The synchronizing effect of slight oscillations of light intensity on activity period of birds.

To settle the question whether the slight daily oscillations of light intensity in high arctic summer have a synchronizing effect or not, experiments with sine wave and square wave light-dark cycles were done. Within a wide range (0.5 till 600 lux) many LD-cycles were tested. All cycles had a maximum:minimum ratio of light intensity of 2:1 resp. 3:2 and a period of 24 h. Between sine wave and square wave light-dark cycles hardly a difference in the synchronizing effect was found. In contrast to this the synchronizing effect of cycles with a ratio of 2:1 was considerably greater than with a ratio of 3:2. Since under 3:2 conditions only a very small number of birds showed a synchronized activity period, it is supposed that, at least in song-birds, this ratio is near the limit of synchronizing capacity. The absolute value of the amplitude of the light cycle seems to have only a small influence on the synchronizing effect, whereas the maximum:minimum ratio has a great influence. With higher light intensities however more and more test-birds showed continuous activity. Because the light intensities in high arctic summer oscillate in a range far exceeding 104 lux, one have to assume that these oscillations of light intensity do not have any synchronizing effect, although at lower light intensities slight oscillations definitely can be successfully, as the experiments have shown.