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 in a planetarium and studied their reaction to fall star patterns. To his surprise, the birds seemed to ignore the artificial heavens on the planetarium dome. Outside it was spring, and the birds always tried to head north. Why?

Something was obviously overriding the instructions provided by the planetarium stars. To test his hunch, Emlen began exposing the birds to periods of simulated daylight that lengthened faster than natural days. Within weeks he succeeded in advancing their biological clocks by six months. Though it was only spring at Cornell, the buntings showed physiological preparations for fall migration. Next Emlen exposed the birds to spring star patterns, which should have dictated a northward passage. But the birds seemed determined to fly south, as if it were fall.

The only star that they did heed was Polaris, the North Star. As long as it appeared, they retained their sense of direction. But when it was removed from the planetarium sky, they seemed hopelessly confused. From these experiments, Emlen concluded that they probably use Polaris, which is visible all year in the northern hemisphere, as a celestial beacon on both legs of their journey.

More important, the tests convinced him that the secret of the buntings’ navigational skill lies in their body chemistry. It tells them not only when to travel but also whether to fly toward or away from the North Star. The most likely agent, he thinks, is a hormone or combination of hormones, secreted in response to varying amounts of daylight as the seasons change. If Emlen can identify these hormones and discover how they work, he may help explain how similar biological clocks work in other animals, including man.