Circadian periodicity in yellow-throated sparrow as demonstrated by intermittent light experiments. 
Groups of adult photosensitive male yellow-throated sparrows were subjected to different intermittent light cycles viz.  2L/2D, 3L/3D, 4L/4D, 8L/8D and 12L/12D besides two control groups held on 8L/16D and 18L/6D photoperiodic treatments.  Testicular growth occurred in 2L/2D, 3L/3D, 4L/4D and 18L/6D but not in 8L/16D, 12L/12D and 8L/16D photoperiodic regimes.  The results of this experiment can be interpreted on the basis of circadian rhythm in photosensitivity in an avian external coincidence model.  Our findings suggest that multiple light flashes are more effective than a single broad pulse of light of equal duration.