Grazing in Oak Woodland: Does it Affect Bird Communities?
Livestock have been grazing oak woodland habitat since the Spanish introducedthem in the sixteenth century. Today, more than 80% of California oak woodlandsare grazed by livestock. Though we can be relatively certain that cattleranching 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 thenumber of bird species and their abundances on two 80-acre plots at theSan Joaquin Experimental Range (SJER), in Madera County, California. Oneof these plots was set aside in 1934 and has not been grazed by livestocksince. The other plot has been moderately grazed, along with most of therest of SJER, since about 1900. Our goal was to attempt to understand thepossible effects of long-term grazing on bird communities in oak-pine woodlandsof the western Sierra Nevada.
More species of birds have nested on the ungrazed plot (38) than on thegrazed plot (33) during our study, although the difference was not statisticallysignificant. We found few differences in the number of territories of individualspecies between the two plots. Eight species were more abundant on the ungrazedplot: 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. Livestockgrazing probably has reduced the number of some of these species in theoak-pine woodlands at SJER, but we have no evidence that grazing is a threatto any of them.
Four species nested on the grazed site only: turkey vulture, Americankestrel, rock wren, and rufous-crowned sparrow. These differences probablyare due to our plots being too small to adequately sample species with largeterritories and to provide specific habitat requirements, such as rockyareas needed by the wrens and sparrows.
The most obvious difference between the grazed and ungrazed plots isthe amount of shrub cover. The ungrazed plot had more than three times theshrub 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: fivespecies of shrub nesters nested on the ungrazed plot, but only three speciesof shrub nesters nested on the grazed plot. The relatively few species dependenton shrubs for nesting on both plots was unexpected.
Tree nesters, including those that build open cup nests and birds thatnest in tree cavities, comprise 75 percent or more of the bird species nestingin our study area. Nonexcavators, birds that nest in natural cavities orcavities excavated by other species, were abundant on both grazed and ungrazedsites and are an important component of the oak woodland bird community,both in terms of number of species and abundance. Most excavated cavitiesare the work of acorn woodpeckers and Nuttall's woodpeckers. Although mostspecies of nonexcavators also use natural cavities formed by limb scarsand heart rot, they still depend on these woodpeckers for nest sites.
European starlings have increased in abundance and extended their rangein the foothill region over the last ten years. Starlings were more abundanton the grazed plot, although not significantly so. All starling territorieson the ungrazed plot were found near the plot edges. Starlings forage mainlyon the ground. They have been seen foraging throughout the grazed plot,but not at all on the ungrazed plot, where apparently the tall grasses andforbs deny them access to the ground. Apparently, starling numbers havebeen increased by grazing. Starlings may compete for nest sites with otherbird species, including western bluebirds and violet-green swallows. Anincrease in starlings might have a negative influence on some native cavity-nestingbird species.
Brown-headed cowbirds regularly associate with cattle and probably aremore 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 expenseof their own young. We found no difference in the number of cowbirds betweenthe grazed and ungrazed plots. Cowbirds often fly long distances betweentheir food sources and nesting areas. This may account for the lack of differencewe noted between the two plots. We have no evidence that cowbird nest parasitismis a threat to any host species at SJER.
Overall, our results do not demonstrate that grazing has led to the lossof any bird species that regularly nests in foothill oak-pine woodland habitat.The availability of trees, especially oaks, to provide nesting and foragingsites is the most important vegetative component contributing to avian biodiversityin this habitat. Whether the increases in starling and cowbird abundanceare a serious concern remains to be evaluated. Although no evidence of suchan effect is evident at this time, competition for nest cavities betweenstarlings and other, native species needs further study.
prepared and edited by Justin Vreeland, Bill Tietje, and Pam Tinnin