It appears that the magnetic navigational sense in migrating species is used as a backup when celestial cues are lacking, Able says. When the birds are forced to rely on the magnetic field, they try to calibrate their magnetic sense using other cues, especially the polarization of daylight at sunset. In the hierarchy of navigation cues, it s my view that sunset information, which seems to be polarized light, takes precedence. Next is the magnetic compass and then the stars which is a very different perspective than we had when I started in this business. The dogma was that birds that migrate in the day use the sun as a compass and birds that migrate at night use the stars.
Stars are indisputably important, particularly in a young bird s learning and development.
Birds learn that stars appear to rotate except for the star that we know to be the North Star, and birds seem to be genetically programmed to know in what direction they should go in relation to the North Star, says Able. But birds face a wide range of conditions as they migrate, and it s not surprising that their navigation systems have a lot of flexibility. Even on clear nights, the visual information is changing. A bird that flies very far south, for example, will find that the star patterns it s familiar with from the northern sky are going to disappear below the horizon behind it and new ones are going to pop up in front of it. If this is its first trip, it will have never seen those stars.