ENTRAINMENT OF CIRCADIAN SYSTEM UNDER VARIABLE PHOTOCYCLES (T-PHOTOCYCLES) ALTERS THE CRITICAL DAYLENGTH FOR PHOTOPERIODIC INDUCTION IN BLACKHEADED BUNTINGS

The present study was designed to investigate the photoperiodic respon ses of blackheaded buntings to various T-photocycles [T = the period l ength of light:dark (LD) cycle]. Two series of experiments were perfor med. In series I, groups of buntings (n = 8-12) were exposed to LD cyc les of 22 to 26 hr with photoperiods of 6, 10, or 11 hr (T22 = 6L:16D, 10L:12D, 11L:11D; T24 = 6L:18D, 11L:13D; T26 = 6L:20D, 11L:15D). Amon g these groups, birds subjected to 11L:11D showed heavy deposition of fat, weight gain, and testicular recrudescence, a response that is cha racteristic of long day (greater than or equal to 12 hr photoperiod pe r day) photostimulation, indicating that a T-photocycle of less than 2 4 hr (T = 22 hr) was photoinductive in buntings when presented with a photophase long enough to interact with the photoinducible phase (phi i), located in the middle of the photocycle. Experiments in series II included an interrupted-night lighting protocol with a main photophase of 6 hr and l-hr light break in photocycles of T = 22 hr, T = 24 hr, and T = 26 hr. The results revealed that a 1-hr light pulse was induct ive when it was introduced in the middle of the photocycles, and, thus , was coincident with the phi i. It is suggested that the critical day length for photoperiodic induction in buntings is altered as a result of entrainment to various T photocycles. (C) 1995 Wiley-Liss, Inc.