Southern Norway hosts an estimated 25,000 wild mountain reindeer, which represents over 90% of the Eurasian population. Wild reindeer often perform extensive migrations, and historical findings in Norway testify to the long standing existence of mass migrations typically between winter pastures further inland and high-altitude summer pastures in more coastal areas. Today, reindeer migrate towards traditional calving grounds in undisturbed areas as the snow begins to melt in early spring. Reindeer typically spend the summer not far from calving grounds, where they move to higher elevations to access nutritious snow-bed vegetation and escape insect harassment. After the rut, reindeer migrate to their winter ranges, which are characterized by relatively little snow cover and wind-blown ridges. There, access to lichens, their main winter food, sustains them through the long winter months. Reindeer migration routes are strongly shaped by topography, including fjords and steep mountains. Historically, Norwegian wild reindeer formed two to three large, intermingling populations. But infrastructure development in more recent decades has fragmented the population into the current 24 isolated sub-populations. Today, most traditional migration routes are lost, and seasonal ranges tend to overlap. In Setesdal-Austhei, one of the southernmost sub-populations, most reindeer maintain a traditional migration between nonoverlapping seasonal ranges. However, the migration route is threatened, as reindeer navigate a narrow bottleneck between a hydropower reservoir with increasingly unstable ice-cover, a road, and recreational development.