Indigo Buntings Learn to Identify North from the Night Sky

"Consider the mechanism through which indigo buntings (a species of migratory songbird) learn to identify north from the night sky. In today’s night sky, Polaris is the North Star, the star in the region of the sky that reliably indicates north. Because the stars change position over time, Polaris will not always indicate true north, nor will true north in the future be predictable from the constellations of today, for the constellations themselves migrate and change over evolutionary time. Indeed, 100,000 years ago there was no Big Dipper, no Orion. Thus, evolution probably could not have built a map of the sky in the brain of the indigo buntings on which Polaris is identified as north, yet indigo buntings have such a map. Presented with a stationary simulacrum of the night sky in a planetarium in autumn, they take off as if to fly south as specified by the North Star, no matter how the planetarium’s night sky has been oriented with respect to true north in the actual world. Indigo buntings must have learned something equivalent to where the North Star is; how could they have done so?

 Steven Emlen (1975) uncovered the mechanism by which nestling indigo buntings achieve this feat. Because the earth rotates on an axis that goes through true north, the North Star, positioned as it is at present above the North Pole, marks the center of rotation for the night sky. Emlen showed that nestling indigo buntings observe the rotating sky and register the center of the rotation as a privileged direction. He showed this by raising indigo buntings in a planetarium in which he could make arbitrary skies rotate around arbitrary centers, and then observed which direction the birds took off in the autumn, when their hormones told them it was time to fly south. There is a critical period for this learning device; if the buntings do not learn the North Star while they are nestlings, they never do. "

Carey, 2009, The origin of concepts, p.15


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