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4 <br /> FORAGING REQUIREMENTS <br /> Swainson's hawk nests in the Central Valley of California are generally found in <br /> scattered trees or along riparian systems adjacent to agricultural fields or pastures. <br /> These open fields and pastures are the primary forage areas. Major prey items for <br /> Central Valley birds include: California voles (Micrutus californicus), valley pocket <br /> gophers (Thomomys bottae), deer mice (Peromyscus maniculatus), California ground <br /> squirrels (Spermophilus beecheyll, mourning doves (Zenaida macroura), ring-necked <br /> pheasants (Phasianus colchicus), meadowlarks (Sturnella neglecta), other passerines, <br /> grasshoppers (Conocephalinae), crickets (Gryllidae), and silphadids (Estep 1989). <br /> Swainson's hawks generally search for prey by soaring in open country and agricultural <br /> fields similar to northern harriers (Circus cyaneus) and ferruginous hawks (Buteo <br /> regalis). Often many hawks may be seen foraging together following tractors or other <br /> farm equipment capturing prey escaping from farming operations. During the breeding <br /> season, Swainson's hawks eat mainly vertebrates (small rodents and reptiles), whereas <br /> during migration vast numbers of insects are consumed (Palmer 1988). <br /> Department of Fish and Game funded research has documented the importance of <br /> suitable foraging habitats (e.g., native grasslands, lightly-grazed pastures, alfalfa and <br /> other hay crops, and combinations of hay grain and row crops) within an energetically <br /> efficient flight distance from active Swainson's hawk nests (Estee pers. comm.). <br /> Recent telemetry studies to determine foraging requirements have shown that birds may <br /> require in excess of 15,000 acres of habitat or range up to 18.0 miles from the nest in <br /> search of prey (Estep 1989). The area needed for foraging is determined by crop types, <br /> agricultural practices, harvesting regimes, prey abundance and availability. Estep <br /> (1989) found that 73.4% of observed prey captures were in fields being harvested, <br /> disced, mowed or irrigated. Some of the preferred foraging habitats for Swainson's <br /> hawks include: (1) Alfalfa - low prey abundance but steady prey accessibility. (2) Fallow <br /> fields - high prey abundance and prey accessibility if not dominated by thistle. (3) Beet <br /> and Tomato fields - largest prey populations but dense cover reduces prey accessibility, <br /> except during harvesting operations when Swainson's hawks have been observed foraging <br /> almost exclusively in these fields from late-July to early-September. (4) Dry-land <br /> pasture provided the primary forage area for 1 radioed pair, and appears to be an <br /> important foraging area. (5) Irrigated pasture provides some forage habitat, especially <br /> during flooding. Unsuitable foraging habitat types include any crop where prey are not <br /> available due to the high density of vegetation, or have low abundance of prey such as <br /> rice fields, vineyards, orchards, and cotton fields. <br /> NESTING REQUIREMENTS <br /> Swainson's hawks nest throughout most of the floor of the Central Valley, <br /> although nesting habitat is fragmented and unevenly distributed. More than 85% of the <br /> known nests in the Central Valley are within riparian systems in Sacramento, Yolo, and <br />