Limosa fedoa) forage in deeper water with their longer bills that can probe for invertebrates in places that medium and small shorebirds can’t.
Medium Shorebirds: Short- and Long-billed dowitchers (Lindronomous griseus, L. scolopaceus) probe the mud with rapid sewing machine-like movements and can forage in xxx deep water with their long bills. Black-bellied Plover (Pluvialis squatorola) on the other hand are short-billed and hunt for insects like flies and other invertebrates living on the surface. They spy prey with their large eyes and rush to snatch the treat from the muddy surface.
Small Shorebirds: Dunlin, (Calidris alpina), Western Sandpiper (C. mauri), and Least Sandpiper (C. minutilla) forage in exposed tidal flats and in shallow waters. Least Sandpipers use early restoration sites, tidal channels, and other partially vegetated intertidal areas.
Tidal Flat Multi-Species Indicator
Tidal Flat Indicator Species Groups
Shorebird numbers in the SF Bay have declined since 2006, reflecting similar declines in nearby habitats such as Bolinas Lagoon and Tomales Bay, as well as across the Pacific Flyway. In recent years, the declines appear to be stabilizing or even increasing slightly for some shorebird groups in some subregions.
- North Bay shorebirds overall declined by 26 to 68%
- Central Bay shorebirds declined the most, by 41 to 81% where there is the least amount of tidal flat habitat.
- South Bay shorebirds declined by 42 to 67%
Methods
Data on wintering shorebirds in tidal flats were collected during high tide surveys across North and South San Francisco Bay. Surveys were conducted annually between November 15 and December 15, with all units surveyed on the same day to minimize shifts in bird distributions.
Shorebirds in the San Francisco Bay are declining, a trend also seen in nearby areas like Bolinas Lagoon and Tomales Bay, and across the entire Pacific Flyway.
Since shorebirds migrate, these declines might be due to broader issues outside the Bay. However, because different shorebird groups in the Bay (with varied breeding grounds and migration paths) are all declining, it suggests that changes to their stopping points during migration or their wintering grounds—such as habitat loss or more predators—could be responsible.
Encouragingly, recent years show a slight recovery for large and medium shorebirds in the North Bay, and for small and medium shorebirds in the South Bay, hinting that habitat management efforts in these areas are working.
Large Shorebirds
Large shorebirds decreased the in the Central and South Bay and the decline in the North Bay may be leveling off.
American Avocet
Marbled Godwit
Willet
Medium Shorebirds
Medium shorebirds decreased the most overall (-74% if we can average across subregions) with the biggest decline in the Central Bay.
Black-bellied Plover
Short-billed Dowitcher
Long-billed Dowitcher
Small Shorebirds
Small shorebirds decreased the least with the largest declines in the Central and South.
Least Sandpiper
Western Sandpiper
Dunlin
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