Researchers along the Russian coast have demonstrated that Eurasian Reed Warblers rely on a geomagnetic map for clues about their migration path. Although it's been suspected since the 19th century that some birds use magnetic field information, experimental evidence has been scarce.
Biologists Dimitry Kishkinev, Nikita Chernetsov, and their colleagues at Biological Station Ribachy devised a system of magnetic coils in their field site on the Baltic Coast in Russia, which is frequented by Reed Warblers. The birds remained several days while the experimenters manipulated the site's magnetic field to mimic the birds' destination in Zvenigorod—in other words, the birds stayed put while the field changed around them. When the warblers finally departed, they flew northwest as if returning from Zvenigorod, rather than northeast as they would have done without the field manipulation. They had been fooled.
The researchers suspect that, during Reed Warblers' first migration to West Africa, they learn the magnetic fields through which they pass; they then fall back on this information during future migrations if they happen to get thrown off course. How they sense magnetic fields, however, is still unknown.
Want to find out more? The paper appears in Current Biology, Kishkinev and Chernetsov et al.: "Eurasian reed warblers compensate for virtual magnetic displacement" dx.doi.org/10.1016/j.cub.2015.08.012