Birds and Oaks in California’s Urban Forest

Daniel A. Airola and Steven E. Greco

Published May 2019 in International Oaks No. 30: 109–116

Abstract

We describe three studies that address the importance of native oaks (Quercus spp.) to birds in the urban forest of Sacramento, California, U.S.A. In one study, we surveyed tree canopy cover and abundance of spring and fall migrant songbirds in an older residential neighborhood. Migrant abundance was strongly and positively correlated with Q. lobata (valley oak) canopy cover, with migrants nearly absent from areas lacking oaks. Migrants foraged in Q. lobata substantially more often (74%) than expected based on its 15% relative canopy cover. In another study, we evaluated abundance of resident and wintering birds across a gradient from young residential neighborhoods to natural woodlands. Urban forest with oak supported eight bird species that were absent from urban forest without oak, and supported substantially higher abundances of eight other species. Highest bird species richness occurred in the mixed oak-riparian community. A third study evaluated acorn harvest and transport by Aphelocoma californica (California scrub-jays) in urban settings. Jays harvested mainly Q. agrifolia (coast live oak) acorns for >121 days and transported them on average >159 m, to a maximum of 665 m, regularly crossing other jays’ territories during transport. They cached an estimated 6,800 and 11,000 acorns at two sites (339 and 845 acorns/season/jay), substantially less than in natural oak woodlands, likely reflecting the increased time required to transport acorns longer distances and the availability of alternative foods. Many seedlings from acorns not retrieved by birds are removed as weeds, but recruitment in less-maintained urban areas provides habitat benefits for other species.

Keywords

urban forest, migratory birds, oaks, Quercus

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