Laserfiche WebLink
and from this estimated 375 birds remained in the state, down from an historical estimate <br /> ranging from 4,200 to 17,100 birds. A number of factors have been suggested to explain <br /> the decline of the species in California: mortality during migration,pesticide poisoning in <br /> the breeding and wintering grounds, eggshell thinning, habitat loss, disturbance, <br /> degradation of habitat, and increased competition with other species of raptors. No single <br /> factor provides an adequate explanation for the observed declines in California. <br /> Furthermore declines have only been reported in California, Nevada, Utah, eastern Oregon <br /> and Washington. <br /> Habitat loss may explain the declines in California. For example,over 90% of the riparian <br /> habitat, native grasslands and seasonal wetlands have been altered or removed by humans. <br /> Yet habitat loss does not explain the loss of breeding pairs in the Mojave Desert andP arts <br /> of Oregon,Washington and Nevada. Changes in habitat structure through grazing and <br /> irrigation may have resulted in a modification of the prey base utilized by this species. <br /> Janes (1984) suggests that the increases in availability of hunting perches (e.g., telephone <br /> poles) have increased interspecific competition between Swainson's hawks and red-tailed <br /> hawks, to the detriment of the former. <br /> Swainson's hawks migrate through the Pacific and Central flyways to grassland regions of <br /> South America. In the Sacramento-Stockton area the birds generally return to breeding <br /> sites by mid-March and migrate in fall from September through mid-October. Birds <br /> nesting in western Canada and states north of California probably move through the Great <br /> Valley during the latter part of migration. Large concentrations "kettles" of Swainson's <br /> hawks can sometime be observed during fall migration. In October 1992, huge numbers of <br /> Swainson's hawks were observed foraging in fields being disked on the Cosumnes River <br /> Preserve. Disking began on 12 October with 5 hawks observed. Over the next 5 days, the <br /> number increased each day peaking at 196 birds on 17 October. A small population of <br /> Buckeye Ranch Resource Plan (November, 1993) <br /> 115 <br />