Birds of Lake Merced

Doug Johnson, Graduate Student, Geography
San Francisco State University

INTRODUCTION

Lake Merced, taken together with the adjoining Fort Funston, comprises key habitat for avifauna. As the only remaining freshwater coastal lake and wetland between Pescadero Marsh 30 miles to the south and Point Reyes 25 miles to the north, Lake Merced provides a valuable stopover for migrants, as well as breeding, nesting, sheltering, and foraging habitat for residents.

The Golden Gate Audubon Society plays a key role in avian management at Lake Merced. Though they have no legal authority to make management decisions, their avid attention as birdwatchers has provided the majority of data to date for studies that aim to inform management decisions. Society volunteers compile casual sightings, conduct formal surveys, and sponsor events such as the annual Christmas bird count. Their amateur expertise is invaluable, as is their strong commitment to preserving habitat for birds. In particular, Dan Murphy of the Audubon Society has been a key coordinator in this effort at Lake Merced.

Overall, there are approximately 48 bird species that call Lake Merced home year round (actually residing there or using it as their main feeding site). Another 8 species depend on Lake Merced during the nesting season (February to July) as a secondary feeding and resting site, and 13 other species may breed or nest at the lake, but are unconfirmed. Other species winter at the lake, and migrants use it as a stopover in the spring and fall (Murphy 2000). For a full list of species, visit the Friends of Lake Merced website.

HABITATS

Birds fill ecological niches in all habitats found at Lake Merced: aquatic, unvegetated shoreline, marsh, arroyo willow riparian, shrub, grassland, mixed conifer/hardwood forest, and eucalyptus grove. The sections below describe in general fashion representative species present in each of these habitat types. Information is combined primarily from two reports: the S.F. PUC’s 1998 Baseline Natural Resources Inventory, and the S.F. Water Department’s 1993 Lake Merced Water Resource Planning Study—see references.

In the aquatic habitat, waterfowl abound. Dabblers such as the American coot (Fulica americana), mallard (Anas platyrhynchos), cinnamon teal (Anas cyanoptera), northern shoveler (Anas clypeata), American wigeon (Anas americana), northern pintail (Anas acuta), and gadwall (Anas strepera), as well as divers like the bufflehead (Bucephala albeola), canvasback (Aythya valisineria), lesser scaup (Aythya affinis), and ruddy duck (Oxyura jamaicensis) rest on open water, especially in late fall and winter. Gulls (Larus species) also raft and bathe. Double-breasted cormorants (Phalacrocorax auritus), terns (Sterna species), and osprey (Pandion haliaetus) forage for fish, while swallows (including bank swallows (Riparia riparia), a state threatened species) forage for insects over open water.

The Gadwall (Anas strepera) is one of many species of waterfowl that use the aquatic habitat of Lake Merced.

Photographed in Palo Alto, California by Jim Rosso, from http://hale.pepperdine.edu/~jlrosso/gadwall.htm

On unvegetated shoreline, waterfowl rest and preen, while killdeer (Charadrius vociferous), sandpipers (Tringa solitaria), migrating shorebirds such as semi-palmated plover (Charadrius semipalmatus) and greater and lesser yellowlegs (Tringa melanoleuca and flavipes), and passerines such as pipits (Anthus species) and red-winged blackbirds (Agelaius pheoniceus) forage on the flats.

In the bulrushes, tules, and smartweed of marsh habitat, waders such as the snowy egret (Egretta thula), green heron (Butorides virescens), and great blue heron (Ardea herodias) forage for small fish, amphibians and invertebrates. The pied-billed grebe (Podilymbus podiceps), American coot, Virgina rail (Rallus limicola), and sora (Porzana carolina) nest in the bulrush. Mallards also use the area to nest and take refuge. The vegetation provides nesting and foraging for red-winged blackbirds, marsh wrens (Cistothorus palustris), and the saltmarsh common yellowthroats (Geothlypis trichas siuosa), a California threatened species. Northern flickers (Colaptes auratus) and black phoebes (Sayornas nigricans) are among the other birds that spend time in emergent wetland vegetation.

The Common Yellowthroat (Geothlypis trichas) is listed as threatened in California.

Photographed at Cronkite Beach, California by Jim Rosso, from http://hale.pepperdine.edu/~jlrosso/yellow.htm

Arroyo willow riparian corridors forms the ecotone between upland (forest, grassland, shrub) and lowland (aquatic and unvegetated shoreline) habitats. As would be expected, these areas have an abundance of bird live. Branches provide perching spots for waterbirds like the double-breasted cormorant, black-crowned night herons (Nycticorax nycticora, and nesting, roosting and foraging resources for Cooper’s hawk (Accipiter cooperii), sharp-shinned hawk (Accipipter striatus), red-tailed hawk, bushtit (Psaltriparus minimus), chestnut-backed chickadee, downy woodpecker, and ash-throated flycatcher (Myiarchus cinerascens). The understory provides habitat for Bewick’s wren (Thryomanes bewickii), California towhee (Pipilo crissalis), and spotted towhee (Pipilo erythrophthalmus).

