Calibration of the Magnetic Compass by Polarized Light Cues

For link to the original paper published in Science, Aug 11, 2006, see "My publications"

Savannah sparrows during exposure to a natural sunset in the Yukon Delta NWR, Alaska

Birds can determine their migratory direction with the help of different compass systems (magnetic compass star, sun and polarized light compass). Because of the changing relationship between the magnetic and geographic reference systems underlying these compasses, bird have to calibrate the different systems with each other on a regular basis. How they do it and which reference system has priority over the others has been the topic of an intense debate over the past decades. In a review on the cue conflict literature (Muheim, Moore and Phillips 2006), we came to the conclusion that most disagreement between different studies can probably be ascribed to differences in cue availabilities during the exposure to the cue conflict. We proposed that birds regularly recalibrate their magnetic compass using polarized light cues from the sky near the horizon at sunset (and sunrise). Thus, apparent inconsistencies in the cue hierarchies (magnetic field, stars, sun, polarized light pattern) used by birds during the premigratory and migratory periods reported by earlier investigators could be explained by differences in access to cues during the exposure to the cue conflict, rather than season of the year. In particular, recalibration of the magnetic compass with respect to sunset (or sunrise) celestial cues occurred during both the premigratory and migratory periods when birds exposed to conflicting information between magnetic and celestial light cues were able to see polarized light cues from the region of sky near the horizon.

During autumn 2005, I tested this hypothesis by carrying out orientation experiments with Savannah sparrows in the Yukon Delta NWR, Alaska (see also Beringia Homepage). I exposed Savannah sparrows for one hour to an artificial polarization pattern shifted ±90° relative to the natural alignment at local sunrise or sunset. The exposures allowed a full view of the surroundings, including the horizon. Before and after the exposure the birds were tested indoors for magnetic compass orientation.

Study site at Kanaryarmiut Field Station in the Yukon Delta National Wildlife Refuge. The indoor orientation experiments were carried out in the weather port on the right.

Orientation funnels lined with type-writer correction paper and covered with an opaque Plexiglas sheet were used to test the magnetic orientation of the Savannah sparrows.

Recalibration of magnetic compass at sunrise and sunset

The birds recalibrated their magnetic compass repeatedly and used sunrise and sunset as independent calibration references. When exposed to the same cue conflict, but with the horizon shielded from view, the birds did not show a shift, thus did not recalibrate their magnetic compass. These results strongly suggest that migratory birds use the polarization pattern near the horizon at sunrise and sunset as underlying calibration reference to regularly update the calibration of their magnetic compass.

Why do birds use polarized light cues from the sunrise or sunset sky and not from other times of day?

At sunrise and sunset, the band of maximum polarization (BMP) passes directly through the zenith and, along with the e-vector, is aligned vertically on the horizon. In contrast to sun position, therefore, the intersections of the BMP with the horizon at sunrise and sunset are independent of topography and horizon height. In addition, since the band of maximum polarization and e-vector are vertically aligned only at sunrise and sunset, their use as a calibration reference would not require a time-compensation mechanism. There is, however, a problem with using sunrise/sunset cues, which is that sunrise/sunset positions shift with season and latitude. The solution we propose, based on earlier work by John Phillips and Jerry Waldvogel, is to average the information from successive sunrises and sunsets as illustrated…

Averaging the intersections of the band of maximum polarization with the horizon during a successive sunrise and sunset from the same location would enable migratory birds to derive an absolute reference system that is “fixed” with respect to the North-South meridian at any location on Earth and that is independent of latitude and time of year.

Periodic updating of the relationship between the polarization patterns at sunrise and sunset (their angular “split” on either side of the meridian) would make it possible that, when bad weather, for example, makes averaging impossible, either the sunrise or sunset cues can be used alone to estimate the reference direction and calibrate other compass systems.

Why should birds averaging instead of using only sunrise or only sunset cues alone as independent calibration references?

Changes in latitude and time of year produce opposite shifts in the alignments of the BMP at sunrise and sunset (figure to right). Such a strategy can result in a gradually curving migratory route (see below) and may be adaptive, but only if always the same cue is used (either sunrise or sunset).

However, in species like the Savannah sparrows that use both sunrise and sunset cues to recalibrate their magnetic compass, failure to average the information from both times of day would produce an “zig-zagging” migratory path (see below), depending on whether the clear skies necessary to see the polarization pattern occurred most recently at sunrise or at sunset. Therefore, the opportunistic use of sunrise and sunset is not adaptive.

A unified theory of compass calibration in migratory songbirds

In conjunction with earlier work showing that the sun and star compasses are secondarily calibrated with respect to magnetic and/or celestial polarized light cues, our findings indicate that sunrise and sunset polarization patterns are used to derive an absolute (geographic) reference system that provides the primary calibration reference for all of the compass systems of migratory birds.

Last updated: 11/11/2007

Read about my adventures during the Beringia expedition to Alaska in autumn 2005!

(click on picture)