Biology: Beacon for Buntings

With uncanny precision, the northern hemisphere's migratory birds fly south in the fall and north in the spring—often to targets that are continents or even oceans away. One theory holds that some birds get their traveling orders from the stars. Not quite, says Cornell Ornithologist Stephen T. Emlen. The cue comes from a "biological clock" set by the birds' internal response to seasonal changes in the length of days.

Emlen's test subject was the indigo bunting, a little songbird and prodigious commuter that flies as far as 6,000 miles a year between Canada and Central America. Emlen put the birds...

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