Photoperiod as a modifying and limiting factor in the expression of avian circannual rhythms

In three species of birds that migrate long distances, the annual rhythms of gonadal activity, molt, and migratory restlessness (Zugunruhe) persist for more than 1 year under certain constant conditions. The most important zeitgeber for these circannual rhythms is the annual cycle of photoperiod, which adjusts the overall period of circannual rhythms to exactly 1 year and also provides for the appropriate adjustment of seasonal activities to the temporal structure of the environment. This is illustrated by results on garden warblers (Sylvia borin) indicating that the longer photoperiods experienced by individuals wintering far south in the African wintering area phase-advance spring migration and the accompanying gonadal development, relative to those of individuals wintering further north. The rate of acceleration is, however, slow enough to prevent a reproductive cycle during the Southern Hemisphere summer. Hence, endogenous circannual components and zeitgeber stimuli constitute a functional entity that provides as a whole for adaptive temporal programming. This idea is further supported by findings in the pied flycatcher (Ficedula hypoleuca), in which a circannual rhythmicity persists only if photoperiod in winter is at least as short as that normally encountered by the species in its wintering grounds slightly north of the equator. In collared flycatchers (Ficedula albicollis), in contrast, rhythmicity continues under much longer photoperiods, consistent with the fact that the wintering area of this species extends to latitudes far south of the equator. It is proposed that the adaptive function of circannual rhythms can be properly understood only if their interactions with environmental factors, particularly those that play a role as zeitgebers, are analyzed in sufficient detail. The biological significance of circannual rhythms may be more apparent in the context of the environmental constraints limiting their expression than in the often rather restricted set of conditions sustaining spontaneous annual cyclicity.     

Avian circannual clocks: adaptive significance and possible involvement of energy turnover in their proximate control

Endogenous circannual clocks are found in many long-lived organisms, but are best studied in mammal and bird species. Circannual clocks are synchronized with the environment by changes in photoperiod, light intensity and possibly temperature and seasonal rainfall patterns. Annual timing mechanisms are presumed to have important ultimate functions in seasonally regulating reproduction, moult, hibernation, migration, body weight and fat deposition/stores. Birds that live in habitats where environmental cues such as photoperiod are poor predictors of seasons (e.g. equatorial residents, migrants to equatorial/tropical latitudes) rely more on their endogenous clocks than birds living in environments that show a tight correlation between photoperiod and seasonal events. Such population-specific/interspecific variation in reliance on endogenous clocks may indicate that annual timing mechanisms are adaptive. However, despite the apparent adaptive importance of circannual clocks, (i) what specific adaptive value they have in the wild and (ii) how they function are still largely untested. Whereas circadian clocks are hypothesized to be generated by molecular feedback loops, it has been suggested that circannual clocks are either based upon (i) a de-multiplication ('counting') of circadian days, (ii) a sequence of interdependent physiological states, or (iii) one or more endogenous oscillators, similar to circadian rhythms. We tested the de-multiplication of days (i) versus endogenous regulation hypotheses (ii) and (iii) in captive male and female house sparrows (Passer domesticus). We assessed the period of reproductive (testicular and follicular) cycles in four groups of birds kept either under photoperiods of LD 12L:12D (period length: 24h), 13.5L:13.5D (27 h), 10.5L:10.5D (23 h) or 12D:8L:3D:1L (24-h skeleton photoperiod), respectively, for 15 months. Contrary to predictions from the de-multiplication hypothesis, individuals experiencing 27-h days did not differ (i.e. did not have longer) annual reproductive rhythms than individuals from the 21- or 24-h day groups. However, in line with predictions from endogenous regulation, birds in the skeleton group had significantly longer circannual period lengths than all other groups. Birds exposed to skeleton photoperiods experienced fewer light hours per year than all other groups (3285 versus 4380) and had a lower daily energy expenditure, as tested during one point of the annual cycle using respirometry. Although our results are tantalizing, they are still preliminary as birds were only studied over a period of 15 months. Nevertheless, the present data fail to support a 'counting of circadian days' and instead support hypotheses proposing whole-organism processes as the mechanistic basis for circannual rhythms. We propose a novel energy turnover hypothesis which predicts a dependence of the speed of the circannual clock on the overall energy expenditure of an organism.     

