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
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
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
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
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.