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FORAGING REQUIREMENTS <br /> Swainson's hawk nests in the Central Valley of California <br /> are generally found in scattered trees or along riparian systems <br /> adjacent to agricultural fields or pastures. These open fields <br /> and pastures are the primary forage areas. Major prey items for <br /> Central Valley birds include: California voles (Microtus <br /> californicus) , valley pocket gophers (Thomomys bottae) , deer mice <br /> (Peromyscus maniculatus) , California ground squirrels <br /> (Spermophilus beecheyi) , mourning doves (Zenaida macroura) , <br /> ring-necked pheasants (Phasianus col.chicus) , meadowlarks <br /> (Sturnella neglecta) , other passerines, grasshoppers <br /> (Conocephalinae) , crickets (Gryllidae) , and silphadids (Estep <br /> 1989) . Swainson's hawks generally search for prey by soaring in <br /> open country and agricultural fields similar to northern harriers <br /> (Circus cyaneus) and ferruginous hawks (Buteo rectalis) . Often <br /> many hawks may be seen foraging together following tractors or <br /> other farm equipment capturing prey escaping from farming <br /> operations. During the breeding season, Swainson's hawks eat <br /> mainly vertebrates (small rodents and reptiles) , whereas during <br /> migration vast numbers of insects are consumed (Palmer 1988) . <br /> Department of Fish and Game funded research has documented <br /> the importance of suitable foraging habitats (e.g. , native <br /> grasslands, pasture lands, alfalfa and other hay crops, and <br /> combinations of hay grain and row crops) within an energetically <br /> efficient flight distance from active Swainson's hawk nests <br /> (Estep pers. comm. ) . Recent telemetry studies to determine <br /> foraging requirements have shown that birds may utilize in excess <br /> of 15 , 000 acres of habitat or range up to 18 . 0 miles from the <br /> nest in search of prey (Estep 1989) . The area needed for <br /> foraging is determined by crop types, agricultural practices, <br /> harvesting regimes, prey abundance, and availability. Estep <br /> (1989) found that 73 . 40 of observed prey captures were in fields <br /> being harvested, disced, mowed, or irrigated. Some of the <br /> preferred foraging habitats for Swainson's hawks include: (1) <br /> Alfalfa - low prey abundance but steady prey accessibility. (2) <br /> Fallow fields - high prey abundance and prey accessibility if not <br /> dominated by thistle. (3) Beet and Tomato fields - largest prey <br /> populations but dense cover reduces prey accessibility, except <br /> during harvesting operations when Swainson's hawks have been <br /> observed foraging almost exclusively in these fields from late- <br /> July to early-September. (4) Dry-land pasture provided the <br /> primary forage area for 1 radioed pair, and appears to be an <br /> important foraging area. (5) Irrigated pasture provides some <br /> forage habitat, especially during flooding. (6) Rice land <br /> appears to provide valuable early season (prior to flooding) and <br /> late season (fall and winter migration periods) foraging habitat. <br /> Unsuitable foraging habitat types include any crop where prey are <br /> not available due to the high density of vegetation, or have low <br /> abundance of prey such as vineyards, mature orchards, and cotton <br /> fields. <br /> VI-16 <br />