5.1 The star compass

The stars are principally of value to nocturnal migrants and, of course, the sky has to be fairly clear. Experiments using caged birds show that their direction of take-off is correct on starry nights, but not on overcast nights when they become disorientated. In a series of classic experiments on North American indigo buntings (Passerina cyanea, Figure 26) where the location and movements of stars and constellations were manipulated in a planetarium, researchers showed that the buntings derive directional information from the pattern of constellations relative to each other and to the celestial pole. In other words, birds do not learn the star patterns themselves but learn to respond to a star pattern that rotates about a single conspicuous star.

Figure 26 Indigo bunting.

The experiments also showed that birds needed experience of the night sky in order to orientate correctly in the planetarium.