Migratory birds show systematic differences in their flight routes between the breeding season (spring) and the post-breeding season (autumn), and these variations are closely linked to optimizing energy efficiency.
During spring, many species significantly reduce the duration of their nocturnal flights, opting for shorter, more direct routes toward breeding sites and reproduction areas. This strategy responds to the need to maximize reproductive success before winter conditions intensify, thereby avoiding periods of high predation risk and food scarcity.
In contrast, autumn poses a different challenge: birds must accumulate the largest possible energy reserves for the return journey to the nest. As a result, they considerably increase the distance covered on each nightly departure, often using specific aerial corridors (such as passes through the Pyrenees or the Asian highlands) where aerodynamic advantages reduce wind resistance.
The variation in flight profile is also influenced by the vertical thermal gradient: in spring, many birds take advantage of rising surface temperatures to fly more efficiently during the day, minimizing energy consumption at night.
Satellite tagging studies have shown that these differences in energy performance can vary considerably between opposite migratory directions, with birds flying south showing greater energy savings during their outbound leg, while those flying north incur higher energy costs due to greater atmospheric turbulence along those routes.
