Time and energy minimization strategy codetermine the loop migration of demoiselle cranes around the Himalayas

Tens of thousands of demoiselle cranes' crossing the Himalayas to the Indian subcontinent have been reported for decades, but their exact spring migration route remained a mystery until our previous study found they made a detour in spring along the western edge of the Himalayas and crossed the Mongolian Plateau to their breeding areas based on satellite telemetry of 3 birds. To corroborate the loop migration pattern and explore whether demoiselle crane's loop migration route is shaped by time- and energy-minimization strategies in spring and autumn and how the temporal and spatial variation of environmental conditions contribute to crane's selection of migration routes, we tracked 11 satellite-tagged demoiselle cranes from their breeding area in China and Russia, simulated 2 pseudo migration routes, and then compared the environmental conditions, time, and energy cost between true and pseudo routes in the same season. Results show that demoiselles' spring migration obeyed time-minimization hypothesis, avoiding the colder Qinghai-Tibet Plateau, benefited by abundant food and higher thermal and orographic uplift along the route; autumn migration follows energy-minimization hypothesis with the shorter route. Our research will contribute to uncover the mechanical reasons why demoiselle crane avoids crossing the giant barrier of the Himalayas in spring, and shapes a loop migration route.