Ascending at Dawn, Descending at Dusk: Could Changes in Light and Oxygen Enhance Radical-Pair Magnetoreception in Migratory Birds?

Long-distance migratory birds have only recently been found to undergo striking diel cycles in flight altitude, ascending by approximately 1.2–3.0 km around dawn and descending near dusk during prolonged flights. These movements expose birds to coordinated changes in atmospheric pressure, oxygen partial pressure, air density, temperature, humidity and light. Thermoregulation, wind conditions, predator avoidance, visual orientation and barrier crossing provide plausible explanations for high daytime flight but may not fully account for its close timing to sunrise and sunset. Here, we consider an additional possibility arising from the combined effects of light and oxygen on cryptochrome photochemistry. Dawn ascent coincides with rapidly increasing daylight and decreasing oxygen partial pressure, potentially affecting light-driven flavin photoreduction and the oxygen-dependent recovery of reduced flavin states. At dusk, descent reverses this combination. We propose that the resulting diel contrast could alter cryptochrome redox-state occupancy and photocycle turnover, thereby modifying the conditions under which a radical-pair-based magnetic compass operates. Such a sensory contribution could act alongside thermoregulatory and ecological benefits. Experiments combining physiologically realistic changes in light intensity, spectral composition and oxygen availability could test whether diel altitude cycles influence cryptochrome photochemistry and magnetic sensitivity in migratory birds.