Dependence of Breeding Birds on the Density of Oaks

Oaks ‘n’ Folks – Volume 2, Issue 2 – November, 1987

The persistence of oak woodlands in California is threatened by suburban development, fuel wood harvest, range management, and poor regeneration. Oak woodlands are extremely valuable to wildlife not only because of the high food value of acorns but also because they support an abundant insect fauna. These insects support large breeding bird populations and provide an important food source to bird migrants on their way to breeding grounds.

The University of California’s Hopland Field Station is located on the eastern slope of the Russian River Valley in southeastern Mendocino County. Researching being conducted here will enable us to determine how tree species’ composition, density and distribution affects individual bird species and bird community composition.

We have established 23 five-hectare plots representing a range of tree densities from 20 to 450 trees per hectare (8-180 trees/acre). Censuses, foraging observations, and nest searches were conducted on each plot during the spring of 1986 and 1987. In addition, all trees and shrubs on each plot were measured and their location mapped.

This study will enable us to identify bird species most sensitive to habitat degradation, to determine habitat specialists, and to clarify ecological interrelationships that exist in oak woodlands. We are currently analyzing data with the eventual goal of inferring management guidelines from our results. This should result in the development of a predictive model to describe the resultant bird community under a variety of habitat conditions. The model could be used to provide advice to private landowners who want to consider bird populations in the management of their woodlands. In addition, if oaks become a commercially harvested species, wildlife surveys such as ours will provide important base line information for developing harvest regulations and predicting effects on wildlife populations.

One obvious result of this study is that oak woodlands support a rich diversity of breeding birds. In addition, bird species richness (the absolute number of species) increases as the elevation increases at the Hopland Field Station. Analysis of our data is still underway.

The California Department of Forestry and Fire Protection is funding this study within the Integrated Hardwood Range Management Program through Humboldt State University. Dr. Barry R. Noon is the principle investigator and Randolph Wilson and myself are graduate assistants.

We hope that the results of this study, as well as other wildlife studies currently being conducted in oak woodlands, will contribute to the wise management and understanding of our oak woodlands and their associated wildlife populations.

prepared and edited by John M. Harper, Richard B. Standiford, and John W. LeBlanc