Covariance Patterns Among Birds and Vegetation in a California Oak Woodland

We sampled characteristics of vegetation and estimated abundances of bird species on 23 plots representing a continuum of tree densities of the blue oak phase of the Coast Range foothill woodland near Hopland, California. Fifty-one bird species were found breeding. Cavity-nesters dominated the bird community in number of species and individuals. Cavitynesters used a variety of tree species for nesting, highlighting the importance of tree species richness. Large deciduous oaks were found to be important as granary trees for acorn woodpeckers, as well as substrates for nest cavity excavation by primary cavity nesters. Large evergreen trees were important in providing natural cavities to many secondary cavity nesting bird species. Both individual bird species and guilds showed few covariations with tree density. We discuss why a guild approach is not always a useful way to describe relationships between bird abundance and vegetation. Effects of spatial scale and plot size on observed bird/habitat relationships are discussed. Approximately eight million hectares in California, or 20 percent of the state's land area, are vegetated by one or more species of oak (Quercus spp.). The rate of harvest of oak trees in California, however, has increased to the point where many oak habitats are threatened. Fuelwood harvest, for example, has increased steadily since 1959, turning sharply upward in 1973 (Menke and Fry 1980, Walt and others 1985). The rate of conversion of oak woodlands to pasture land and for agricultural, residential and commercial development has also increased markedly since 1973 (Bolsinger 1987). Unfortunately, the harvest of oaks may severely reduce habitat quality for many wild animals, or preclude some wildlife species entirely. A further problem is that many oak woodlands are experiencing very poor regeneration and extremely low seedling and sapling survival. The exact cause for the low recruitment is unknown, but has been attributed in some cases to excessive predation by small mammals (Griffin 1980, Knudsen 1987) and overgrazing by deer and cattle (Bowyer and Bleich 1980, Griggs 1987). In the face of these threats and an increasing trend of exploitation, it is particularly disturbing to discover that there remain significant gaps in our understanding of the relationship between California wildlife and oak woodlands. Muick and Bartolome's (1985) listing of studies conducted in California Presented at the Symposium on Oar Woodlands and Hardwood Rangeland Management. October 31-November 2, 1990. Davis, California. Wildlife Biologist, Redwood Sciences Laboratory, Forest Service. U.S. Department of Agriculture, Arcata, California; Wildlife Biologist, Six Rivers National Forest. Forest Service, U.S. Department of Agriculture, Eureka, California; Research Ecologist, Redwood Sciences Laboratory, Forest Service, U.S. Department of Agriculture, Arcata. California. 126 oak habitats between 1953 and 1985 included l6 bird studies, 15 mammal studies, and only one reptile and amphibian study. Few of these studies addressed the results of habitat disturbance or habitat loss on the wildlife community. California oak woodlands are particularly rich in bird speciesapproximately 110 species of birds can be observed during the breeding season (Verner 1980). Verner (1983) reported that oak woodlands in North America rank among the top three habitat types in the number of bird species for which they provide breeding habitat. In order for a resource manager or agency to make recommendations on the management of oak woodlands for birds, more information is needed on how birds respond to variation in the vegetation structure and composition of these habitats. For example, at this time the density, size, or spatial distribution of trees required to meet the needs of the bird community on managed woodlands is unknown. It is important that the needs of the wildlife community be considered along with economic and aesthetic considerations in the management of California's oak woodlands. Two silvicultural options which can easily be targeted for management are the residual density of trees remaining after the harvest and tree species composition. Tree density and tree species composition can be optimized for a variety of purposes including regeneration potential, forage production, aesthetic quality, and wildlife value. This paper reports on the relationship among various attributes of the breeding bird community and the vegetative community, primarily tree density and tree species composition. The relationship of the bird community to the vegetative community is defined by changes in bird species composition and changes in the abundance of particular bird species. In addition, we described the nest site selection patterns of the cavity nesting birds and granary tree selection by acorn woodpeckers. Selection was explored in terms of used and available tree species and tree diameters.