Circannual clocks in avian reproduction and migration

Many behavioural and physiological functions of organisms are adjusted to the periodic changes in their environment, particularly to those related to the natural day and year. This adjustment is often achieved through the action of endogenous daily (circadian) and annual (circannual) clocks. Studies of the control of avian moult, migration and reproduction have played a major role in understanding how biological clocks function and interact with rhythms in the environment. Investigations on tropical birds such as the East African subspecies of the Stonechat ( Saxicola torquata axillaris ). and long-distance migrants like the Garden Warbler ( Sylvia borin ). have provided the longest records of circannual rhythms, some of them running for more than 12 years, with periods ranging from about 9 to 13 months. Avian circannual rhythms are organized in a characteristic way for a particular species or population, and cross-breeding experiments have shown that some of the differences found among them are genetically determined. In African Stonechats circannual rhythms guarantee that seasonal events occur at the appropriate times of the year and in the characteristic sequence. They also control a "reproductive window" that provides the temporal framework for breeding. The width of this window is rather inflexible but the performance of a bird within this framework (e.g. whether it breeds once or twice per season) is subject to modification by environmental conditions. In migratory birds circannual programs are involved in determining the time course, distance and direction of migration. Circannual rhythms are synchronized with and modified by environmental factors in a complex way, but the endogenous mechanisms usually respond to environmental cues such that an optimal adjustment to season and latitude is guaranteed.     

Circannual clocks in avian reproduction and migration

Many behavioural and physiological functions of organisms are adjusted to the periodic changes in their environment, particularly to those related to the natural day and year. This adjustment is often achieved through the action of endogenous daily (circadian) and annual (circannual) clocks. Studies of the control of avian moult, migration and reproduction have played a major role in understanding how biological clocks function and interact with rhythms in the environment. Investigations on tropical birds such as the East African subspecies of the Stonechat ( Saxicola torquata axillaris ) and long-distance migrants like the Garden Warbler ( Sylvia boriri ) have provided the longest records of circannual rhythms, some of them running for more than 12 years, with periods ranging from about 9 to 13 months. Avian circannual rhythms are organized in a characteristic way for a particular species or population, and cross-breeding experiments have shown that some of the differences found among them are genetically determined. In African Stonechats circannual rhythms guarantee that seasonal events occur at the appropriate times of the year and in the characteristic sequence. They also control a "reproductive window" that provides the temporal framework for breeding. The width of this window is rather inflexible but the performance of a bird within this framework (e.g. whether it breeds once or twice per season) is subject to modification by environmental conditions. In migratory birds circannual programs are involved in determining the time course, distance and direction of migration. Circannual rhythms are synchronized with and modified by environmental factors in a complex way, but the endogenous mechanisms usually respond to environmental cues such that an optimal adjustment to season and latitude is guaranteed.     

Photoperiodic control of seasonality in birds

This review examines how birds use the annual cycle in photoperiod to ensure that seasonal events--breeding, molt, and song production--happen at the appropriate time of year. Differences in breeding strategies between birds and mammals reflect basic differences in biology. Avian breeding seasons tend to be of shorter duration and more asymmetric with respect to changes in photoperiod. Breeding seasons can occur at the same time each year (predictable) or at different times (opportunistic), depending on the food resource. In all cases, there is evidence for involvement of photoperiodic control, nonphotoperiodic control, and endogenous circannual rhythmicity. In predictable breeders (most nontropical species), photoperiod is the predominant proximate factor. Increasing photoperiods of spring stimulate secretion of gonadotropin-releasing hormone (GnRH) and consequent gonadal maturation. However, breeding ends before the return of short photoperiods. This is the consequence of a second effect of long photoperiods--the induction of photorefractoriness. This dual role of long photoperiods is required to impart the asymmetry in breeding seasons. Typically, gonadal regression through photorefractoriness is associated with a massive decrease in hypothalamic GnRH, essentially a reversal to a pre-pubertal condition. Although breeding seasons are primarily determined by photoperiodic control of GnRH neurons, prolactin may be important in determining the exact timing of gonadal regression. In tropical and opportunistic breeders, endogenous circannual rhythmicity may be more important. In such species, the reproductive system remains in a state of "readiness to breed" for a large part of the year, with nonphotic cues acting as proximate cues to time breeding. Circannual rhythmicity may result from a temporal sequence of different physiological states rather than a molecular or cellular mechanism as in circadian rhythmicity. Avian homologues of mammalian clock genes Per2, Per3, Clock, bmal1, and MOP4 have been cloned. At the molecular level, avian circadian clocks appear to function in a similar manner to those of mammals. Photoperiodic time measurement involves interaction between a circadian rhythm of photoinducibility and, unlike mammals, deep brain photoreceptors. The exact location of these remains unclear. Although the eyes and pineal generate a daily cycle in melatonin, this photoperiodic signal is not used to time seasonal breeding. Instead, photoperiodic responses appear to involve direct interaction between photoreceptors and GnRH neurons. Thyroid hormones are required in some way for this system to function. In addition to gonadal function, song production is also affected by photoperiod. Several of the nuclei involved in the song system show seasonal changes in volume, greater in spring than in the fall. The increase in volume is, in part, due to an increase in cell number as a result of neurogenesis. There is no seasonal change in the birth of neurons but rather in their survival. Testosterone and melatonin appear to work antagonistically in regulating volume.     

