Quarterly Journal of PRBO Conservation Science. Number 128, Spring 2002. $2.50 Vulnerable songbirds breeding in remnant tidal marsh habitat are the focus of multifaceted PRBO research —and of this special Observer issue. ▼ Insights into the health of a threatened ecosystem Tidal Marsh Song Sparrows of San Francisco Bay Introduction Nadav Nur, PhD an Francisco Bay is the largest Much of it consists of small remnant estuary on the Pacific coast of the habitat patches in a sea of diked or SUnited States. Its wetlands pro- developed baylands. It is often abutted vide crucial habitat for a wide range of by freeways and shopping centers and Alameda Song Sparrow species. Millions of shorebirds and sometimes criss-crossed by manmade waterfowl use the estuary and adja- ditches and power lines. scientific foundation for the design and cent baylands, either as wintering A number of distinctive organisms evaluation of restoration projects. grounds or as “refueling” areas during have evolved to take advantage of this The Tidal Marsh Bird Project is fall and spring migrations up and productive habitat and are restricted to one component of a larger effort by down the Pacific Flyway. In addition, it. In San Francisco Bay, these include prbo—our San Francisco Bay Habitat many bird species and other wildlife birds—California Clapper Rail, Project—to study, maintain, and sus- inhabit San Francisco Bay wetlands California Black Rail, Alameda Song tain the broad array of bird species that year-round. A lack of alternative habi- Sparrow, San Pablo Song Sparrow, depend on the entire mosaic of habitats tats makes them vulnerable, especially Suisun Song Sparrow, and Saltmarsh in the Bay. Our focus ranges from the species that depend upon tidal Yellowthroat; plants, such as Suisun shorebirds feeding on invertebrates in marshes fringing the estuary. thistle and soft bird’s-beak; and other tidal mudflats to cormorants foraging Salt-water marshes—subject to wildlife, such as the salt-marsh harvest in the open waters of the Bay. tidal action, marked by extensive net- mouse and San Francisco garter snake. While our tidal marsh work works of sloughs and channels, and The effect of extensive habitat loss includes all the birds that rely upon characterized by salt-tolerant plants and degradation has been to put these this habitat, we have focused much such as pickleweed and gumplant— populations at risk. As a consequence, attention on three unique subspecies represent a vital ecosystem. Among management agencies and the scientific of nesting Song Sparrows. Studying their ecological functions, marshes community have placed a premium on these birds can be fruitful in advanc- enhance the Bay’s water quality by fil- developing programs to conserve ing the nascent science of restoration. tering toxic compounds out of the remaining populations and restore The threats they face—habitat frag- runoff from surrounding uplands. depleted species, through maintaining mentation, changes in salinity, expo- In the early nineteenth century, tidal and restoring tidal marsh habitat. sure to pollutants, non-native invasive marsh was the dominant habitat type in Against this backdrop, prbo’s species, and more—are ones faced by San Francisco Bay. Its historic acreage Tidal Marsh Bird Project began in the whole community of tidal marsh has been reduced by about 85%, due to 1996. Its aims are: to provide manage- fauna. In this Observer issue, we sum- development as well as diking for agri- ment agencies with critical information marize some of our recent findings. culture and salt evaporation ponds. about the status of tidal marsh-depend- Even what remains of this habitat today ent species; to guide recovery efforts for Nadav Nur, PhD, is Director of PRBO’s has been severely altered and degraded. these populations; and to establish a Population Ecology Program. Peter LaTourette Page 2 PRBO Conservation Science, Spring 2002 While all three subspecies face significant threats, the one requiring the most urgent attention is the Alameda Song Sparrow. Significant Findings Nadav Nur, PhD, and Hildie Spautz Song Sparrows do not appear to o provide management agen- respond to vegeta- cies with information about the tion height and Tstatus of tidal marsh-dependent density of plant species, and how best to protect and stems (unlike the San Francisco facilitate recovery of their populations, Marsh Wren and prbo conducts standardized surveys of Salt Marsh Yellow- Song Sparrows each year at over 50 throat). They do marshes throughout the San Francisco respond positively Bay Estuary. Some results to date: to shrub cover, o Virtually every Bay tidal marsh has mostly gumplant one or more pairs of Song and coyote brush, Sparrows (Yellowthroats and Black and negatively to Rails are found in some marshes Juncus (rushes). Figure 1. Distribution of three subspecies of tidal marsh Song but not others). o There are strong Sparrows on San Francisco Bay. o There is marked variation in Song associations Sparrow population density among between Song Sparrow