Migratory behaviour has been hypothesized to be driven by fitness benefits arising from environmental factors that vary spatially at the landscape scale and temporally on an annual cycle. If migration has a fitness payoff, it follows that barriers to migration may have a measurable demographic cost. My thesis is that migratory behaviour has a demographic payoff that is mediated by access to energy during the season of greatest resource limitation. I tested this proposition using data from wild and semi-domestic reindeer populations in Norway. In Chapter 1, I developed a method for estimating the spatiotemporal distribution of digestible energy available to reindeer across a 385,000 km2 study area at a spatial resolution of 30x30m and a temporal resolution of 16-day intervals. In Chapter 2, I used GPS locations of wild reindeer to determine that with enough freedom of movement, wild reindeer move between summer ranges with later snow melt and low human infrastructure and winter ranges with high digestible energy and short winters. However, range fragmentation reduced that ability of reindeer to exploit environmental gradients seasonally. In Chapter 3, I developed a path model that links the migratory behaviour of semi-domestic reindeer to population growth rates and equilibrium densities. I found that migratory displacement increased equilibrium density by 0.0155 ± 0.0025 reindeer/km2 for every kilometer of distance between seasonal home ranges, which is ecologically significant in a system where the median equilibrium density is 0.78 reindeer/km2 and where migration ranges from a negligible seasonal displacement to more than 100 km. Future research aimed at quantifying the annual dynamics in energy balance, rather than just energy availability or energy costs separately would build on the current understanding of the energetic drivers of ungulate migration.