Partial Migration in a Population of Greater Prairie-Chickens in Northeastern Colorado

-Seasonal movements of radio-marked Greater Prairie-Chickens (Tympanuchus cupido) were examined in northeastern Colorado during 1986-1989. Many birds migrated between breeding and winter areas; all appeared to display fidelity to both breeding and winter sites. The average date of migration from winter to breeding areas was 20 February for males and 27 March for females. The average date of migration from breeding to winter areas was 4 July for females and 28 July for males. Much of the variability in timing of migration from breeding areas for females was related to brood status; average date of migration was 10 June for females without broods and 26 August for females with broods. Average migration distance between winter and breeding ranges differed significantly by sex: 9.2 km for females and 2.7 km for males. When migration samples were expanded to include distances between breeding and late summer ranges (as suggested by timing of movements), females migrated an average distance of 10.6 km, while males migrated 2.9 km. Seasonal movements of Greater Prairie-Chickens appear to represent a partial migration with both obligatory and facultative components. Received 3 June 1991, accepted 23 February 1992. MIGRATION has been defined as "a regular round trip within a life-span of the individual" (Sinclair 1983:241), while patterns of migration have been categorized as "annual," "partial," or "differential" (Terrill and Able 1988). Species of Tetraoninae are particularly variable with respect to migration patterns. While some species such as Willow and Rock ptarmigan (Lagopus lagopus and L. mutus) may be annual migrants in certain regions (Weeden 1964, Irving et al. 1967), most are considered either nonmigrants or partial migrants with only some individuals migrating. However, extensive research on movements of Blue Grouse (Dendragapus obscurus; Wing 1947, Mussehl 1960, Zwickel et al. 1968), Spruce Grouse (D. canadensis; Herzog and Keppie 1980, Schroeder 1985), White-tailed Ptarmigan (Lagopus leucurus; Hoffman and Braun 1975), and Common Capercaillie (Tetrao urogallus; Rolstad 1989) indicate that patterns of partial migration may be the rule rather than the exception, particularly among females. Explanations for partial migration include seasonal and/or sexual differences in habitat selection, and dispersal differences due to ag3 Present address: Washington Department of Wildlife, P.O. Box 1077, Bridgeport, Washington 98813, USA. gressive interactions, breeding success, and/or heritability (Lack 1944, Biebach 1983, Smith and Nilsson 1987). For example, Blue Grouse migration has been attributed to elevational and/ or seasonal differences in habitat selection (Wing 1947, Mussehl 1960, Zwickel et al. 1968), and Spruce Grouse migration appears to be related to fidelity to specific winter and breeding areas, and to variability in dispersal movements during their first spring (Herzog and Keppie 1980, Schroeder 1985, 1988). Despite similarities in patterns of movement for Blue Grouse, Spruce Grouse, White-tailed Ptarmigan, and Common Capercaillie, possible explanations for movement have not been thoroughly examined in other species of Tetraoninae. Greater Prairie-Chickens (Tympanuchus cupido) are among the most mobile of the species in the Tetraoninae, with seasonal movements up to 170 km (Hamerstrom and Hamerstrom 1949). Early reports indicated that Greater Prairie-Chicken movements were particularly extensive among females and typically related to seasonal differences in habitat selection (Schmidt 1936, Hamerstrom and Hamerstrom 1949) and regional food availability, such as the abundance of acorns (Quercus spp.; Cooke 1888, Gross 1930, Leopold 1931). However, Gross (1930) suggested that introduction of corn throughout the range of Greater Prairie-Chick-