Abstract
Swainson’s hawk (Buteo swainsoni) has experienced population declines throughout much of its range and the most drastic decline has occurred in California. Once abundant throughout California, the range of the Swainson’s hawk has been reduced. Two disjunct populations remain in the state. A small population occurs in the Modoc Plateau and a larger population is concentrated in the four county region of Sacramento, San Joaquin, Yolo, and Solano Counties. This is considered the core breeding area for the species in California. Swainson’s hawk is listed as a State Threatened Species by the California Department of Fish and Wildlife and a bird of conservation concern by the United States Fish and Wildlife Service. Loss of suitable foraging and nesting habitat are major contributors to the hawk’s protected status in the state. Understanding how Swainson’s hawk utilizes the landscape for foraging and nesting is a critical aspect in the effort to recover the species. The purpose of this study was to determine foraging habitat preference in a 194 square kilometer study area in the Lower Cache Creek Resources Management Area in central Yolo County and to identify predictive factors of nest presence. A secondary objective was to identify and measure nest tree characteristics. From March 2010 through August 2010 strip-transect road surveys were conducted to observe and document Swainson’s hawk foraging behavior and the land use with which it was associated. Six routes were driven twice a week. All observed raptors were recorded as well as their associated behavior, and associated land use. Only the Swainson’s hawk foraging observations were analyzed. An attempt to census the nesting population was done by intensively searching the study area by vehicle and on foot. Land use within 500 m and 1600 m buffers were analyzed around nest trees with ArcView GIS 10.1 and then compared to nest buffers established around random trees to determine if predictors of nest presence existed. Swainson’s hawks preferred to forage in hayfields, alfalfa, and tomato-row crops in greater proportion than their availability. They used almond orchards, walnut orchards, grain crops, residential, and “other” cover types significantly less than expected based on their availability. Nest trees were significantly closer to paved roads, human dwelling and red-tailed hawk nests than random trees. Diameter at breast height (DBH), crown width, and height were all significantly greater that random trees. Several habitat categories predicted nesting presence including agricultural-industry, annual grass, natural habitats, eucalyptus-conifer, idle fields, and hay. Negative predictors included; increased distance from roads, human dwellings, and red-tailed hawk nests, as well as row crops, agricultural-urban, aggregate mining, irrigated field crops, corn, and grain crops. The study area has a good proportion of high quality foraging habitat and preferred crops for Swainson’s hawk. While land use influences where Swainson’s hawks nest, it may not be the only determinant of nest presence. Nesting may also be limited by availability of large trees and competition with other species. Further investigation should be conducted to determine additional factors that influence Swainson’s hawk nest presence.