Discrimination of artificial starry sky by pigeons.

Night migratory birds use the star compass for their navigation. Here, I examined discrimination of a starry sky by a nonmigratory bird, the pigeon. Four pigeons were trained in an operant chamber to discriminate between an artificial starry sky created using software (Stella Theater Pro) and a black sky without stars. After they learned the task, they received three generalization tests. Test 1 presented a starry sky with different classes of stars (i.e., the number of stars was changed). Test 2 presented a starry sky at different times (i.e., a horizontal shift of the sky). Test 3 presented a starry sky at different latitudes (i.e., a vertical shift of the sky). The pigeons displayed a clear generalization gradient with a peak shift to the sky with more stars in Test 1 and a peak close to the original sky in Test 3, but almost a flat gradient in Test 2. Therefore, pigeons demonstrated stimulus control using the pattern of the stars, and they were more sensitive to the vertical shift than to the horizontal shift.