Star compass learning: how long does it take?

Young night-migratory birds establish a functional star compass during ontogeny by searching the starry night sky for its centre of rotation and interpreting this as “north”. The birds then use the learned location of the rotational centre to calibrate their magnetic compass. Here, we examine the required duration of this learning process. We exposed three groups of hand-raised inexperienced European Robins Erithacus rubecula to a rotating artificial star pattern for 3 weeks, 1 week, or 1 night, respectively, during ontogeny. A control group was exposed to the same, but stationary, artificial star pattern for 3 weeks. During their first autumn migration, we tested the birds’ orientation under the stationary star pattern as well as in the local geomagnetic field with no stars visible. Only the group that had the longest exposure time to celestial rotation during ontogeny was able to show orientation in the appropriate direction during autumn migration in the local geomagnetic field in relation to the memorized former centre of celestial rotation. This suggests that these birds had calibrated their magnetic compass relative to the star pattern seen during ontogeny. All other groups showed inappropriate or random orientation both under the stationary star sky and in the local geomagnetic field, suggesting that 7 nights of observing celestial rotation are not sufficient for young European Robins to establish a star compass and to calibrate their magnetic compass accordingly.