Open-File Report 2026–1064
Ecosystems Mission Area—Land Management Research Program and Species Management Research Program
Prepared in cooperation with the California State Coastal Conservancy, California Wildlife Foundation, California Department of Fish and Wildlife, U.S. Fish and Wildlife Service, and South Bay Salt Pond Restoration Project
Suggested citation:
Ackerman, J.T., Hartman, C.A., and Herzog, M., 2026, Monitoring nesting waterbirds for the South Bay Salt Pond Restoration Project—2024 breeding season: U.S. Geological Survey Open- File Report 2026–1064, 27 p., https://pubs.usgs.gov/publication/ofr20261064
Associated data for this publication:
Ackerman, J.T., Hartman, C.A., and Herzog, M.P., 2025, Waterbird nest abundance in south San Francisco Bay: U.S. Geological Survey data release, https://www.sciencebase.gov/catalog/item/6806bca9d4be027c777d34ba
Abstract
The San Francisco Bay supports thousands of breeding waterbirds annually and hosts large populations of American avocets (Recurvirostra americana), black- necked stilts (Himantopus mexicanus), and Forster’s terns (Sterna forsteri). These three species have relied largely on former commercial salt ponds in south San Francisco Bay, which provide wetland foraging habitat and island nesting habitat. The South Bay Salt Pond Restoration Project is in the process of restoring as much as 15,100 acres of these former salt ponds to tidal marsh and tidal mudflats. Although this restoration is expected to have numerous benefits, including providing habitat for tidal wetland- dependent species, improving water quality, buffering against storm surge, and protecting inland areas from sea level rise, the reduction in former salt- pond habitat and nesting islands may negatively affect breeding waterbirds. To address the reduction in former salt- pond habitat available to waterbirds, the South Bay Salt Pond Restoration Project will maintain some pond habitat for wildlife and provide enhancements such as the construction of new islands for nesting. The South Bay Salt Pond Restoration Project follows an adaptive management plan in which waterbird response to the changing landscape is monitored over time to ensure that existing breeding waterbird populations are maintained.
In this report, we provide results of waterbird nest monitoring in south San Francisco Bay during the 2024 breeding season and present these results in the context of annual nest monitoring in south San Francisco Bay since 2005. Overall, Forster’s tern nest abundance in 2024 (1,808 nests) was the highest recorded between 2005 and 2024, and it maintained the high abundance first observed in 2022 (1,727 nests), which reversed the historically low abundance observed during 2015–17. In contrast, nest abundance remained at or near 20- year lows for American avocets (222 nests) and black- necked stilts (126 nests) in 2024, but both species had small increases in their nesting population sizes compared to 2022. In 2024, there were only 3 Forster’s tern, 5 American avocet, and 3 black- necked stilt major colony nesting sites, which is down from the annual averages of 6.6, 12.4, and 6.6 observed during 2005–09.
Nest success (73 percent for American avocets, 54 percent for black- necked stilt, and 64 percent for Forster’s terns) increased compared to 2022 (30 percent for American avocets, 29 percent for black- necked stilt, and 53 percent for Forster’s terns) and during 2005–10 (37 percent for American avocets, 24 percent for black- necked stilt, and 61 percent for Forster’s terns). Nest success in 2024 was above (American avocets and black- necked stilts) or slightly below (Forster’s terns) baseline values established for the South Bay Salt Pond Restoration Project. Average egg- hatching success was lower for American avocets (86 percent) and Forster’s terns (86 percent) and similar for black- necked stilts (96 percent) than the values observed during 2005–10. Average clutch sizes for American avocets (3.87 eggs), black- necked stilts (3.88 eggs), and Forster’s terns (2.73 eggs) were greater than what was observed in 2022 and during 2005–10. Average nest- initiation dates in 2024 were substantially earlier among all three species (April 19 for American avocets, April 25 for black- necked stilts, and May 12 for Forster’s terns) than in 2022 (May 4 for American avocets, May 13 for black- necked stilts, and May 20 for Forster’s terns) and during 2005–10 (May 15 for American avocets, May 3 for black- necked stilts, and May 30 for Forster’s terns). Finally, the enhanced managed ponds with newly constructed islands (Ponds A16 and SF2) supported 52 percent of American avocet nests, 47 percent of black- necked stilt nests, and 94 percent of all the Forster’s tern nests recorded in south San Francisco Bay in 2024.
In cooperation with the California State Coastal Conservancy, California Wildlife Foundation, the USFWS’s Don Edwards San Francisco Bay National Wildlife Refuge, the California Department of Fish and Wildlife’s Eden Landing Ecological Reserve, and the South Bay Salt Pond Restoration Project, we provide a summary of nesting ecology data for Forster’s terns, American avocets, and black-necked stilts in areas of the South Bay Salt Pond Restoration Project for the 2024 breeding season. Moreover, we compare current (2024) nest abundance, distribution, and reproductive success to historical data collected during 2005–19 and 2022. We also report on gull hazing efforts in 2024 to deter nesting by California gulls at high-priority sites on the Don Edwards San Francisco Bay National Wildlife Refuge. These results provide the most recent assessment of the breeding waterbird populations and nesting population trends in south San Francisco Bay.