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Swainson's hawks have begun to overwinter in the California Delta in the vicinity of <br /> Terminus. Why these birds adapted to wintering in the Northern Hemisphere is still <br /> unknown. <br /> Swainson's Hawk Life History <br /> Swainson's hawks nest almost exclusively in trees, with a few recorded on cliff faces and <br /> on the ground (Bent 1937). Nesting trees are typically located on the edges between <br /> woodlands and either grasslands or shrub habitats,or in isolated trees and clumps in open <br /> country. In most cases the tree selected is the largest in the area, and the nest has a <br /> panoramic view (Bent 1937). Surveys in California revealed that Swainson's hawks <br /> nested almost exclusively in large, sparsely vegetated flatlands characterized by valleys, <br /> plateaus, broad foodplains, and desert expanses. None have been found nesting in dense <br /> woodlands unless they were adjacent to vegetated plains or certain agricultural croplands <br /> (Bloom 1980). In the Great Valley, Swainson's hawk nests are located in emergent valley <br /> oak (71.9% of the nest located); cottonwoods (9.4%), willow (3.1%) and ornamental <br /> plantings and roadside trees (15.6%) (Estep 1989). Schlorff and Bloom (1984)reported <br /> 92% of the nests they located were in valley oaks and cottonwoods. There are no records <br /> of nesting in live oaks in the Great Valley. In the Great Valley, nest trees typically occur in <br /> four settings: riparian edge, lone trees, roadside trees, or farmhouse trees (Estep 1989). In <br /> some cases Swainson's hawks will nest in large trees in an urban setting (pers. obs.). <br /> Nest trees near human habitations are commonly used throughout the species'range. In <br /> North Dakota, approximately 75% of all nest sites used were directly or indirectly <br /> produced by human activities, shelterbelts, abandoned and active farmsteads, and rights- <br /> of-way (Gilmer and Stewert 1984). Estep found 35% of the nests located by his survey <br /> were within 0.2 miles of a county road or highway. The reproductive success of pairs <br /> Buckeye Ranch Resource Plan (November, 1993) <br /> 116 <br />