Having to cross the Sahara in the middle of migration means a bird can't count on food for some 5 nights of flying. So, how does a first-timer know to take on extra fuel?
For thrush nightingales traveling from Sweden to southern Africa, the cue for a life-saving eating binge may come from changes that birds sense in the magnetic field, propose Thord Fransson of the Swedish Museum of Natural History in Stockholm and his colleagues.
They built lab equipment to simulate strengths and directions of the magnetic field along the route to Africa. When their magnetic field mimicked that in northern Egypt–the place to pack on fat for the trip across the desert–birds gained extra weight, the researchers report in the Nov. 1 Nature.