ii. Magnetic compass

Humans cannot sense it, but the Earth is covered with directional magnetic charges, which form a magnetic field. At the Earth's magnetic north, these magnetic field lines lie at a 90 degree angle to the horizontal, while at the equator they lie at 0 degrees to the horizontal. For those who can detect it, this provides a simple compass identifying the direction of the poles or the equator. Birds were shown to use a magnetic compass by Wiltschko in 1968, who experimentally reversed magnetic north in special cages using electrical coils; this also reversed the directional preferences of migratory robins². Unlike the sun or stars, such magnetic information is constant and always available. However, it can’t provide any information at the equator.

iii. Stargazers

On a starry night, or even a partially overcast night, the stars provide a celestial reference system, once used by sailors to plot their course home. Birds also use star maps to navigate. When Emlen released Indigo buntings in a planetarium, they used these artificial constellations to orientate themselves. When he changed the stars' configuration by 180 degrees, the Indigo buntings also changed their orientation by the 180 degrees ¹⁴. The buntings relied particularly on the constellations around Polaris, the North Star, to navigate. The North Star is an important reference point, since it maintains its position close to the north celestial pole, whilst the other stars appear to migrate around it.

Birds need to observe the starry sky and its rotation during juvenile development, otherwise they are unable to use the stars to navigate².