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i Microtus sp.) The abundance of harriers often fluctuates cyclically with the abundance of this prey item(Hamerstrom, 1979). THREATS AND MANAGEMENT CONCERNS Prior to the ban on DDT, pesticides were a major factor in the decline of northern harrier populations. Pesticides were especially important in contributing to the decrease of northern harrier populations in the eastern United States. Since the ban of DDT, residual pesticide contamination is no longer the primary threat to northern harrier populations. Habitat loss is now the major problem facing northern harrier populations. Marshes, potholes and wetlands are being drained and native grasslands are being converted to agricultural uses. This is particularly true in the west (Evans, 1982). Because of these habitat losses, northern harrier populations are assumed to be declining slowly or stable. SELECTED REFERENCES Beedy, Edward C. and Stephen L. Granholm. 1985. Discovering Sierra Birds. Yosemite Natural History Association and Sequoia Natural History Association. 229 pp. Bent, A.C. 1937. Life histories of North American birds of prey. Part I, Dover Publications,Inc.,New York. 409 pp. California Department of Fish and Game. 1987. Special animals. Natural Diversity Data Base. 18 pp. Ehrlich, Paul R., David S. Dobkin and Darryl Wheye. 1988. The Birder's Handbook. Simon and Schuster,Inc.,New York. 798 pp. Evans, David L. 1982. Status reports on twelve raptors. U.S. Dept. of the Interior, Fish and Wildlife Service, Special Scientific Report- Wildlife No. 238, Washington. 68 pp. Hamerstrom,Frances. 1979. Effect of prey on predator. Auk 96:370-374. Hamerstrom, Frances, Frederick N. Hamerstrom and Charles J. Burke. 1985. Effect of voles on mating systems in a central Wisconsin population of harriers. Wilson Bull. 97(3): 332-346. McCaskie,Guy, Paul DeBenedictis,Richard Erickson and Joseph Morlan. 1988. Birds of Northern California: An annotated field list (reprinted with supplement). Golden Gate Audubon Society,Berkeley,CA. 108 pp. National Geographic Society. 1987. Field Guide to the Birds of North America, Second Edition. National Geographic Society, Washington. 464 pp. Newton,Ian. 1979. Population ecology of raptors. Buteo Books,Vermillion. 399 PP.