Year of Award


Document Type


Degree Type

Doctor of Philosophy (PhD)

Degree Name

Fish and Wildlife Biology

Department or School/College

College of Forestry and Conservation

Committee Chair

Mark Hebblewhite

Commitee Members

Winsor H. Lowe, L. Scott Mills, Michael S. Mitchell, Thomas R. Stephenson


University of Montana


Despite their potential conservation importance, the demographic implications of migratory behavior remain poorly understood. Sierra Nevada bighorn sheep (Ovis canadensis sierrae; hereafter “Sierra bighorn”) are federally-endangered and partially migratory. In summer, Sierra bighorn share high-elevation summer ranges, but in winter some individuals migrate to lower elevation for winter while others remain resident at high-elevation. Lower elevations have better forage in winter, but these areas also carry an increased risk of predation from Sierra bighorn's primary predator, the cougar (Puma concolor). We should therefore expect differences in winter conditions to result in demographic differences between migrants and residents.

First, I developed new software tools in an open-source R package ‘migrateR’ for classifying migratory behavior, including novel techniques for identifying altitudinal migration. Applying these tools to Sierra bighorn showed that migratory behavior in this taxon is extremely flexible in both status (migrant v. resident) and tactics (e.g. timing, duration of movements), with individuals frequently switching migratory status between years.

I tested for status-specific differences in winter resource use and selection by migrants and residents using resource selection functions across three scales. Migrants and residents showed scale-specific differences in resource selection offering contrasting solutions to a forage-predation tradeoff. Residents avoided predation risk at the coarsest scale, but focused on forage in fine-scale selection, whereas migrants selected for forage at the coarsest spatial scale and focused on avoided predation risk at finer scales. This pattern of selection resulted in migrants gaining better access to forage. The amount of migrant habitat predicted differences in the prevalence of migration across eight populations.

Lastly, I tested causes and consequences of migratory behavior in Sierra bighorn. Migratory propensity increased with winter severity. Individuals that were still lactating in fall were highly likely to migrate, but the strength of this effect declined with body mass. I failed to find an effect of winter elevation on adult female survival. Finally, Sierra bighorn were more likely to be observed with a lamb following residency than following migration. These results suggest that where residency is viable, residents make greater per-capita contributions to population growth than do migrants.



© Copyright 2015 Derek Benjamin Spitz