The Magnetic Field

The theory of birds using the Earth's magnetic field as a means of navigation took some time to become seriously considered, mainly due to the weakness of the Earth's magnetic field and as a result it took much convincing and experimentation to convince the science community that birds could at the very least feel the magnetic field. The first concrete evidence that birds could be affected came from Wiltschko (1972), when he discovered he could predictably alter European Robins using a magnetic field. This initial breakthrough led to many more studies regarding the influence of magnetic fields on bird species. Southern (1972), quickly discovered in a study that disturbances in the Earth's magnetic field was having an effect on the orientation of ring-billed gulls. During his study on pigeons using the sun for navigation, Keeton (1979), also trained his pigeons to operate using the magnetic field on cloudy and overcast days when the sun was not overly visible.

Now that all of this research was being conducted, the question quickly turned to how the birds were using the magnetic field. Wiltschko (1972), followed up on his initial study on Robins and stated that birds did not use the polarity of the magnetic field, instead they would take north as the direction in which the most acute angle was formed between the magnetic and gravity vectors, as shown in the diagram below.

Keeton (1979), also followed up on his research by attaching bar magnets to the back of his pigeons on overcast days to see if they could still navigate. The answer was a resounding no as the pigeons did not have any idea where they were going. 

While it was proven that birds do use the Earth's magnetic field as a navigation source, it was also proven that magnetic navigation is not their first choice of navigation as they tend to use the sun and stars as their first method.