The extensive coyote brush and blackberry that form the shrub habitat is generally found on steeper slopes. Many birds, like Allen’s hummingbirds (selasphorus sasin), Brewer’s blackbirds (Euphagus cyanocephalus), and song sparrows (Melospiza melodia) nest in the vegetative cover. American goldfinches (Carduelis tristis), white-crowned sparrows (Zonotrichia leucophyrys), and golden-crowned sparrows (Zonotrichia atricapilla) make use of the vegetation, and ground nesters like California quail (Callipepla californicus) and California towhee (Pipilo crissalis) make there home under the cover.

The annual grasses and iceplant of the grassland provide foraging for Brewer’s blackbirds, American kestrels (Falco sparverius), American crows (Corvus brachyraynchos), turkey vultures (Cathartes aura), and sparrows.

The coniferous and hardwood forest, with its vertical layered tree canopy and varied understory of shrubs and herbs, provides foraging for chestnut-backed chickadee (Parus refescens), scrub jay (Aphelocoma coerulenscens), downy woodpecker (Picoides pubescens), hairy woodpecker (Picoides villosus), American robin (Turdus migratorius), and olive-sided flycatcher (Contopus borealis). Predators like red-shouldered hawk (Buteo lineatus) and red-tailed hawk (buteo jamaicensis) roost and find refuge on branches, and cavities provide nesting sites for tree swallows (Tachycineta bicolor) and violet-green swallows (Tachycineta thalassina). The similarity in habitat features between the forest, riparian, and marsh habitats results in significant overlap between the species found in these areas.

Great Blue Heron (Ardea Herodias) roosting in forest.

From http://www.kodak.com/digitalImaging/samples/birdsPix.shtml

One final habitat type that is not always officially recognized is the non-native eucalyptus grove. Cormorants and black-crowned night herons use high branches as roosting sites, and wintering and migrating warblers forage for insects in the grove. This is a factor that would need to be considered if removal of the trees was proposed.

ISSUES

The California Department of Fish and Game maintains a Natural Diversity Data Base that lists species of special concern, which includes species that are listed (or candidates for listing) as threatened, endangered, or of special concern at the federal or state level. The species present at Lake Merced include three that are listed in the data base: the bank swallow, the California black rail, and saltmarsh common yellowthroat.

The bank swallow (Riparia riparia) makes nests in soft waterside cliffs. Their former habitat at the Lake Merced was destroyed when the lake was reconfigured. They continue to nest at nearby Fort Funston, and Lake Merced now serves as a critical food source during breeding season.

The saltmarsh common yellowthroat (Geothlypis trichas) is a year-round resident. Their bulky nests made of weed stems, grass, bark and ferns are found amongst tules and willow thickets.

The California black rail (Laterallus jamaicensis coturniculus) forages for insects, seeds, and aquatic invertebrates that can be gleaned from the ground among the marsh’s rushes. The last official sighting at Lake Merced dates to 1937, but Audubon Society members claim to have seen it there.

In addition, the American peregrine falcon (Falco peregrinus anatum) was federally listed until recently, so individuals that hunt at Lake Merced merit continued attention.

The Peregrine Falcon (Falco peregrinus anatum) has made a strong recovery, in part due to its ability to adapt to urban prey such as pigeons.

"Belle" copyright 1996 by Ray Congdon, found at http://www.scn.org/earth/tweeters/images/birds9.html

The 1993 Lake Merced Water Resource Planning Study reports that other special status birds occur occasionally at the lake, including brown pelicans, ospreys, Cooper’s hawks, sharp-shinned hawks, loggerhead shrikes, and tricolored blackbirds.

One issue that has been of concern for decades is potential lead poisoning from submerged lead shot that results from trap shooting over the lake from the Pacific Rod and Gun Club. One study (Ecology & Environment 1993) found that levels found in aquatic macroinvertebrates were not sufficient to be harmful, and that the four bird species they looked at—mallards, ruddy ducks, ring-necked ducks, and American coots—showed low levels of lead. The only significant danger is for diving ducks that eat by straining benthic sediments. If they happen to ingest a lead BB, the shot will dissolve in their gullet and kill the bird. If the bird is ingested by a predator or scavenger, secondary poisoning will occur. Shooters are now required to use non-lead shot, and the lake has been dredged in efforts to remove old shot.

Shore nesters can be greatly affected by changing lake levels. In recognition of this, current operating plans call for lake levels to be held steady between April 15 and July 15, and for lake levels to be changed at certain maximum rates at other times to allow organisms time to move (Feasibility Evaluation 1998)[references]. Of course, the declining health of aquatic life (also linked to water levels, pollutants, and other factors—see Water Quality and Fish sections), a primary food source for many bird species, is significant as well.

Recreation potentially impacts the integrity of bird habitat. Physical intrusions from boaters may disturb shore-nesting birds. The noise of the Rod and Gun Club and police shooting range may limit nearby bird presence.

Ground nesters and others are in potential predation danger from feral cats, though the level of their presence and impact does not seem to be well-documented.

CONCLUSION

When looking at California’s landscape from a "bird’s eye view" Lake Merced is an important habitat for both residents and migrants. The site’s wildlife values deserve dedicated attention. In addition, bird watching is an increasingly popular hobby, with extensive commercial support (see, for instance, the Birdwatching.com website). As urban residents grow more interested in Nature-based outdoor recreation, the importance of natural and cultural resources at Lake Merced, of which birds are a main aspect, will also grow. It is one of San Francisco’s most important remaining wild places.

LINKS
Birdnet

Cornell Laboratory of Ornithology