ABUNDANCE AND DISTRIBUTION OF THE COMMON RAVEN AND AMERICAN CROW IN THE SAN FRANCISCO BAY AREA, CALIFORNIA

Abstract

During the breeding season of 1999, we surveyed from roads in the San Francisco Bay area to determine the regional abundance and distribution of the Common Raven (Corvus corax) and American Crow (C. brachyrhynchos). Ravens concentrated along the outer coast and occurred in relatively low numbers in some interior areas, whereas the number of crows increased significantly from the outer coast to interior and bayshore locations. Both species occurred in significantly greater densities along urban and suburban survey routes than along rural routes, but dramatic exceptions were evident in some areas. Data from Breeding Bird Surveys, Christmas Bird Counts, and breeding bird atlases yielded similar distribution patterns. Breeding Bird Surveys and Christmas Bird Counts revealed strong regional increases in both species, but annual trends varied substantially at local scales. This variation, as well as significant abundance variation among survey routes, suggested considerable local differences in either habitat suitability or capacity for further population growth. Our results suggested that raven and crow populations may increase in both rural and developed areas undergoing rapid urbanization and that local conditions, rather than whether the habitat is rural or urban, may influence regional patterns. Abundances of the Common Raven (Corvus corax) and American Crow (C. brachyrhynchos) have increased substantially over much of North America (Marzluff et al. 1994, Boarman and Heinrich 1999, Sauer et al. 2001). The Common Raven occurs throughout California but is scarce in much of the Central Valley, the central coast from the Santa Lucia Moun- tains south to northwestern Ventura County, and irrigated portions of the Coachella, Imperial and lower Colorado River valleys in the southeast corner of the state. The American Crow is widespread but absent from some drier western parts of the San Joaquin Valley, interior foothills, and the southeast- ern deserts (Small 1994). Throughout their range, ravens appear to be invading agricultural areas to a greater extent than urban areas (but are common in many urban areas), whereas crows appear to be invading urbanized areas more rapidly than agricultural areas (Marzluff et al. 1994, Marzluff and Restani 1999). Both species are resident through most of the San Francisco Bay area, a highly urbanized region with large sections that remain undeveloped or used for agriculture. Both species have increased in number recenfiy in the southern portion of the region (Coston 1998). The overall status and distribution of these corvids within the San Francisco Bay area, however, have not been addressed. To determine the abundance and distribution of ravens and crows in the San Francisco Bay area, we surveyed the region during the spring of 1999 (Figure 1). Because field observations were limited to repeated road surveys along particular routes within a single season, we compared the results with existing Breeding Bird Survey (BBS; Sauer et al. 2001), Audubon Chrisbnas