Effect of daytime light intensity on daily behaviours and concurrent hypothalamic gene expressions in migratory redheaded bunting

Animals use photic cues to time their daily and seasonal activity. The role of photoperiod has been much investigated in seasonal responses, but the role of light intensity is less understood in passerine finches. We investigated if and how daytime light intensity influences photoinduced migratory phenologies and hypothalamic mRNA expressions in a Palearctic-Indian migratory finch, redheaded bunting (Emberiza bruniceps). Photoperiodic manipulations were employed to induce winter-nonmigratory (NM), premigratory (PM), and migratory (MIG) states in photosensitive buntings. In each life history state, the birds were further subjected to 0.055 (low), 0.277 (medium), or 1.11 W/m² (high) (N = 5 each) light intensity treatment. The low daytime light intensity dampened the locomotor activity rhythm and delayed the onset of Zugunruhe. We found life history-dependant but not light intensity-dependant changes in body mass, fat score, and testis volume. Plasma corticosterone levels were increased under the low-light intensity group in the migratory state. The buntings were foraging throughout the night in the migratory state, aiding body fattening. Front and back sleep were drastically reduced during the migratory phase under all three light intensities. In the migratory state, we found elevated hypothalamic IL1B and IL6 expression in medium and high-light intensity groups, which had significantly reduced sleep duration. In the winter nonmigratory state, the expression of CAMK2 correlated with daytime activity and active wakefulness of buntings. The decreased GHRH expression correlates with the reduction in total sleep in migrating buntings. Overall, daytime light intensity emerges as a key factor that fine-tunes the photoperiodic response and regulates active and sleep behaviour in migratory buntings.