Termination of seasonal breeding in the photoperiodic weaver bird

Termination of seasonal breeding in the photoperiodic weaver bird does not occur as a result of photorefractoriness, and yet, in nature birds become refractory. Earlier studies indicated a seasonal variation in photosensitivity of this bird during the reproductive cycle and it was suggested that setting of the photoperiodic response threshold at a higher level might explain gonadal regression. Such a possibility was now examined by keeping birds in constant 13L/11D (the near maximal day length at Varanasi, India) and 15L/9D simultaneously for over 8 mo. While breeding in birds in 15L/9D continued, gonads regressed in 13L/11D indicating that the day length which was stimulatory in spring is no more sufficient to permit gonadal activity. Cyclicity in 13L (and in artificial 12L as well as natural daylength) may be explained on the basis that these schedules exceed the basic photoperiodic response threshold—hence the initiation of gonadal development. Once the gonads are developed due to decline in photosensitivity (caused perhaps by sex hormones) gonadal regression occurs, thus resulting in reproductive periodicity.