Tidal Songbirds

Nadav Nur, PhD

The tidal marshes fringing San Francisco Bay are home year-round to songbirds - less conspicuous than wintering shorebirds but unique and endemic. Both the Song Sparrow and Common Yellowthroat have evolved subspecies that occur only in Suisun, San Pablo, and San Francisco bays. During spring and summer, a given marsh may be thick with nesting sparrows, their territories crowded into vegetation slightly taller than the surrounding soggy landscape. Yet far fewer of these native Bay Area birds sing from marshlands today than in the recent past: they have lost a lot of ground. This account of new research on tidal songbirds is adapted from a 1997 PRBO report by Nadav Nur, Steve Zack, Jules Evens, and Tom Gardali.

The San Francisco Bay region has undergone dramatic change in the past 150 years. From the 19th-century gold mining in the Sierra Nevada, which flushed untold tons of sediments into the rivers, through the present-day urbanization around the shoreline, large-scale developments have drastically affected San Francisco Bay. Water diversion systems, agricultural transformation, and creation of salt ponds are among the factors that have altered the size and function of the bay and adjoining estuary, which drains the great Central Valley. Every natural habitat in the region has been affected, perhaps none more than tidal saltmarsh: only an estimated 15% of original tidal marsh habitat remains. This represents a decrease from 194,000 acres in the mid-19th century to 29,300 acres at present.

Tidal marshlands are vital but fragile ecosystems, enriched by the many nutrients that mix in estuarine waters and built up by their own rapidly growing plant life. Conversion to pastures, factories, and salt evaporation ponds has diminished the richness of Bay Area biodiversity. In much of San Francisco Bay's remaining tidal marshland, levees built by duck clubs block the tidal flow and have changed the composition of this native habitat.

A number of resident species are affected. Many of these creatures, even the more mobile ones such as birds, are sedentary in nature and rely solely on tidal marsh habitat. Some survive on San Francisco Bay in small, isolated breeding populations (more subject to extinction), because their habitat is so heavily fragmented.

Concern for tidal marsh birds prompted PRBO to document their predicament starting in 1985, when research associate Jules Evens, together with Gary Page, Rich Stallcup, and others, began studies of threatened Black Rails in the Bay Area. Recently, as regional efforts to plan for the future of bayshore habitats take shape, PRBO has felt compelled to also examine the songbirds that are obligate dwellers in tidal marshes. We wanted to identify the critical aspects of marsh habitat that support these birds and discover whether any local subspecies are threatened due to low numbers and/or reproductive success.

Songbirds in Question

It has long been known that distinct subspecies of Song Sparrow and Common Yellowthroat dwell in San Francisco Bay tidal marshes; we have good reason to suspect that they are particularly affected by habitat loss. The Alameda Song Sparrow (Melospiza melodia pusillula), for example, occurs only along the southern and eastern fringes of San Francisco Bay. Some 10% of its original habitat remains. The Samuel's Song Sparrow (M. m. samuelis) dwells in San Pablo Bay, where some 22% of the original tidal marsh remains; the Petaluma River marsh is a stronghold for this subspecies. The Suisun Song Sparrow (M. m. maxillaris) is restricted to the easternmost bay region, especially Suisun Bay. Only 13% of the estimated original marsh remains there. The subspecies of Common Yellowthroat found throughout the San Francisco Bay region is the Saltmarsh Yellowthroat (Geothlypis trichas sinuosa), characterized by a darker back than the migratory yellowthroats and by its brown flank feathers that appear in the fall. Distributed spottily in the Bay region, its population size today is poorly known.

Each of these bird taxa, along with the Black Rail, has been considered a "Species at Risk" under the Federal Engangered Species Act. In other words, they are candidates for protection as "threatened" or "endangered" but cannot yet be listed - or protected - because there is not enough information to support such classification.

To fill the need for scientific data on tidal marsh birds' status, PRBO launched a new project in 1996. With funding from the "Success with Species at Risk Initiative," administered then by the National Biological Survey (now part of the U.S. Geological Survey), we designed a three- to five-year study of tidal marsh birds. While the second year of field work now continues, we report here some significant 1996 findings. This article concentrates on songbirds - the sparrows and yellow-throats that few Californians realize are tied to the fate of San Francisco Bay tidal marshlands. A feature on the bay's Black Rail populations, also addressed in our tidal marsh study, will appear in the next Observer.