density and about 12,000–18,000; Suisun Song marshes, even within a sub-region landscape features. In particular, Sparrows, 40,000–66,000; San Pablo such as San Pablo Bay. density is greater in marshes that Song Sparrows, 70,000–85,000. These o In San Francisco Bay marshes, are larger, not isolated from other numbers are greater than previously inhabited by the Alameda Song marshes, and not adjacent to urban estimated for San Pablo and Suisun Sparrow, we estimate an average areas. This information is of great Song Sparrows but similar to previous density of 5.2 birds per hectare value—both to inform current estimates for the Alameda Song (100 meters2). San Pablo and habitat management practice and Sparrow. While all three subspecies Suisun Bay marshes, inhabited by for prioritizing new acquisition and face significant threats, the subspecies the other two subspecies, demon- restoration programs. requiring the most urgent attention of strate average densities of 14.9 Our estimates of density allow us management agencies is the Alameda birds per hectare. to estimate the total breeding popula- Song Sparrow, which is also the most o Habitat characteristics may explain tion size for each of the three sub- genetically distinctive subspecies of spatial variation in abundance. species: Alameda Song Sparrows, the group. Tidal Marsh Fragmentation: Edge Effects Having found nest predation to be a main cause of Song Sparrows’ nest failure, we investi- Bay edge gated the effects of marsh edge proximity on predation rate and overall nest success. For five San Pablo and Suisun Bay marshes, we looked at the fate of more than 1,800 Song Sparrow nests monitored between 1996 and 2000. Nests located closer to the Bay edge fledged young sig- nificantly more often than did nests located closer to upland marshland edges. Our analyses, however, suggest that the edge characteristics of an entire marsh may be more important than the location of nests within a marsh. Landscape-level conservation planning is needed in upland edge order to identify conservation/restoration sites most hospitable to Song Sparrows and other marsh-dependent species.—Diana Stralberg, meters Geographic Information Systems Specialist, and Yvonne Chan, Seasonal Biologist Figure 2. Song Sparrow nests at China Camp State Park (San Pablo Bay) in 1997. Black dots are nests that successfully fledged young. Red stars are nests that were depredated. Black triangles are nests that failed for other reasons. prbo graphics PRBO Conservation Science, Spring 2002 Page 3 A growing concern for all wildlife reliant on San Francisco Bay is the accumulation of chemical pollutants in the water, sediments, and food web. Contaminants and Nest Success Right:A PRBO field Hildie Spautz biologist examines an he San Francisco Bay ecosys- inactive marsh nest site. tem is much altered from the one eggs that he collected where tidal marsh Song T from nests on the Sparrows evolved, several thousand Richmond–San Rafael years ago. A growing concern for all Bridge. Eggs from wildlife reliant on San Francisco Bay is 1999 and 2000 have the accumulation of chemical pollu- been tested for nine tants in the water, sediments, and food chemical contaminants. cern in avian eggs, and eggs that failed web. Sources of toxic compounds, Cormorants’ eggs had much higher to hatch had selenium levels similar to especially in the northerly arm, San levels than sparrows’ for eight of the those of viable eggs, indicating that egg Pablo Bay, include runoff from agricul- nine compounds (e.g., dde, pcbs, and mortality is probably not directly relat- ture and discharges from industries mercury), likely reflecting differences ed to higher levels of environmental fringing the Bay. between their ecological niches. selenium in parental diets. P ’s study of Song Sparrows’ rbo Sparrows are restricted to tidal marsh- In field observation of nests reproductive success in San Pablo and es and consume plant material and throughout their cycle, we document Suisun Bays has found reproductive invertebrates; cormorants, higher on the proportion of eggs that hatch. We rates to be disturbingly low. Predation the food-chain, forage for fish in the have found no statistically significant is a main cause of nest failure. differences between tidal marsh The probability that a nest sur- nests and those at prbo’s vives to fledge at least one young Palomarin Field Station (belong- averages less than 20% per year; ing to the Marin Song Sparrow for some marshes, the rate is subspecies), or between San 10% or less. Nest survival must Pablo and Suisun Bay eggs. No approach 25% to yield long-term detectable pattern of reduced sustainability of the populations. hatchability in Bay marshes at As shown in Figure 3, there is this time is good news! great variability among years and San Pablo Song Sparrows and among marshes, and San have higher rates of overall nest Pablo Song Sparrows display success (young that fledge) than higher reproductive success than Suisun Song Sparrows, due pri- their Suisun neighbors.