Birds carry more than one compass
Migratory birds possess at least three biological compasses: one based on the sun, one on the stars, and one on Earth's magnetic field [2]. These compasses give birds a sense of direction even when one cue is unavailable—for example, on cloudy nights when the sun and stars are hidden, the magnetic compass can still provide a heading [2][4].
Having several compasses does not mean birds use them all at once. The evidence suggests they switch between cues depending on what is locally available, which is why navigation can continue under very different sky conditions [2][5].
Local cues steer the journey, not just the start
Navigation by local information has been studied in wild birds for well over a century, and the core idea is that birds read environmental signals as they travel rather than following a single pre-set route [1]. Topography and winds are among the external cues that influence orientation, especially for experienced migrants [2].
Simulation work testing geomagnetic navigation strategies found that a single simple rule—such as flying a constant heading or following a magnetic gradient—does not fully account for observed migration paths. Instead, the results point to a multifactorial navigational strategy in which birds respond to several local cues together [3].
Inexperienced and experienced birds rely on cues differently
Young birds on their first migration leave on a predictable date and fly a roughly predictable route, which suggests a strong innate program guiding early movement [5]. Experienced migrants, by contrast, appear to make more use of external information such as topography and wind as they refine their route [2].
This difference matters for the question of moment-to-moment decisions: a first-time migrant may depend more heavily on inherited compass instructions, while an older bird can adjust its heading in response to what it encounters locally [2][5].
How birds combine cues in real time is still being worked out
The physiological and geophysical mechanisms behind bird navigation are not fully settled. Early work noted that normal migration often occurs on cloudy nights when neither sky nor ground is visible, raising the question of which senses remain useful in those conditions [4].
The evidence supports the broad picture—multiple compasses plus local environmental cues—but the precise way birds weigh and integrate those cues from moment to moment is still an active question, and simulation studies are one way researchers are testing which strategies could produce the routes birds actually fly [3].
Sources used in this answer
Navigation in birds and other animals
Navigation by local information in wild birds has been studied since the 19th century, based on the assumption that birds use environmental cues available to them.
Migratory orientation in inexperienced and experienced avian migrants
Migratory birds possess three biological compasses based on the sun and other cues, and experienced and inexperienced migrants differ in their use of external cues such as topography and winds.
Simulation experiment to test strategies of geomagnetic navigation during long-distance bird migration
Simulation experiments using GPS and geomagnetic data indicate that simple constant-heading or bi-gradient strategies do not fully explain migration, pointing instead to a multifactorial navigational strategy.
The physiology and geophysics of bird navigation
Bird navigation depends on physiological sensory capabilities and geophysical cues such as sounds and wind patterns, and normal migration commonly occurs on cloudy nights when sky and ground are not visible.
Conceptual approaches to avian navigation systems
Young birds of a given population leave on a predictable date and fly a roughly predictable route, reflecting an endogenous program, while navigation also draws on celestial, geomagnetic, olfactory, and visual cues.