THE PHOTOPERIODIC CONTROL OF RE PRODUCTIVE CYCLES IN BIRDS

Abstract

annual cycles to that of the synchronizing role of a Zeitgeber, in the sense of Aschoff (1955), for an innate quasi-annual reproductive periodicity. Photoperiodism, thus defined, is a part of the large and fascinating field of annual biochronometry. Whether an organism has an annual cycle that is generated by an annual environmental periodicity or simply phased or synchronized by it, the most conspicious reliable annual en vironmental periodicity at mid- and high-latitudes is the annual change in day length. A comparison (Fig. 1) of the vernal change in day length with another annually periodic environmental factor, air temperature, serves to emphasize the relative reliability of day length in the timing of annually periodic events. Although photoperiodically controlled annual cycles are widespread among animals (Emme, 1958,1960, 1962; Farner, 1959, 1961b; Danilevskii, 1961; B?nning, 1963), it is among migratory temperate-zone birds that the most precise development of photo periodic mechanisms, and the greatest degree of dependence on them in the timing of annual cycles, have developed. However, even a casual review of reproductive periodicity in birds reveals that day length is only one of several or many environmental sources of information that may be used in the control of reproductive cycles. It should only be re emphasized that, because of its great reliability, day length is very extensively used by species of mid- and high-latitudes; indeed, in many it is almost exclusively important in the timing of the annual repro ductive period. Knowledge of photoperiodic control of annual reproductive cycles is not novel. By the seventeenth century, it was well known to Dutch bird netters that the song ordinarily associated with vernal reproductive activity could be obtained in autumn by holding males on reduced light through spring and early summer, and then exposing them to the long days of late summer (Hoos, 1937). This practice (muiten) was used to produce singing decoys for netting autumn migrants. An almost identical * A Sigma Xi-RESA Lecture, 1963-64. Material used in this lecture has been drawn extensively from previously published results of investigations supported by the National Science Foundation, the National Institutes of Health, the Office of Naval Research, and by funds made available for biological and medical research by State of Washington Initiative Measure No. 171. Much of the material on flash light ning and interruption of dark periods is from previously unpublished results of experi ments supported by the Office of Naval Research through Contract Nonr 1520 (00). 137