Fig. 2. Results from experiment 2. (A,B) Food intake, (C,D) body mass (shown as per cent change relative to values on day 0), and (E, F) change in testis volume after 21 days relative to that on day 0. At the beginning of the experiment, testes of birds in all groups were minimum in size (approximately 0.33 mm³). Of five groups of photosensitive birds (N=6 in each), group I was exposed to an 8 h:16 h L:D photoperiod and fed ad libitum, and groups II-V were subjected to 16 h:8 h L:D photoperiod and given an ad libitum or restricted (5 h) food supply: group II, ad libitum; group III, zt 0-5; group IV, zt 5.5-10.5; group V, zt 11-16; zt=0 indicates the beginning of the lights-on period. The experiment ran for a period of 3 weeks. Values are means ± S.E.M. except in A and B, where food intake bird^-1 day^-1 refers to values calculated from a group value, obtained from a group of birds (N=5-6) housed within one cage (see Materials and methods). * indicates a significant difference (P<0.05) relative to the value on day 0 (one-way repeated-measures ANOVA or paired t-test; ** indicates a significant difference within a group as a function of the duration of the experiment (one-way repeated-measures ANOVA); indicates a significant difference between groups on the indicated days; + indicates a significant difference with respect to the long-day controls.