Our goals were to furnish land managers and conservation biologists with the best estimates to date of population size for the four passerine subspecies. For Song Sparrows we are also determining productivity, so that, if any local subspecies proves threatened or endangered, we will know where it produces enough young to replenish its population. In each of 19 study sites in 16 different Bay Area marshes, we also examined landscape features that might bear on the birds' abundance - especially those affected by management action.

Preliminary Findings: 1996

Song Sparrows were present at every marsh included in this study; in San Pablo and Suisun bays, densities were moderate to high, averaging 18.2 and 22.8 birds per hectare respectively. In San Francisco Bay, home of the Alameda Song Sparrow, densities on average were much lower - about four to six birds per hectare.

Distribution of the Saltmarsh Yellowthroat appeared much less regular: at about half of all marshes, we detected none. The exception to this was Suisun Bay, where most sites had reasonable numbers of yellowthroats. Causes for the spotty distribution remain to be identified: it may represent recent extirpation or reflect the bird's failure to recolonize certain areas over time.

We were especially interested in identifying the characteristic features of marsh areas with relatively high abundance of birds. We found that channels in the marsh are important, both for sparrows and for yellowthroats (see Figure 1). Yellowthroats were also associated with greater amounts of peppergrass, cord grass, and Scirpus (bulrush and tule). Although pickleweed (Salicornia) is often the dominant plant in tidal marshland, the more pickleweed, the fewer yellowthroats were present. For Song Sparrows, greater abundance was associated with greater vegetation cover, but we found no one or more plant species to have a marked negative or positive correlation.

How important are the area (size) of a given marsh and its isolation in supporting these birds? We found a significant correlation for Song Sparrows (but not yellowthroats) between bird density and marsh area, but this was only true for isolated marshes, which we defined as more than one kilometer from the next closest tidal marsh. For these marshes, larger area meant greater sparrow densities (Figure 2).

Marsh area may yet prove important for both species in non-isolated marshes, those less than one kilometer from the next marshland. Our study in 1996 included very few marshes that were both small and non-isolated, and few small marshes of any kind from San Pablo or Suisun Bay. In 1997, we are surveying small marshes, both isolated and connected, in the two northern bays, in order to better address the importance of marsh area and isolation.

A better way to measure marsh isolation than using the arbitrary threshold of one kilometer's distance would be to use a bird's view of isolation. This will require information about dispersal distances for Song Sparrows and yellowthroats, and this year we are studying dispersing juvenile Song Sparrows that we have color-banded.

Regarding Song Sparrow nesting success, at three of four marshes about one in three eggs produced a fledgling, a 30% success rate comparable with that of Song Sparrows at Palomarin and elsewhere. The exception was Southampton Bay at Benicia State Park, where the sparrows' nest success is only 16% - likely too low to sustain their population. PRBO's nest monitoring methods are designed to determine productivity while causing no harmful disturbance.

Our estimates of total population size for all three Song Sparrow subspecies contain some warning notes. Based on our census data and calculations of habitat extent, we estimated 58,800 Suisun Song Sparrows, 66,600 Samuel's Song Sparrows, and only 8,250 Alameda Song Sparrows. We know from our Palomarin research that up to 25% of adult Song Sparrows present during breeding season are non-territorial "floaters." What this means for the Alameda Song Sparrow is that its entire population may hold as few as an estimated 6,200 breeding adults. This is cause for concern and immediate further research, in case this subspecies is already endangered.

There is much work ahead, especially to secure the necessary protection for endemic tidal marsh songbirds. Surveys at more marshes, and attempts to cover a greater diversity of potential habitat at each marsh, are needed in order to derive reliable estimates of population density. Future surveys need to target specifically the yellowthroat and Alameda Song Sparrow. PRBO will continue making scientific contributions to the conservation of endemic songbird subspecies that still hold ground in San Francisco Bay tidal marshes.