Although it is known that birds can return to their breeding grounds with remarkable precision, it has remained a mystery how they know when and where to stop migrating. Here, using nearly a century of Eurasian reed warbler (Acrocephalus scirpaceus) ringing recoveries, we investigated whether fluctuations in the Earth’s magnetic field predict variation in the sites to which birds return. Ringing recoveries suggest that magnetic inclination is learned before departure and is subsequently used as a uni-coordinate ‘stop sign’ when relocating the natal/breeding site. However, many locations have the same inclination angle. Data from populations with different migratory directions indicate that birds solve this ambiguity by stopping at the first place where the right inclination is encountered on an inherited return vector.
