It's time to go-Drivers and plasticity of migration phenology in a short-distance migratory ungulate

Recurring events like migrations are an important part of the biological cycles of species. Understanding the factors influencing the timing of such events is crucial for determining how species face the pervasive consequences of climate change in highly seasonal environments. Relying on data from 406 GPS-collared Alpine ibex Capra ibex monitored across 17 populations, we investigated the environmental and individual drivers of short-distance migrations in this mountain ungulate. We found that vegetation phenology, including spring growth and autumn senescence, along with snow dynamics-snowmelt in spring, onset of snow cover in autumn-were the main drivers of the timing of migration. In spring, ibex migration timing was synchronized with the peak of vegetation green-up, but more in males than in females. Specifically, a peak of green-up occurring 10 days later delayed migration by 6.4 days for males and 2.7 days for females. This led to increased differences in migration timing between sexes when the peak of green-up occurred early or late in the season. In addition, ibex delayed migration timing when the length of the spring season was longer and when the date of snowmelt on ibex summer ranges occurred later. Similarly, in autumn, prolonged vegetation senescence and delayed onset of snow cover led to later migration. Overall, we observed a high degree of behavioural plasticity, with individuals responding to inter-annual variations in vegetation and snow phenology, even though the extent of these adjustments in migration dates was lower than the magnitude of the interannual changes in environmental conditions. Nonetheless, females could be less plastic than males in their timing of spring migration, likely due to the parturition period following migration forcing them to trade off foraging needs with predation risk. As the identified drivers of ibex migration are known to be and will continue to be largely impacted by climate change, the capacity of ibex to respond to such rapid changes could differ between sexes.