1. Introduction
Bird migration is one of the most remarkable biological phenomena. Some species travel tens of thousands of kilometers twice a year with remarkable precision. Spirals play a role in this process in two ways: as altitude-gaining technique (thermals) and as navigation trajectory in certain conditions.
2. Thermals and Ascending Spirals
Thermals are columns of warm air rising from the sun-heated ground surface. Soaring birds — raptors, storks, cranes — exploit them by spiraling inside the ascending column. By circling tightly, they stay in the rising zone and gain altitude without expending muscular energy. They then glide to the next thermal, gradually losing altitude.
3. Navigation and Magnetic Compass
Migratory birds use several navigation systems: Earth's magnetic field (magnetic compass based on magnetite crystals or free radicals in the eye), stars (stellar compass learned during youth), the sun (solar compass with time compensation), and visual landmarks. The magnetic compass detects field inclination, not polarity, allowing it to distinguish equator from poles.
4. V-Formations and Aerodynamics
The V-formations of geese and cranes are not spirals, but they illustrate how birds exploit aerodynamic structures created by their companions. Each bird flies in the wingtip vortex of the preceding bird, reducing its drag by 20 to 30%. The optimal position in the formation is slightly behind and to the side, in the upwash zone of the vortex.
5. Conclusion
The spiral in bird migration is primarily a technique for exploiting thermals — an optimal aerodynamic solution for gaining altitude in a rising air column. It illustrates how physical constraints (aerodynamics, energy) shape biological behaviors.