ROLE OF LIGHT IN THE PROGRESSIVE PHASE OF THE PHOTOPERIODIC RESPONSES OF MIGRATORY BIRDS

¹ Department of Biological Sciences, Northwestern University, Evanston, Illinois

1. To determine the roles of light and darkness in the photoperiodic responses of birds, effective photoperiods were combined with normally inhibitory dark periods to give cycles longer than 24 hours as follows: 12L-16D; 12L-20D; 16L-16D; 16L-22D; 16L-32D. The experiments were performed in winter and spring, and tested both induction and maintenance of the gonadal and fat responses.

2. The results indicate that the photoperiod and not the dark period determined the response. Long dark periods per se, therefore, are not inhibitory.

3. Comparison of the rate of response in previous experiments with similar photoperiods but different dark periods shows a greater rate of response with longer dark periods. The response to a given light-dark cycle may, therefore, be a function of the photoperiod and the subsequent dark period.

4. The results also indicate that the gonadotropic and lipogenetic effects of each stimulatory light-dark cycle are "stored," or cannot be negated by long intervening dark periods.

5. The relation of these findings to theories of the mechanism of response to light and dark is discussed.