Research Summary

Shorebird Use of Mudflats in Elkhorn Slough

The wetland ecosystem within the Elkhorn Slough watershed provides critical feeding and roosting habitat for migrating, wintering, and resident shorebirds. This region is one of the most important migratory stopover areas in California for shorebirds. The intertidal mudflat zone is their primary feeding area, where a phenomenal number of invertebrate species live at or beneath the mud surface, providing a critical food source for shorebirds.

However, the dynamic nature of this intertidal zone, due to the ever-fluctuating tide, precludes the continuous availability of food in this habitat. In response to this, shorebirds exhibit remarkable adaptability that enhances their survival during periods of resource limitation. Variation in migration patterns, feeding strategies, diet, and habitat use effectively reduces competition among the shorebird species and allows for profitable utilization of intermittent food resources.

For the last three years, Sarah Connors, a graduate student at Moss Landing Marine Laboratories and former graduate research fellow at ESNERR, has been studying shorebird use of mudflats in the Elkhorn Slough watershed. On foot or by boat, Sarah surveyed all the different mudflat regions within the slough watershed 2-3 times each month for 2 years to determine the abundance of each shorebird species present in each area. Collecting that information is helping her to gain a better understanding of three main questions:

1) What are the physical variables that characterize mudflat regions used by each shorebird species and determine relative importance of the different mudflat regions for each species?

Does each shorebird species in the Elkhorn Slough watershed use particular mudflat regions based on a specific set of physical, spatial and temporal components? By measuring physical variables at the time that each shorebird survey is conducted, Sarah will be able to test whether one may be able to predict with confidence the shorebird species that will be likely to use a given region. The physical components that she measures include salinity, substrate composition, water level, tide height, and weather variables (temperature, wind speed and direction); temporal variables include tidal phase (ebb, low, or flood), moon phase, time of day and day length.

2) Do mudflat regions that are available during high tide support a higher density of shorebirds during high tide (when other mudflats are flooded) than low tide?

Elkhorn Slough's wetland complex comprises several notable mudflat regions, encompassing areas exposed to full tidal influence, such as the main channel of Elkhorn Slough and Parson's Slough, as well as areas with restricted tidal flow, including Moro Cojo Slough, North Marsh, and the salt ponds. Although these mudflats may appear to be very similar, each is a distinct habitat, unique in physical and biological characteristics. The heterogeneity among these mudflat units creates a mosaic of habitats that provides diverse, alternately available feeding grounds. Tidally restricted mudflats throughout the Elkhorn Slough system provide excellent opportunities for habitat enhancement. For example, Sarah has found that by carefully manipulating the water level in North Marsh, a tidally restricted marsh, the region is available to birds at high tide when most other mudflat regions are flooded, providing a place for feeding and roosting; such areas are especially critical during migration when birds are continually seeking available foraging habitat to maximize food intake.

3) Has the distribution of shorebirds along the main channel of Elkhorn Slough changed since the late 1970's due to alteration in habitat structure?

Habitat composition along the main channel of Elkhorn Slough is changing dramatically due to continued erosion and tidal scour. Extensively degraded portions of salt marsh have become mudflats. Reduction in salt marsh vegetation, where shorebirds roost and forage, may prove detrimental to shorebird populations; conversely, the evolution of these salt marshes into mudflats may produce valuable foraging habitat. Researchers have suggested that sediment grain size, which is finer with increasing distance from the mouth of the slough inland, has become coarser since the 1970's as a result of increased tidal flow. This may contribute to changes in invertebrate density and species composition in the slough, and consequently, alter shorebird distribution in the intertidal zone as well. Sarah will be comparing present shorebird distribution in Elkhorn Slough to distribution 20 years ago (data collected by Bernadette Ramer) in an attempt to gain a better understanding of the potential effects of ongoing erosion and changing habitat structure on shorebirds.

Sarah has finished her shorebird surveys and is currently analyzing her data. Her findings will provide a better understanding of shorebird use of mudflats within ESNERR. With this information, these wetlands can be more effectively managed to optimize habitat quality for shorebirds as well as for many other species.