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Two points:

1 - I remember a study from long ago where an experment was done with homing pigeons. They had placed a small electromagnetic coil around their heads (or neck). When released to fly home, the birds flew around confused and not one of them got back home. The conclusion... they used the earths magnetic field for navigation.

2 - That had to be one unique GPS device. For them to plot the pigeon, that device had to calculate the position (no problem), record the info for retrieval later (no problem) or transmit the birds position real time to a receiver (no porblem). The problem for me seems to be that no matter how you collect the date, that poor pigeon is loaded down with one hell of a backpack for a small bird...