Seabirds and Shorebirds
III. Shorebirds and other Coastal Avifauna
The shoreline and coastal wetlands that border the MBNMS are also relatively important to birds. The Elkhorn Slough, which is centrally located in the MBNMS, attracts the third largest concentration of shorebirds in California, surpassed only by the much larger Humboldt and San Francisco bays. For some species, San Francisco Bay and Elkhorn Slough, along with estuaries of the Point Reyes National Seashore and Pescadero Marsh (Figure 1) may comprise an integral unit, with birds commuting between them (Ramer et al. 1991).
Ten shorebird species are dominant on intertidal mudflats of Elkhorn Slough and the Salinas River mouth. Among the small species are the Least and Western sandpipers, Dunlins, Sanderlings and both Short- and Long-billed dowitchers. Larger shorebird species include Black-bellied Plover, Willet, American Avocet, Marbled Godwit and Long-billed Curlew. Most of these species also occur, at least occasionally, on outer sand beaches. Sanderlings, Willets and Marbled Godwits are the species most commonly observed feeding along these beaches (Davis and Baldridge 1980). Primary prey include sand crabs (Emerita analoga;) some birds such as the Sanderling also rely heavily on the beach isopod Excirolana sp. (Estelle 1991).
The brackish and salt marshes of the MBNMS are utilized as roosting and feeding habitat by many of the shorebirds mentioned above during high tides (Davis and Baldridge 1980). They are also frequented by both the most visible and secretive of coastal avifauna. The largest avian inhabitants of the wetlands --the Snowy Egrets, Great Egrets, the Great Blue Herons and Black-crowned Night Herons--feed on a variety of large vertebrate and invertebrate prey, including rodents, snakes, shore crabs and fishes (Davis and Baldridge 1980).The rarely observed Sora Rail also inhabits these marshes; other rail species which once used MBNMS marshes have declined dramatically (Roberson 1985; and see Special Status Species Table).
Horned and Eared grebes, American Coots, and a host of ducks dominate the coastal bird assemblage that uses the shallow, tidal waters of local sloughs and estuaries. Ducks include the diving ducks, such as Goldeneyes and Buffleheads, and dabbling ducks, such as Pintail, Mallard and Cinnamon Teal (Davis and Baldridge 1980).
A rocky shoreline occurs in the southern and northern portion of the MBNMS. The dominant species in this habitat are the resident Black Oystercatcher, which actually feeds heavily on mussels and limpets (Hahn and Denny 1989), and Black Turnstones, which feed on barnacles, periwinkles, limpets, isopods and amphipods (Glynn 1965). These birds are most abundant during fall and winter, and during this period are accompanied by small numbers of Ruddy Turnstones, Surfbirds and Wandering Tattlers (Shuford et al. 1989). A variety of species commonly considered land birds also feed along rocky shores, including Belted Kingfishers, Black Phoebes, American Crows, Yellow-rumped Warblers, Brewers Blackbirds and European Starlings. Amphipods and kelp flies are important prey, except for the piscivorous Kingfisher (Davis and Baldridge 1980).
At the top of the seabird and shorebird trophic pyramid are such coastal raptors as Peregrine Falcons and Short-eared Owls (Roberson 1985, Shuford et al. 1989). Bald Eagles and Osprey formerly nested along the Big Sur coast and near Watsonville, respectively (Roberson 1985), and probably also hunted along the shoreline and sloughs within the MBNMS.
Shorebirds reach their greatest densities during October through March in Elkhorn Slough (Ramer et al. 1991;Figure 3), which is centrally located along the MBNMS shoreline. This seasonal peak reflects the presence of individuals moving to and from northern breeding grounds, as well as large numbers of overwintering birds. However, many shore and marsh species occur year-round, including several that breed locally. The American Avocet, Black-necked Stilt and Killdeer commonly nest in local sloughs (Roberson 1985). Much less common are the Black Oystercatcher, whose nests are sparsely distributed along rocky coasts (Sowls et al. 1980), and the Snowy Plover, which nests in coastal dunes (Warriner et al. 1986). American Coots, Mallards, Pintails, and Cinnamon Teal breed in marsh vegetation; egrets and herons nest in trees near coastal wetlands at Elkhorn Slough and other locations in the MBNMS (Roberson 1985, Buchanan 1990).
The MBNMS coast supports or supported a number of special status species, including the endangered Peregrine Falcon (note federal delisting in progress), and California Species of Special Concern such as the Short-eared Owl, Bald Eagle and Osprey (see Special Status Species Table). The MBNMS coast is also critically important to California populations of two species which have decreased significantly in number during the past two decades-- the Snowy Plover, a threatened species inhabiting sandy beaches and dunes (Warriner et al. 1986), and the Black Oystercatcher occurring along the rocky coast (Warheit et al. 1984, Warheit and Lindberg 1988). In estuarine habitats of the region, the endangered Clapper Rail has severely declined and may have been extirpated (Roberson 1985).
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