Effects of duration and time of food availability on photoperiodic responses in the migratory male blackheaded bunting (Emberiza melanocephala)

The effects of the duration and time of food availability on stimulation of the photoperiodic responses (fattening and gain in body mass, and growth a nd development of testes) were investigated in the migratory blackheaded bu nting (Emberiza melanocephala). Two experiments were performed. Experiment I examined the effects of a reduction in the duration of food supply in bun tings that were subjected to long day lengths (16h:8h L:D) and received foo d ad libitum (group I) or for restricted durations, coinciding with the end of the lights-on period, of 8h (group II) and 4h (group III). Buntings of group I gained in body mass, whereas there was a mixed response in group II (half the birds gained and half lost body mass), and all birds of group II I lost body mass. There was no effect on testis growth in groups I and II, but testes grew more slowly in group III. Experiment 2 investigated the eff ects of both the duration and the time of food availability. Of five groups of birds, group I was exposed to an 8h:16h L:D photoperiod, and groups II- V were exposed to 16h:8h L:D. Whereas birds of groups I and II received foo d ad libitum, those of groups III-V were fed only for 5h, at zt 0-5 (group III), zt 5.5-10.5 (group IV) or zt 11-16 (group V), where zt = zeitgeber ti me and zt 0 refers to the beginning of the lights-on period. Apart from dur ation, the timing of food availability also had an effect on photoperiodic stimulation under the 16h:8h L:D photoperiod. Birds that were fed ad libitu m fattened and gained in body mass, whereas among restricted feeding groups , only birds in the group fed during the first 5h (zt 0-5, group III) showe d a significant increase in body mass (albeit considerably lower than in th e ad libitum group). Birds fed during the middle 5h (A 5.5-10.5, group IV) showed an intermediate response, and those fed during the last 5h (zt 11-16 , group V) lost body mass. Testicular growth was suppressed in birds that w ere fed for 5h in the evening, but not in those fed for the same period in the morning or in the middle of the long day. Taken together, these results show that the duration of food supply and/or the time of day at which food is available affect photoperiodic stimulation of fattening and gain in bod y mass as well as the growth and development of gonads in the blackheaded b unting.