During overcast, however, the orientation of the clock shifted and control pigeons were both homeward bound and there was no significant difference between the groups. Thus, he concluded that the sun is used to obtain directional information when it is visible, but that pigeons must have a secondary compass sense. Keeton hypothesized that pigeons may have the ability to detect the earth's magnetic field. In 1971, he attempted to disrupt this detection by gluing a bar magnet onto the pigeons. He found that these pigeons were often unable to navigate home from unfamiliar release sites under overcast conditions but experienced little difficulty on sunny days when compared to control birds that were equipped with a brass bar.
Open circle indicates vanishing bearing of one control bird.
Solid circle indicates vanishing bearing of one bird with magnet. In five out of the six releases, the mean vanishing bearing of birds with magnets was either random or not homeward oriented while control birds were oriented well in all but one release. Thus, the magnets often interfered with the normal navigation mechanism of the pigeons. Keeton concluded that there seem to be two compass systems utilized during navigation with the primary system relying on the position of the sun and the secondary one on the earth's magnetic field. From these magnet experiments came an interesting finding regarding the development of the compass sense in pigeons.