John Kelly and Emiko Condeso
Audubon Canyon Ranch, Cypress Grove Preserve
Although shorebirds are clearly using the new wetlands, it would be valuable to know if the increased habitat provided by the Giacomini Wetlands Restoration Project will actually increase the size of wintering shorebird populations in Tomales Bay. Continued monitoring of bird use is likely to provide an answer. However, based on current results, which reflect sometimes dramatic variation in shorebird numbers in both the Giacomini Wetlands and throughout Tomales Bay, it is too soon to know.
Frequent movements of shorebirds from the Giacomini Wetlands to and from other areas in southern Tomales Bay challenge interpretation of monitoring data. However, efforts to distinguish between changes that are related to restoration versus those caused by other factors are distinctly improved by the fact that Tomales Bay probably supports two distinct wintering populations of most species.
Past research has shown that shorebirds concentrate at the two ends of Tomales Bay--at Walker Creek delta to the north and Lagunitas Creek delta to the south--where suitable intertidal feeding areas are the largest. However, it is difficult to know how many birds are likely to occur in any one place. Shorebirds use feeding areas throughout the bay and numbers vary dramatically over time and space. In addition, Tomales Bay shorebirds forage in nearby seasonal freshwater wetlands that become suitable during winter. These areas include the dune slack meadows of Lawson's Landing at the north end of the bay, the extensive, seasonally wet lowlands east of Tomales, and, prior to restoration, the seasonally wet pastures on the Giacomini Ranch. To further challenge our understanding of shorebird use, baywide numbers vary with other, extrinsic influences, such as reproductive rates in the arctic, waves of migrant traffic moving through the area, and storm-driven, midwinter flights to other regions.
In spite of the substantial variation in shorebird use of Tomales Bay, observations of flock movements and experiments with banded birds (Kelly 2001) have revealed an important aspect of shorebird use: most species are represented by two distinct wintering populations--same species, but different groups of individuals--that occupy opposite ends of the bay. This information allows us to monitor shared (regional) variation in winter shorebird abundance and test predictions about changes in the proportional use of southern Tomales Bay after restoration by using abundances in northern Tomales Bay as a control for comparing abundances in southern Tomales Bay before and after restoration. Information about what might have occurred in southern Tomales Bay without restoration comes from the pre-restoration shorebird survey data collected by ACR since 1992, 16 years prior to full breaching of the former Giacomini Ranch levees.
So far, with the exception of fewer-than-expected numbers in early winter of 2008 and late-winter 2011, shorebird numbers in southern Tomales Bay closely match predictions before restoration. However, these results are only anecdotal because they do not account for annual variation in regional numbers since the 2008 restoration. Because the effects of habitat restoration on shorebird use are continuing and are likely to build over time, winter shorebird numbers could increase in future years.
However, where exceedances of predicted conditions are observed is in abundances of particular shorebird species after restoration of the Giacomini Wetlands. The number of Greater yellowlegs in southern Tomales Bay has been slightly greater than expected. Since the restoration began, flooding in the northeast portion of the Giacomini Wetlands has resulted in persistent shallow ponding, which favors Greater yellowlegs. In Southern Tomales Bay, greater-than-expected numbers of Dowitchers (primarily Long-billed Dowitchers) occurred during the last two winters. Dowitchers wintering in this area of California typically reflect a predominance of Long-billed dowitchers; Short-billed dowitchers are more abundant during migration. The late winter decline in 2011 (see main story) was associated with a virtual absence of Dowitchers using Tomales Bay.
With the exception of low numbers at the end of the 2010-2011 winter, the number of Least sandpipers in southern Tomales Bay has been slightly greater in 2009 and 2010 than predicted without restoration. The same was true of Western sandpipers, with numbers exceeded unrestored conditions in 2009 and 2010, but not 2011. Annual influxes of Black-bellied plovers into Giacomini have coincided with greater-than-predicted numbers in southern Tomales Bay. The late winter declines in 2010 and 2011 were regional as numbers plovers dropped to nearly zero throughout the bay.
Shorebird use of the Giacomini Wetlands is likely to continue evolving, with changes in the relative abundances of species as drainage patterns, vegetation, and patterns of tidal exposure continue to evolve. Whether these changes will increase the number of shorebirds in Tomales Bay or, alternatively, provide benefits limited to the enhanced use of alternative (restored) feeding areas, remains a mystery. However, this mystery is likely to be resolved during the next few-to-several years by continued monitoring of shorebird use.
One aspect of the restoration process that is likely to benefit shorebirds is the likely population growth of amphipods, polychaete worms, tiny clams, and other intertidal invertebrate prey, as restored tidal feeding areas continue to mature. However, this process is gradual.
Shorebird numbers in Tomales Bay often decline in response to increased winter rainfall, increased stream flow, and lower salinities (Kelly 2001a, 2001b). Periods of heavy winter runoff occur frequently in the Giacomini Wetlands, and heavy runoff can kill, smother, or wash away invertebrate prey. Reduced salinities can cause estuarine prey to sink deeper into the mud, beyond the reach or foraging shorebirds. Storm-driven deposition of sediment and debris can degrade the quality of shorebird feeding areas. The sensitivity of shorebirds to such processes is associated with dynamic variation in their use of southern Tomales Bay, which is influenced strongly by runoff from Lagunitas Creek.
Small sandpipers, such as Western Sandpipers and Dunlins, forage on the thin, slimy layer of biofilm that forms on tidally exposed mudflats (Kuwae et al 2008) and provides a nutritious, mucilaginous mixture of microbes and detritus. Heavy runoff during harsh winters might dramatically alter the formation and availability of biofilm for small shorebirds. Therefore, the benefits of restoring shorebird foraging habitat in a seasonally dynamic area such as the Giacomini Wetlands are difficult to determine.
Dynamic hydrographic conditions in the Giacomini Wetlands are likely to be associated with dramatic variation in shorebird use. Although we are seeing clear benefits and increased use of the Giacomini wetlands by shorebirds, we don't yet know how or when or to what extent this area will benefit these birds. Shorebirds might benefit strongly from the new wetland feeding areas, but numbers are likely to climb and fall dramatically if feeding areas are periodically degraded by heavy winter storms and flooding.
Kuwae, T., P. G. Beninger, P. Decottignies, K. J. Mathot, D. R. Lund, and R. W. Elner. 2008. Biofilm grazing in a higher vertebrate: the Western Sandpiper Calidris mauri. Ecology 89:599-606. (218 KB PDF)