Waterbird response to modified hydrology of Central Valley wetlands, California

Wetlands of California's Central Valley have historically been home to one of the largest concentrations of waterbirds in North America. The Grasslands Ecological Area (GEA) embodies the largest contiguous wetland remaining in California. These wetlands are highly modified and intensively managed in order to produce sufficient food and cover for wintering wildlife, especially waterfowl. Food generated for wildlife in the area consists mainly of moist-soil managed plants, primarily swamp timothy (Crypsis schoenoides ). Wetland management determines habitat structure, composition and productivity, which affects waterbird use. The wetlands of GEA drain into the San Joaquin River (SJR), where salinity levels have become a concern. Therefore, wetland managers were requested to delay drawdown of their wetlands by 4-6 weeks, so that drain water enters the SJR when it is at assimilative capacity. However, wetland managers are concerned that a change in the hydrological regime may cause retention of salts, negatively impacting wetland plant productivity, and ultimately reduce food availability for waterbirds that rely on the GEA for non-breeding habitat.;My study aimed to assess the response in late-winter through spring avian use of wetlands subjected to delayed drawdown during 3 years. I quantified differences in wetland use by geographic location (block), management type, season, and year for waterbird species richness and four functional waterbird groups (dabbling ducks, surface-diving birds, shorebirds, and large wading birds). Late-winter through spring use by dabbling ducks and surface-diving birds was not affected by delayed drawdown. However, delayed drawdown may positively affect shorebird and large wading bird populations by providing extended periods of resource availability (food, cover, and roosting sites) during spring migration. Cumulative impacts from multiple years of modified hydrology remain unknown. Furthermore, I did not study impact during fall or early winter. This study suggests that 1-2 years of delayed wetland drawdown imposed on a wetland unit would not adversely affect waterbird use during late-winter through spring. Nevertheless, wetlands subjected to delayed hydrology should be rotated to avoid cumulative impacts, or further long-term research should be conducted. A diversity of wetland types should be maintained in the GEA to ensure the greatest diversity of waterbirds is supported.