Regulation of seasonality in the migratory male blackheaded bunting (Emberiza melanocephala)

Abstract

-The present study was carried out on a Palearctic-Indian migratory species, the blackheaded bunting (Emberiza melanocephala), to understand the importance of photoperiodism and circannual rhythms in determining seasonality in changes in body mass and testis size in birds. An initial experiment determined the effects of duration and intensity of light on photoperiodic induction. The birds were exposed to different photoperiods (hours of light:hours of darkness; 11.5L:12.5D, 12L:12D, 12.5L:11.5D and 13L:11D) at the same (~ 450 lux) light intensity, and to 13L:11D at different light intensities (50-, 100-, 400-, 800-and 1000-lux). The induction and subsequent regression of photoperiodic responses were dependent upon duration and intensity of the light period until these reached threshold. A second experiment investigated if an endogenous seasonal rhythm underlies photoperiodism in buntings. Birds maintained since February on a 8L:16D photoperiod (a non-inductive short day length invariably used to ensure photosensitivity in photoperiodic species) were subjected periodically to 16L:8D (a long day length), one group every month from mid-March to mid-August. The magnitude of long day response in body mass and testes decreased as the duration of the short days progressed, but testicular response was restored in birds that were exposed to long days in July and August. The birds exposed simultaneously to short, long, and natural day lengths for 32 weeks underwent an induction-regression cycle under long days and natural day lengths, but not under short days in which a decrease in body mass occurred after about 20 weeks. The last experiment examined the importance of latitudinal migration on photoperiodism, by comparing the response to long days of three groups which included birds from populations those were held in the outdoor aviary for 1 or 2 years at 27° N and those immediately arrived from their breeding grounds (~ 40° N). There was no difference in the photoperiodic induction among the three groups, indicating that neither experience to changing photoperiods during a migratory journey, nor to long photoperiods at breeding grounds, were critical for a subsequent response (initiationtermination-reinitiation) cycle. Taken together, these findings suggest that (1) the blackheaded bunting has its own endogenous timing program, which is regulated by the photoperiod, and (2) the photoperiodic programs of bunting are flexible enough to accommodate variations in the amplitude of environmental cycles. Thus, it appears that photoperiodism has evolved independently of the evolution of migration in this species.bunting / intensity / photoperiod / photoperiodic response system / photosensitivity / seasonality

Temperature alters the photoperiodically controlled phenologies linked with migration and reproduction in a night-migratory songbird

“…Buntings in the hightemperature group on 13 L did not fully fatten and gain weight (figure 3d,e), but they attained full testicular maturation (figure 3f ). This is not surprising as body fattening and testicular recrudescence in blackheaded buntings are reported to be the separate photoperiodic phenomena [5,20,21]. Our results (figure 3) further suggest that temperature affected both the timing and duration of a life-history stage in the blackheaded bunting.…”

“…We discount this, as results from our previous studies on the blackheaded buntings do not suggest a change in the sensitivity to long days during the months of the year in which the current experiments were performed [5,20,21]. In a detailed study in which photosensitive blackheaded buntings were subjected to long days at monthly intervals for six months, beginning from midMarch, there were no significant difference in fat deposition and weight gain, and in testicular growth among groups that were exposed to 16 L:8 D in mid-March, mid-April and mid-May [21]. Our observations on the blackheaded buntings (figure 3) contrast with those on the white-crowned sparrows and European starlings in which the high temperature had been found to accelerate the rate of photoperiodic induction (figure 3) [14,15].…”

“…One possibility is that the high temperatures used in our study acted as a stressor to photoperiodic induction, as blackheaded buntings do not experience such temperatures in the wild. We would not favour this however, as a 138C temperature gradient we used in current experiments is close to that which buntings probably experience during their migratory and reproductive life-history stages [21]. Buntings start autumnal migration before temperature probably goes below 208C, and start spring migration when temperature is about 308C [20,23].…”

“…Because 11.5 L was noninductive in both temperature groups, we re-examined our hypothesis in the experiment 2, by increasing the daily light period by 0.5 h. A 12 h light per day is weakly inductive when compared with longer days, e.g. 13 h light per day [21]. Beginning on 14 April 2008, when daylight and middaytime outdoor temperature were approximately 12.75 h and 398C, respectively, birds (n ¼ 6 per group) were exposed to 12 L:12 D at low (278C) and high (408C) temperatures for four weeks.…”

Effect of photoperiod length on body mass and testicular growth in the house sparrow (Passer domesticus) and brahminy myna (Sturnus pagodarum)

“…In the present study, we attempted to address this by examining the impact of a strong light stimulus on body mass and gonadal growth of two birds: the house sparrow (Passer domesticus) and brahminy myna (Sturnus pagodarum). Both species are photosensitive and use photoperiod in the regulation of their gonadal cycle [12,13,23,24]. We subjected them simultaneously to a long day length corresponding to what these species experience in the wild and to an unusually long day length that these species never experience in the wild.…”