Christian John , Tal Avgar, Karl Rittger, Justine A. Smith, Logan Stephenson, Thomas Stephenson, Eric Post

Climate change reduces snowpack, advances snowmelt phenology, drives summer warming,
alters growing season precipitation regimes, and consequently modifies vegetation phenology
in mountain systems. Altitudinal migrants cope with seasonal variation in such conditions by
moving between seasonal ranges at different elevations, but vertical movements may be
complex and are often not unidirectional during the spring migratory season. We uncover
drivers of vertical movement variation in an endangered alpine specialist, Sierra Nevada
bighorn sheep. We used integrated step-selection analysis to determine factors that promote
vertical movements, and factors that drive selection of destinations after vertical movements.
Our results reveal that high temperatures consistently drive uphill movements, and provide
some evidence for the contribution of precipitation events to downhill movements.
Furthermore, bighorn select destinations that have a high relative index of forage growth and
maximize delay since snowmelt. These results indicate that although Sierra bighorn seek out
foraging opportunities related to landscape phenology, they compensate for short-term
environmental stressors by undertaking brief vertical movements. Migrants may therefore be
impacted by future warming and increased storm frequency or intensity, both in terms of their
fine-scale vertical movements, and in terms of tradeoffs between forage access and predation
risk.