Oaks ‘n’ Folks – Volume 12, Issue 2 – September, 1997
Red-tailed hawks are a common resident of much of California’s oak woodlands.Large, powerful raptors, red-tailed hawks are a member of a group of birds called Buteos, sometimes called “buzzard hawks” for their soaring nature. The most broadly distributed Buteo in North America, red-tailed hawks range from Alaska to eastern Canada and as far south as Panama. Red-tailed hawks can stand up to 25 inches tall with a wingspan of nearly 50 inches.
The nesting ecology of red-tailed hawks has been described for many areas in North America, from Puerto Rico to the northeastern United States to the upper Midwest to the desert southwest. No studies have documented nesting habitat of red-tailed hawks in California oak woodlands.
If oak woodlands continue to be degraded or destroyed, potential shortages of preferred nest sites may limit populations of red-tailed hawks. Describing the nesting habitat of red-tailed hawks in oak woodlands of central California will provide baseline information to help land managers evaluate human effects on red-tailed hawk habitat and to help conserve that habitat.
From January to June 1992 to 1995, 44 active nests of red-tailed hawks were located on three private livestock ranches in San Luis Obispo County and on Camp Roberts in San Luis Obispo and Monterey Counties. To describe the habitat of nesting sites, 17 variables were measured in summer and fall on nest trees and in circular, tenth-acre plots centered on nest trees.Six variables described the nest exclusively, 5 variables described characteristics of the tree in which the nest was located, and 6 variables described the vegetation characteristics of the tenth-acre plots. To determine if red-tailed hawks were using nest sites that differed from other available but unused sites, these same variables (except the nest-specific variables) were measured on trees not used for nesting (i.e., no nest was present), and in tenth-acre plots centered on the unused tree. One each of these unused trees was randomly located between 400 and 800 ft from each nest tree.
Nests were found in seven species of trees: 25, 8, and 1 in blue, valley,and coast live oaks, respectively; 4 in western sycamore, 3 in gray pine,2 in eucalyptus, and 1 in Fremont cottonwood. Hawks displayed no preferential selection for nest trees of particular species when compared with available proportions of unused tree species.
Nest trees were substantially taller, had larger diameter trunks, and had broader canopies than their unused counterparts (Table 1). Similarly,nest trees occupied a dominant canopy position relative to the trees in the surrounding tenth-acre plot more often than unused trees relative to trees in their surrounding plots. Of 33 plots in which trees other than the central tree occurred, nest trees were approximately twice the height of surrounding trees within the plot. The density and percent canopy cover of trees and shrubs were similar between nest and unused sites. Ground cover of herbaceous vegetation and downed wood also were similar.
On average, nests were nearly 15 horizontal feet from the trunk of the nest tree and were approximately midway between the trunk and the dripline. Sixty percent of nests were in the top 20 percent of the nest tree. Most nest trees faced north (60%) or east (25%).
Red-tailed hawks selected large trees for nesting. In another study conducted at Camp Roberts, 1,233 tree diameters of randomly selected trees were measured.Fewer than 6 percent of those trees were as large as our smallest nest tree(17 inches dbh), indicating a strong preference by red-tailed hawks for the largest trees in central-coastal California oak woodlands.
Tall trees probably provide several advantages to nesting red-tailed hawks: protection from predators, a sturdy base for the nest, and a superior vantage, which may facilitate vigilance and accessibility to hunting areas.Tree species probably is unimportant to red-tailed hawks when selecting a site for nesting, as long as the tree’s architecture, size, and location on the landscape are suitable. In some areas of North America where large trees are not present, red-tailed hawks will use other substrates such as cacti and utility poles. Whether hawks would use smaller trees if larger trees are removed from oak woodlands, or if the use of small trees would adversely affect red-tailed hawk populations, is unknown.
The sustainability of much of California’s 8 million acres of oak woodland is a significant concern. In some areas, some of the 19 native oak species are not regenerating well. Valley oak and blue oak, among those most-used for nesting by red-tailed hawks in this study, are two species that are regenerating particularly poorly. Most oak woodland in California is privately owned and is subject to various anthropogenic activities including grazing,firewood cutting, and development pressures. Livestock grazing probably has little direct effect on nesting habitat of red-tailed hawks, but-at local scales-may affect their distribution and abundance to the extent that it impedes tree regeneration and influences rodent populations. Grazing may increase populations of ground squirrels, which are likely an important prey item for red-tailed hawks in this study area.
Of greater concern, however, is the direct removal of trees to accommodate development pressures and intensive agricultural cropping. Red-tailed hawks may tolerate large-parcel development, but increased noise, human activity,pets, road construction, and reductions of local prey populations make this tolerance of development unlikely. Firewood cutting, livestock grazing,land conversion, and other anthropogenic activities should maintain a component of the largest trees in oak woodlands.
Table 1. Selected habitat characteristics of nest trees and surrounding(tenth-acre) sites of red-tailed hawks, and of unused trees and sites in oak woodlands in central California, 1992-1995.
prepared and edited by Richard B. Standiford and Pamela Tinnin
Justin K. Vreeland, Staff Research Associate, Integrated HardwoodRange Management Program