North Sky Guides Birds

Abstract

> THE SMALL indigo bunting apparently steers his way 2,000 miles south every year by looking over its shoulder at the northern sky. This night-flying immigrant does not seem to rely on any one star or constellation for his direction, said Stephen T. Emlen of the University of Michigan. Rather he recognizes the entire pattern of stars within 35 degrees of the North Star, and can be confused only when that part of the sky is blocked. Mr. Emlen's experimental results, reported at the American Institute of Biological Sciences meeting, College Park, Md., diverged from most theories currently used to explain bird migrations. Birds supposedly find their direction from a single star or group of stars and then compensate for the time of night, Mr. Emlen said. In other words, they are thought to have a built-in clock for recognizing where the star should be at any one hour. However, in testing this theory under artificial planetarium skies, Mr. Emlen found he could not confuse the bunting by moving the stars ahead as much as six hours. They still oriented themselves south in the fall and north in the spring, despite the fact the constellations were far west of their normal positions. The birds were not much bothered by having the entire southern sky disappear even when they were headed in that direction. But when Mr. Emlen shut off the northern sky, the birds simply stopped orienting altogether and did nothing. Mr. Emlen then proceeded to find out whether the bunting depends for direction on any single cue, such as the Milky Way, the Big Dipper, Cassiopeia or the North Star. He was unsuccessful, which leads him to believe the birds must take their direction from the geometrical pattern formed by all these cues together. He also found that different birds take slightly different cues. Some were disoriented by one test, others by a second. However, the majority lost their direction when the circumpolar area was blocked. At the same session on animal perception, William Hermnkind of the University of Miami, Coral Gables, Fla., revealed that fiddler crabs have an optical system whereby they can measure polarized light in the sky and find their way from sea to shore. Polarized light is only one of three cues they use, but it is an important one, said Mr. Hermkind. The other two are the shore itself and the position of the sun. By putting the crabs in a pan of water and covering it with a polaroid filter, Mr. Herrnkind found he could make the crabs change direction according to the position of the filter.