Executive Summary
The South Bay Salt Pond Restoration Project (SBSPRP) is restoring over 15,000 acres of former salt evaporation ponds to a mix of tidal marsh and ponded wetland habitats. These wetlands provide habitat for many waterbirds, including migrating Red-necked Phalarope (Phalaropus lobatus) and Wilson’s phalarope (P. tricolor). Sustaining baseline population goals for wildlife populations requires understanding how species are responding to restoration actions over time. While many waterbird guilds have increased in abundance from pre-restoration baselines, phalarope numbers have declined >50% below baseline numbers. The purpose of this ongoing study is to understand observed declines in phalarope numbers within the SBSPRP area and their relationship with broader population trends and phalarope movements. This report serves as a data summary and coarse-scale assessment of phalarope monitoring efforts in the South San Francisco Bay during the 2024 migration.
From June 17, 2024 to September 26, 2024, we conducted 8 rounds of phalarope migration surveys at 32 sites (18 SBSPRP managed ponds, 7 Cargill-managed salt production ponds, and 7 sites outside of these areas).
The surveys found that counts of Wilson’s Phalaropes peaked at 787 phalaropes on 07/16 and counts of Red-necked Phalaropes peaked at 1,082 phalaropes on 09/12. Across all sites, a total of 4,835 phalaropes were counted throughout the summer (2433 Red-necked, 1,352 Wilson’s, and 1,050 that could not be identified to species). Compared to last year, this was -11,819 fewer sightings of phalaropes (-11,634 Red-necked, -286 Wilson’s, and +101 that could not be identified to species). Looking only at the 25 sites within SBSPRP area, a total of 3,161 phalaropes were counted throughout the survey season (1938 red-necked, 1,157 Wilson’s, and 66 that could not be identified to species). Within only the SBSPRP and salt pond complexes from the early July through end of September surveys, the average number of sightings of phalaropes per survey was 448. This was -91.6% below the revised 2005-2007 baseline of 5,324, and counts therefore remain below the Adaptive Management Plan trigger
threshold of a decline of 50% or more below baseline values.
We gathered and analyzed foraging data for the first time this year and assessed how salinity affected foraging behavior. Based on a literature review on phalarope prey ecology, we designed six new salinity bins: near-fresh, brackish, saltwater, and low, medium, and high hypersaline. The overwhelming majority of observations of Red-necked and Wilson’s Phalaropes were of foraging, rather than roosting, birds. Phalaropes demonstrated a strong association with a relatively wide band of moderately high salinity (30-150 ppt), which closely aligned with the reported tolerance of Ephydra and Artemia spp. reported in the literature. Surprisingly, Wilson’s Phalaropes were most commonly observed at sites with salinity matching the marine environment (30-50 ppt), despite their tight association with hypersaline lakes.
Behavioral observations showed that brackish (2-30 ppt) and marine (30-50 ppt) ponds had significantly fewer foraging attempts and successful prey captures. Outside of low hypersaline ponds (50-80 ppt) appeared to provide the best habitat, with medium hypersaline ponds (80-150 ppt) trending towards lower numbers of successful prey captures and high hypersaline ponds (>150 ppt) having significantly lower foraging attempts. Sunnyvale WPCP, the only near-freshwater (0-2 ppt) pond used by phalaropes, was an outlier site not comparable to most ponds in the South Bay because it is a sewage treatment pond with unnaturally high nutrients and had notably higher rates of foraging attempts and successful prey captures.
Continuing observations of phalaropes outside of the original set of 31 sites indicate the current spatial coverage of sampling may be insufficient. We reanalyzed historical count data from the entire 2002-2024 record of waterbird surveys conducted for the SBSPRP, and identified 17 new sites (in addition to one that was already added, E5C) that are potentially important habitat. While some of these sites, such as those that have been breached to tidal action, are likely no longer suitable, others appear likely to remain habitat. We identify six sites in particular that are high-priority for adding to the set of surveys.
Suggested citation: Van Schmidt, Nathan D., Amy Parsons, and Theresa Michalke. 2025. South San Francisco Bay phalarope 2024 migration survey and foraging study report. Report prepared for the South Bay Salt Pond Restoration Project.