Avian resource depression or intertaxonomic variation in bone density? A test with San Francisco Bay avifaunas

Previous analyses of the Emeryville Shellmound fauna suggested that ever-expanding late Holocene human populations of the San Francisco Bay area depressed a wide range of vertebrate taxa, including cormorants (Phalacrocorax), geese (Anserinae), and large shorebirds (e.g., Numenius, Limosa). Far-reaching implications for prehistoric human behavior, historical ecology, and modern conservation biology have resulted from that work. We test the generality of the bird-based conclusions here by documenting temporal trends in avifaunal abundances from five additional sites located on the San Francisco Peninsula. Analyses of these data reveal patterns identical to those found at Emeryville, showing significant declines through time in the relative abundances of both geese and cormorants between about 2000 and 700 B.P. New photon absorptiometric-based density data for cormorants, geese, and ducks (Anatinae) are used to determine if the declines through time in the relative abundances of geese and cormorants are correlated with variation in the extent of density-mediated attrition observed in these faunas. The data and analyses presented here suggest that while density-mediated attrition played a role in structuring element abundances in these avifaunas, there is no evidence that it varied systematically through time to produce the revealed patterns in taxonomic abundances. Resource depression thus remains a viable hypothesis for the declining abundances of large-sized waterbirds during the late Holocene of the San Francisco Bay region. The new bone density data we present should be useful to analyses of archaeological and paleontological avifaunas wherever these or related taxa are found around the world.