BREEDING STATUS AND HABITAT USE OF BLACK-NECKED STILTS AND AMERICAN AVOCETS IN SOUTH SAN FRANCISCO BAY

In light of recent and proposed restoration projects that will affect bird numbers in San Francisco Bay, California, we assessed the status of breeding populations of the Black-necked Stilt (Himantopus mexicanus) and American Avocet (Recurvirostra mericana) in South San Francisco Bay in May 2001. We counted 1184 stilts and 2765 avocets. Considering only birds observed exhibiting breeding behaviors, our low estimates of breeding birds in the south bay were 270 stilts and 880 avocets, but the true numbers are probably closer to the number of stilts and avocets we actually counted. Our estimates of the breeding population fall within the range of similar estimates from the south bay 20-30 years ago. We know of no other sites on the Pacific coast of the United States with breeding populations of stilts and avocets whose sizes approach those of the South San Francisco Bay. The greatest numbers of stilts and avocets bred on salt ponds in the south bay; lesser numbers bred in a combination of fresh and salt marshes. The observed use by stilts and avocets of available habitat differed significanfiy from expected use. Stilts used tidal marshes and salt ponds approximately in order of availability, whereas avocets made greater use of salt ponds. Within marshes, stilts most often used vegetated areas followed by mudfiat/open water habitat, whereas for avocets the pattern was reversed. Within salt ponds, both species were most often observed on islands, but their order of use of other microhabitats in salt ponds differed. We observed little use of tidal fiats by breeding stilts and avocets. The San Francisco Bay estuary (hereafter, the bay) is recognized as a site of hemispheric importance to shorebirds (Harrington and Perry 1995) because it holds over 500,000 shorebirds (Page et al. 1999). Over 90% of the bay's wetlands, especially tidal marsh, have been filled or diked over the past I50 years to create agricultural ands and salt-evaporation ponds (Harvey et al. 1988, Goals Project 1999). Salt ponds now cover over 12,000 ha around San Francisco and San Pablo bays (Goals Project 1999), the majority being in South San Francisco Bay (hereafter, the south bay). 2 Western Birds 34:2-14, 2003 BLACK-NECKED STILTS AND AMERICAN AVOCETS IN SAN FRANCISCO BAY The presence of American Avocet bones in native American middens suggests that this species was in the bay prior to the first published report in 1884 (Grinnell et al. 1918, Howard 1929). The first breeding record was an observation of downy young in 1926 (Gill 1977), a year prior to Grinnell and Wythe's (1927) listing the bird as an irregularly common visitor to the bay. No additional documentation of breeding by avocets exists prior to Martin's (1939) discovery of young in Santa Clara County in 1937. By 1952, Sibley (1952) considered the species to be a common resident but listed only scattered breeding records for 1941 and 1950. In 1972, Gill (1972) estimated 1800 breeding pairs in the south bay. Subsequent studies by Moss (1980) and Rigney and Rigney (1981) estimated 800 and 650 breeding pairs, respectively. Grinnell et al. (1918) noted that Black-necked Stilts appeared sparingly in the bay, and Grinnell and Wythe (1927) reported the first nesting there by this species. The Black-necked Stilt remained an uncommon summer resident and rare winter visitant in the south bay through the early 1950s (Sibley 1952). Numbers of nesting stilts increased over the next two decades with breeding populations in the south bay estimated at 400-500 pairs in 1971 (Gill 1972) and 600-650 pairs in 1981 (Rigney and Rigney 1981). The creation of salt ponds has been credited with increasing breeding and nonbreeding populations of Black-necked Stilts and American Avocets in the bay (Gill 1977, Harvey et al. 1988), providing roosting, foraging, and nesting habitat for both species. Here, we report on a spring survey in 2001 that examined the abundance, distribution, behavior, and habitat use of stilts and avocets in the south bay. We compare our results to those from prior surveys in the early 1970s and 1980s and discuss our findings with respect o various active or proposed restoration projects that may affect future breeding populations of these birds. STUDY AREA AND METHODS We surveyed stilts and avocets in San Francisco Bay south of the San Mateo Bridge (Figures 1 and 2) where other studies have focused and where the majority of the estuary's tilts and avocets breed (Gill 1977, Harvey et al. 1992). Two survey teams started on opposite sides of the bay and, in general, moved south to complete coverage of the study area. We searched salt ponds and other wetlands in their entirety for adult stilts and avocets. We also surveyed tidal fiats adjacent o salt ponds and marshes that border the bay as far out as we could see. Although we tried to cover all wetlands, some private salt-crystallization ponds were not accessible (see Figure 1 or 2). In addition, outer Bair Island and its immediate vicinity, including adjacent idal fiats, were not surveyed because of difficulties in access. We surveyed 9613 ha of salt ponds, 4068 ha of tidal or diked marshes, 575 ha of other diked wetlands, and approximately 4039 ha of tidal fiats. Study Period and Survey Technique We surveyed for 120 hours from 15 to 25 May 2001, during the peak breeding period for stilt and avocets in the south bay (PRBO unpubl. data; see also Robinson et al. 1997, Robinson et al. 1999). Two teams of two or BLACK-NECKED STILTS AND AMERICAN AVOCETS IN SAN FRANCISCO BAY