Early efforts to understand the processes underlying bird migration

"The problem of how migrating birds and homing pigeons navigate still remains unsolved in spite of intensive research efforts in the past few years. Over twenty years ago, Gustav Kramer suggested that in order to fly from an unfamiliar place to a geographically distant home site, a bird would need information analogous to a map (on which to read its own position and that of home), and a compass (to choose the appropriate direction indicated by the map). It is now well established that both migrating song birds and homing pigeons (on which much of the experimental work is done) have up to three types of compass-based on the Sun's azimuth, star patterns and the resultant of the Earth's magnetic field , but the nature of the map remains elusive ... [T]he big remaining mystery surrounding the magnetic compass, is how the bird's sensory system detects the Earth's magnetic field."

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Navigational tasks for migrating birds and homing pigeons require both a compass to choose direction and map-like information to determine position relative to a distant home. Gustav Kramer proposed the map-and-compass framework decades ago. Migratory songbirds and homing pigeons possess up to three compass mechanisms: cues from the Sun's azimuth, star patterns, and the resultant of the Earth's magnetic field. The specific form of the positional 'map' remains unknown. The major unresolved question about the magnetic compass concerns how avian sensory systems detect and transduce the Earth's magnetic field for orientation.

Read at Nature