Termination of seasonal breeding in a weaver finch, Ploceus philippinus: Role of photoperiod

Seasonal gonadal regression in the photoperiodic weaver finch Ploceus philippinus occurs in the late summer despite the presence of long days. Earlier studies indicated a decline in photosensitivity in late summer, which might explain the failure of birds to respond to long days, resulting in gonadal regression. Results obtained in the present study suggest that 1) birds become refractory only to the stimulatory day lengths experienced in nature (longest day at 25° N 13.34 hr) and not to all long day lengths as in many other photoperiodic passerine species 2) birds become refractory in accordance with the day length they experience during the growth phase of the gonadal cycle. Thus birds held on 12 L/12 D become refractory to 12 L/12 D but are photosensitive to longer day lengths. Similarly, birds on natural day length become photorefractory to the longest day but are sensitive to higher day length, viz 15 L/9 D. 3) The long summer day lengths experienced in nature might cause the seasonal decline in photosensitivity.