Oaks ‘n’ Folks – Volume 12, Issue 1 – February, 1997
Livestock have been grazing oak woodland habitat since the Spanish introduced them in the sixteenth century. Today, more than 80% of California oak woodlands are grazed by livestock. Though we can be relatively certain that cattle ranching is a less damaging use for this land than housing subdivisions,what are the effects of grazing on wildlife?
Using data we have collected from 1985 through 1995, we compared the number of bird species and their abundances on two 80-acre plots at the San Joaquin Experimental Range (SJER), in Madera County, California. One of these plots was set aside in 1934 and has not been grazed by livestock since. The other plot has been moderately grazed, along with most of the rest of SJER, since about 1900. Our goal was to attempt to understand the possible effects of long-term grazing on bird communities in oak-pine woodlands of the western Sierra Nevada.
More species of birds have nested on the ungrazed plot (38) than on the grazed plot (33) during our study, although the difference was not statistically significant. We found few differences in the number of territories of individual species between the two plots. Eight species were more abundant on the ungrazed plot: Cooper’s hawk, great-horned owl, hairy woodpecker, western kingbird,common raven, wrentit, California thrasher, and Bullock’s oriole. However,all of these species nest in grazed areas in other parts of SJER. Livestock grazing probably has reduced the number of some of these species in the oak-pine woodlands at SJER, but we have no evidence that grazing is a threat to any of them.
Four species nested on the grazed site only: turkey vulture, American kestrel, rock wren, and rufous-crowned sparrow. These differences probably are due to our plots being too small to adequately sample species with large territories and to provide specific habitat requirements, such as rocky areas needed by the wrens and sparrows.
The most obvious difference between the grazed and ungrazed plots is the amount of shrub cover. The ungrazed plot had more than three times the shrub cover of the grazed plot, and cover of the most abundant shrub species,wedgeleaf ceanothus, was nearly seven times higher on the ungrazed plot.The difference in shrub cover was reflected in the bird community: five species of shrub nesters nested on the ungrazed plot, but only three species of shrub nesters nested on the grazed plot. The relatively few species dependent on shrubs for nesting on both plots was unexpected.
Tree nesters, including those that build open cup nests and birds tha tnest in tree cavities, comprise 75 percent or more of the bird species nesting in our study area. Nonexcavators, birds that nest in natural cavities orcavities excavated by other species, were abundant on both grazed and ungrazed sites and are an important component of the oak woodland bird community,both in terms of number of species and abundance. Most excavated cavities are the work of acorn woodpeckers and Nuttall’s woodpeckers. Although most species of nonexcavators also use natural cavities formed by limb scars and heart rot, they still depend on these woodpeckers for nest sites.
European starlings have increased in abundance and extended their range in the foothill region over the last ten years. Starlings were more abundant on the grazed plot, although not significantly so. All starling territories on the ungrazed plot were found near the plot edges. Starlings forage mainly on the ground. They have been seen foraging throughout the grazed plot,but not at all on the ungrazed plot, where apparently the tall grasses and forbs deny them access to the ground. Apparently, starling numbers have been increased by grazing. Starlings may compete for nest sites with other bird species, including western bluebirds and violet-green swallows. An increase in starlings might have a negative influence on some native cavity-nesting bird species.
Brown-headed cowbirds regularly associate with cattle and probably are more abundant today than they would have been in the absence of grazing.Cowbirds are nest parasites, which lay their eggs in the nests of host species.These host species in turn raise the cowbird’s young, usually at the expense of their own young. We found no difference in the number of cowbirds between the grazed and ungrazed plots. Cowbirds often fly long distances between their food sources and nesting areas. This may account for the lack of difference we noted between the two plots. We have no evidence that cowbird nest parasitism is a threat to any host species at SJER.
Overall, our results do not demonstrate that grazing has led to the loss of any bird species that regularly nests in foothill oak-pine woodland habitat.The availability of trees, especially oaks, to provide nesting and foraging sites is the most important vegetative component contributing to avian biodiversity in this habitat. Whether the increases in starling and cowbird abundance are a serious concern remains to be evaluated. Although no evidence of such an effect is evident at this time, competition for nest cavities between starlings and other, native species needs further study.
Kathryn Purcell and Jared Verner
prepared and edited by Justin Vreeland, Bill Tietje, and Pam Tinnin