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Swainson's hawk is monogamous, and pairs remain together year after year until one dies. Nesting begins well after arrival in Central California,usually in May. Incubation lasts from 28 to 35 days, and nestlings are fledged at between four and eight weeks of age. Nest sites are solitary trees or small groves adjacent to agricultural fields or open grassland (Bloom, 1984). Many of the nesting areas are associated with riparian woodland. Schlorff and Bloom (1984) report that 82 percent of nests in their study were located within one mile of riparian forests. Estep (1989) confirms this fmding with an estimate of 78 percent nesting in riparian areas. Nests are usually located near the tops of tall trees, sometimes up to 90 feet above the ground (Mallette and Gould, 1978). Nest locations are normally within easy flying distance to agricultural fields with abundant rodent pray. Major prey items include rodents(California meadow voles,pocketgophers,California ground squirrels), birds (meadow larks, mourning doves, young ring-necked pheasants), and larger insects such as grasshoppers and Jerusalem crickets. Foraging range depends on both the abundance and availability of prey. Thus, a given piece of land could have a very high mouse population,but if the vegetative cover there was such that these rodents could not be detected from an aerial perch, then the site would be of little value to this or any other soaring hawk species. The actual home range area over which foraging occurs also varies greatly with land use. Craighead and Craighead (1956) recorded maximum foraging range in Wyoming as varying between 180 and 1,056 acres. In Central California, Estep (1989) recorded a range from 30 to 16,000 acres. Bechard (1982) found a direct correlation between foraging home range and cultivated land. Where the latter occupied more of the acreage in an area, the foraging area was consistently larger. Perhaps the most pertinent current estimate of foraging requirements are those given in the 1990 California Department of Fish and Game Mitigation Guidelines for Swainson's Hawk in the Central Valley of California. This document states that the minimum acreage needed to support a nesting pair is 1,200 acres and that all acreage within a ten mile radius of an active nest is considered potential foraging habitat. In addition to the observations made during this study, there are two additional reports which show Swainson's hawk utilization of Brovelli woods. One is the Brovelli Woods Raptor Survey(Guse, 1990) which was conducted over only one half of a full daylight period in late June, but still reports two possible nest sites in Brovelli Woods. The other source is the Habitat Conservation Plan for the Swainson's Hawk in San Joaquin County(Jones& Stokes Associates, 1990)which reports three nesting territories for the project site and places all 25 nesting territories in the Dry Creek Swainson's Hawk subpopulation within CDFG's estimated average foraging radius of 10 miles of the site (Figure 4.7-14). The project site falls within the range of this subpopulation group. One other state-listed threatened species was observed during the course of the study. This is the greater sandhill crane (Gnus canadensis tabida). Mixed flocks of both greater and lesser sandhill cranes were observed on two occasions in the pasture north of the project site and once on the project site. The latter observation was made after the early growth of annual grass and forb species had begun to grow above the previous season's dead grass mat. It is also possible that on some occasions this subspecies utilizes the Tracy Lake basins for both feeding and roosting given the fact that numerous sightings have been made on lands adjacent to the site (Gifford, 1992). Because work was temporarily halted during the major wintering period for sandhill cranes in San Joaquin Valley, no crane roosting in the shallow bay areas of Tracy Lakes was documented. Like most other large water birds, cranes are very refractory to human presence, and their use of any foraging or roosting site is predicated on the relative absence of humans. 19