Avian community use and Occ upancy of California Oak Savanna

Abstract

California oak savanna is a habitat of sparse tree canopy that extends from northern Baja California to southern British Columbia and is under threat from land-use pressures such as conversion to agriculture, overgrazing, urban development, and fire suppression. Bird-conservation plans have been drafted for the region's oak woodlands. Yet it is unclear whether birds use California oak savanna at different frequencies than they do neighboring oak habitats. In the foothills of the central and northern Sierra Nevada, California, we explored patterns of avian community structure and habitat occupancy in four habitats: blue oak (Quercus douglasii) savanna with a well-developed grass and forb layer, blue oak savanna with a well-developed shrub layer, and two habitats with a denser canopy, blue oak woodland, and montane hardwood. Additionally, we assessed the effect of habitat characteristics on avian community structure and occupancy. Avian communities were uniquely grouped among the four habitats. Five species of management and conservation concern-the Western Kingbird (Tyrannus verticalis), Western Bluebird (Sialia mexicana), Lark Sparrow (Chondestes grammacus), Western Meadowlark (Sturnella neglecta), and Bullock's Oriole (Icterus bullockii)-were predicted to occupy oak savanna habitats at frequencies higher than in oak woodland or montane hardwood. Shrub cover was the most influential habitat characteristic shaping the avian community and was negatively associated with occupancy of the five savanna-affiliated birds. The distinctive structure and occupancy patterns observed for species of concern in California oak savanna suggest that birds perceive this as unique habitat, highlighting the need for its conservation. © The Cooper Ornithological Society 2013.