As you can hear in the interview with Jon Hagstrum, the earth constantly emits low frequency sounds as a result of movements within its crust - these are known as infrasounds. It's thought that, similar to the earth's magnetic fields, each area of the planet has its own distinct sonic signature. Jon Hagstrum believes that some birds like pigeons can actually hear these infrasounds and as such, recognise where it is over the planet.
Once again, truly astonishing and ground-breaking science.
Jon Hagstrum was at the Royal Institute of Navigation and you can hear all the talks here.
There is also an interesting article on navigation in the Independent. Steve Connor spoke to Professor Peter Hore, a physical chemist at Oxford University, who thinks photochemical reactions allow birds to use the Earth's weak magnetic field.