Many North American Tetraoninae exhibit migratory behavior (Hoffman and Braun 1975, Herzog and Keppie 1980, Cade and Hoffman 1993, Schroeder and Braun 1993), either as partial (only some individuals migrate) or differential (age or sex classes differ in timing and/ or distance) migrants between seasonal habitats (Terrill and Able 1988). Tetraoninae may migrate because of elevational or seasonal differences in habitat availability and selection, or fidelity to seasonal-use areas (Herzog and Keppie 1980, Schroeder and Braun 1993), and these migrations are typically 16 km were uncommon), allowing most individuals to find required habitats within a relatively small annual range. In contrast, the ecology of migratory populations is more complex because individuals must cope with long-distance movements to suitable seasonal ranges (Dalke et al. 1963, Berry and Eng 1985, Connelly et al. 1988, Wakkinen 1990). Although Sage Grouse do not undertake migrations at a scale similar to some avian species such as Anseriformes (Bellrose 1980), Neotropical migratory Passeriformes (Finch and Stangel 1993), or Charadriiformes (Krementz et al. 1994), their movements still m et most ecological definitions of migration (e.g., Sinclair 1983). Connelly et al. (1988), Wakkinen (1990), and Roberts n (1991) investigated the ecology of a migratory Sage Grouse population inhabiting a xeric environment (< 25 cm annual precipitation) in southeastern Idaho. Grouse used contiguous areas for wintering, breeding, and nesting but moved as far as 82 km to summer range at either higher or lower elevations. Grouse return to winter range beginning in late fall (Robertson 1991). Movements to summer range presumably occur because Sage Grouse breeding and nesti g ranges lack mesic habitats, such as wet meadows and iparian areas, that provide succulent vegetal foods for birds throughout summer (Autenrieth 1981, Connelly et al. 1988). Although reports of distance moved from breeding to summer ranges are common, little information regarding relationships between timing of thes movements and vegetal characteristics or environmental factors are available. Nest fate also may influence the ability of females to initiate migration to summer range because successful females may delay migration (Schroeder and Braun 1993). Migratory birds may rely on exogenous (environmental) factors (Terrill 1990), endogenous circannual rhythms (Berthold 1990, Gwinner 1990), or a combination of both (Krementz et al. 1994) for migrational timing cues. Sage Grouse appear to initiate migration to summer range at different times within and among years (Connelly et al. 1988), suggesting the cue for migrational timing may be influenced by exogenous factors. However, no empirical data exist to document timing and progression of plant desiccation, how weather variables might influence desiccation, or how migrational timing may be influenced by plant moisture content. We hypothesized that (1) cumulative annual precipitation and temperature influence timing of spring and summer vegetal desiccation, (2) vegetal moisture content would be higher in plants collected at sites used by Sage Grouse than at random sites, and (3) annual timing of Sage Grouse migration is related to vegetal moisture content and nest fate. 1 Received 15 February 1996. Accepted 24 July 1996. 2 Present address: U.S. Army Corps of Engineers, Waterways Experiment Station, Environmental Laboratory, 3909 Halls Ferry Rd., Vicksburg, MS 39180, e-mail: fischer@ex 1 .wes.army.mil
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