The conservation of bison (Bison bison) in Yellowstone National Park from near extinction to a high of 5000 animals has led to societal conflict regarding overabundance and potential transmission of brucellosis to cattle with widespread economic consequences. As abundance increased during 1971-1996, more bison migrated from the Hayden and Pelican valleys to the lower-elevation Madison headwaters area and, eventually, outside the park (Meagher 1998, Fuller et al. 2007). Meagher (1998) and others (Taper et al. 2000, Gates et al. 2005) concluded these migratory movements were stress-related responses to decreased food availability as bison fully occupied habitat in the Pelican and Hayden valleys and, subsequently, the Firehole and Madison river drainages. This hypothesis implies that these areas have a relatively fixed capacity for wintering bison and, as a result, a larger proportion of bison migrate to the Madison headwaters and elsewhere as bison numbers increase beyond this capacity. However, density-independent factors such as genetic predisposition, individual asymmetries (e.g., age, sex), and stochastic variations in climate that influence food availability may also affect migration (Lundberg 1987). We used data collected during 1970-1971 through 2005-2006 to quantify annual variations in the magnitude and timing of migration by central herd bison, identify potential factors driving this variation, and evaluate if the proportion of migrants increased with abundance.