This piece has been forwarded to me by one of our editors: Simon Raggett. He gives us the quantum twist. You will also find much written here on the subject of avian navigation based around gravity from our editor Antonio Nafarrate, who has been in communication with the authors of this piece. Please follow the link below.
A curious finding is reported in the 15 November New Scientist. Trained homing pigeons became disorientated when flying over a crater where the force of gravity was lower than normal.
Gravity is the weakest force and the intermediating particles are undiscovered, the force usually being understood in terms of the curvature of spacetime. Our normal understanding of brains does not allow for detection of presumably very minor fluctuations in the gravitational field, but it can be speculated that the radical pair entanglement in avian brains might somehow be up to this. It might also be worth considering that water courses, mineral deposits etc might create fluctuations in the gravitational field.
The reference is Journal of Experimental Biology, DOI: 10.1242/jeb.108670