Light intensity alters the stimulatory effects of long photoperiod, but does not alter critical photoperiod in the Indian Weaver Bird (Ploceus philippinus)

Abstract

Day length regulates annual reproductive cycle in many birds including subtropical Indian Weaver birds (Ploceus philippinus). We investigated whether light intensity modulated the effects of the duration of photoperiod in inducing testicular recrudescence- regression cycle, and whether such effects are influenced by the seasons. Two experiments were performed in which Weaver birds were exposed to near threshold and long photoperiods at different light intensities at two times of the year. In experiment 1, Weaver birds were exposed to 11.5 L:12.5 D, 12 L:12 D and 13 L:11 D photoperiods with light intensity 0.04 and 3.4 Wm−2 for a period of 40 weeks, beginning in May (pre-breeding season). Experiment 2 compared light intensity effects on photoperiodic induction in two different seasons, and differed from the experiment 1 in using a different grade of light intensities. Weaver birds were exposed to 11.5 L:12.5 D, 12 L:12 D and 13 L:11 D at 0.05 and 2.1 Wm−2 light intensity for 24 weeks, beginning in April (pre-breeding season) and September (post-breeding season). Observations on testis size and wing primaries molt were recorded weekly (molt) or at 4-week intervals (testes). There was a significant effect of light intensity on photoperiodic induction of testicular growth in birds exposed to 13 h, but not to 11.5- and 12 h photoperiods. Further, testis recrudescence under a stimulatory photoperiod was faster in birds exposed during pre-breeding, compared to post-breeding. Thus, light intensity influences the rate of testicular maturation and molt, but does not alter the critical day length for photoperiodic induction in Indian Weaver birds. This may be adaptive for birds in using light intensity to modulate the rates of their physiological processes as needed in the wild.