LAUSR

Although it is known that birds can return to their breeding grounds with exceptional precision, it has remained a mystery how they know when and where to stop migrating. Using nearly a century’s worth of Eurasian reed warbler (Acrocephalus scirpaceus) ringing recoveries, we investigated whether fluctuations in 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 or 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. Description Where to stop Many migratory species undertake incredibly long migrations throughout a year. Much research has focused on understanding how these animals know where to go and how to return, and most work has focused on understanding how animals know—or learn—routes and directions. Equally important, however, is how they know when and where to stop migrating. Wynn et al. looked at a century of leg ringing and banding data in Eurasian reed warblers and found that the birds rely on the magnetic inclination as a “stop sign” to return to within meters of their breeding site. —SNV A century of ringing records implies that reed warblers use the Earth’s magnetic field as a stop sign to identify their natal breeding sites.