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Professor Stephen T. Emlen
Professor Stephen T. Emlen of Cornell University placed indigo buntings in funnel-shaped boxes (seen here on stepstools behind the round star projector) under a planetarium sky and demonstrated that birds learn to select the proper migratory direction according to star patterns. Photo courtesy of Cornell University.
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While investigation of bird migration and navigation has long incorporated such techniques as direct observation, capture and marking, experimental investigation markedly increased in the mid-twentieth century. Many experiments have examined the orienting behavior of birds kept in captivity, called Zugunruhe, or migratory restlessness. This behavior includes rapid fluttering of the wings and hopping in the direction towards which related birds in the wild would be migrating.

Shortly after the Second World War, Gustav Kramer used mirrors in a laboratory in Germany to shift the sun's image by ninety degrees and found that European starlings shifted their orientation in the same direction. From this research it was theorized that when birds maintain their direction over the course of a day, as the sun traverses from east to west, they are able to compensate for the lapse of time by adjusting to the changing angle of the sun. Subsequent research has shown that melatonin secretions from the light-sensitive pineal gland on the top of the bird's brain aids this solar orientation, and that birds can detect polarized light from the sun through the atmosphere even during cloudy days and evenings. Birds that migrate at night also take directional information from patterns of polarized light emanating from the setting sun.

In the 1950's, German ornithologists Franz and Eleanore Sauer placed caged warblers into a planetarium and slowly rotated the star ceiling over time to resemble the night sky on the birds' normal migration route. The birds continued their orienting, wing fluttering, and restless behaviors until they "arrived" at the night sky location at which they would normally end their migration. In this way, Sauer demonstrated that the birds had used the changing night sky patterns to orient migratory behaviors.

Building on the earlier planetarium studies, Cornell University behavioral ecologist Stephen Emlen demonstrated that developing birds learn to recognize star patterns that, over time, become a reference system for migratory way-finding. During the 1960's he conducted experiments with caged indigo buntings, in which a star ceiling was manipulated to simulate spring or fall skies, inducing physiological states of migratory readiness in the buntings. The birds were placed in cages having inked blotting pads on their floors. Paper funnels that opened wider at the top enabled the birds' view of the starry ceiling. Responding to the "seasonal changes" in the sky, the buntings quivered and jumped in the direction of their intended migration, leaving inked marks on the side of the funnel. Quantitative records were made of the frequency of jumping in the migratory direction, "northward" in spring, "southward" in autumn. As Emlen turned off some star projections in the planetarium sky, the birds' orientation became poorer. Birds that were raised in conditions that prevented their seeing star patterns were unable to select the normal migration direction. Birds that were raised under a planetarium sky that rotated around the star Betelgeuse in the constellation Orion oriented to Betelgeuse as if it were in the north when they were later placed under a normally oriented night sky.

During the mid-twentieth century, scientists began to find wide evidence that many animals also rely on the Earth's magnetic field for navigation. F.W. "Fritz" Merkel and Wolfgang Wiltschko (1964) placed birds in a large cement cage that screened out all environmental clues; the birds still were able to properly orient. When they put the birds into a large steel cage, which interrupted magnetic lines of force around the cage, the birds oriented randomly.

In the 1970's Cornell scientist William Keeton attached brass bars to homing pigeons. Placed in an unfamiliar location, they generally oriented themselves in the direction of home, whether it was a sunny or overcast day. When a magnetic bar was attached to the pigeons, they oriented normally on a sunny day, but were disoriented on an overcast day. The magnet appeared to cause disorientation when they were unable to orient by the sun. A later study used coils around the birds' heads that produced a magnetic current which affected their orientation: if it flowed in a clockwise direction, the usual direction of the magnetic field in the northern hemisphere, the birds flew directly home on either a cloudy or sunny day; if the current was reversed and flowed counterclockwise, the birds flew directly home on a sunny day, but flew 180 degrees in the wrong direction on an overcast day. The experiments indicated that geomagnetic forces are used for orientation, along with information from the angle of the sun.

Migrating birds also appear to orient by wind direction. University at Albany ornithologist Kenneth Able used radar to track nocturnal migrants and reported that birds frequently flew in the wrong direction by using wind cues when stars were unavailable. Other studies have indicated that birds also use smell to guide them to their nesting sites.

A clear message emerging from studies of bird orientation and navigation is that they do not rely on a single source of information for migration cues. Instead, they use overlapping cues from a variety of sources.

Cabinet, by Martha Glowacki
Martha Glowacki, Starry Transit, 2004, wood, glass, brass, copper, bird carcasses, paper, pigments (detail).
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Starry Transit: An Installation by Martha Glowacki, MMoCA's sculptural installation at University of Wisconsin-Madison's Washburn Observatory, interprets the phenomenon of bird migration as it has been investigated by scientific explorers. Inspired by Emlen's planetarium studies demonstrating that night flying migratory birds use star patterns as navigational aids, Glowacki constructed an installation that permits visitors to the observatory to participate in the tradition of research and discovery into natural mysteries.