In November 1989, Audubon Canyon Ranch (ACR) began an ongoing program to monitor seasonal populations of shorebirds (Scolopacidae and Charadriidae) on Tomales Bay. Three baywide counts, treated as replicates, are completed within each Nov.-Dec. and Jan.-Feb. census period, to estimate early winter and late winter population levels, respectively. An additional baywide count is conducted each August and April in an attempt to record peak numbers of fall and spring migrants. Tomales Bay is divided into 11 count areas, which are covered simultaneously by teams of qualified volunteer field observers. Each count requires 60-90 minutes to complete. Some counts have been coordinated to coincide closely with other counts conducted at Bolinas Lagoon and Point Reyes area (Gary Page, Point Reyes Bird Observatory), and at Bodega Harbor and Estero Americano (Peter Connors, Bodega Marine Lab). Tomales Bay supports 11,000 to 18,000 shorebirds in early winter, and up to a third of the wintering shorebirds along coastal Marin and Sonoma Counties. Most wintering populations on Tomales Bay decline gradually through winter and may show an increase during spring migration (April). The most abundant wintering shorebird species on Tomales Bay are Dunlin, Western Sandpiper, Least Sandpiper, Marbled Godwit, Willet, and Black-bellied Plover. Shorebirds concentrate primarily at the north and south ends of the bay, near the mouths of Walker and Lagunitas creeks. Restricted daily movements of winter flocks between the north and south Tomales Bay suggests that Tomales Bay may support two wintering populations of some shorebird species.