Is food availability a circannual zeitgeber in tropical birds? A field experiment on stonechats in tropical Africa

Equatorial stonechats (Saxicola torquata axillaris) in Africa are seasonal breeders like their temperate-zone conspecifics (S.t. rubicola). Their annual cycle in gonadal size and function is controlled by an endogenous circannual rhythmicity that has been shown to run for up to 10 years in a constant equatorial photoperiod under laboratory conditions, with a period deviating from 12 months. In nature, however, this rhythm is synchronized with the actual year. Because photoperiod is essentially constant at the equator, it is likely that other environmental factors act as zeitgebers. The authors test whether food availability affects reproductive cycles of free-living East African stonechats. The authors offered supplemental food to the birds 2 months before the regular onset of the breeding season. Supplementally fed males started to sing and display earlier than males of control pairs that did not receive extra food. Although the supplemented food advanced the onset of the breeding season in the pairs that were fed, the onset of the postnuptial molt following the breeding season was not correspondingly shifted. Furthermore, in the year following the experiment, all pairs initiated breeding at the same time. The authors conclude that food availability does not act as a zeitgeber, but rather as a factor that modifies the timing of reproduction without affecting the underlying rhythmicity. The authors propose that this is adaptive under environmental conditions that are relatively constant within a given year but may vary considerably between years. The zeitgeber synchronizing the endogenous rhythmicity remains to be identified.     

A tropical bird can use the equatorial change in sunrise and sunset times to synchronize its circannual clock

At higher latitudes, most organisms use the periodic changes in day length to time their annual life cycle. At the equator, changes in day length are minimal, and it is unknown which cues organisms use to synchronize their underlying circannual rhythms to environmental conditions. Here, we demonstrate that the African stonechat (Saxicola torquatus axillaris)-an equatorial songbird-can use subtle solar cues for the annual timing of postnuptial moult, a reliable marker of the circannual cycle. We compared four groups that were kept over more than 3 years: (i) a control group maintained under constant equatorial day length, (ii) a 12-month solar time group maintained under equatorial day length, but including a simulation of the annual periodic change in sunrise and sunset times (solar time), (iii) a 14-month solar time group similar to the previous group but with an extended solar time cycle and (iv) a group maintained under a European temperate photoperiod. Within all 3 years, 12-month solar time birds were significantly more synchronized than controls and 14-month solar time birds. Furthermore, the moult of 12-month solar time birds occurred during the same time of the year as that of free-living Kenyan conspecifics. Thus, our data indicate that stonechats may use the subtle periodic pattern of sunrise and sunset at the equator to synchronize their circannual clock.     

A phase response curve for circannual rhythm in the varied carpet beetle <Emphasis Type="Italic">Anthrenus verbasci</Emphasis>

We know that entrainment, a stable phase relationship with an environmental cycle, must be established for a biological clock to function properly. Phase response curves (PRCs), which are plots of phase shifts that result as a function of the phase of a stimulus, have been created to examine the mode of entrainment. In circadian rhythms, single-light pulse PRCs have been obtained by giving a light pulse to various phases of a free-running rhythm under continuous darkness. This successfully explains the entrainment to light-dark cycles. Some organisms show circannual rhythms. In some of these, changes in photoperiod entrain the circannual rhythms. However, no single-pulse PRCs have been created. Here we show the PRC to a long-day pulse superimposed for 4 weeks over constant short days in the circannual pupation rhythm in the varied carpet beetle Anthrenus verbasci. Because the shape of that PRC closely resembles that of the Type 0 PRC with large phase shifts in circadian rhythms, we suggest that an oscillator having a common feature in the phase response with the circadian clock, produces a circannual rhythm.     

Seasonality in a temperate zone bird can be entrained by near equatorial photoperiods

Birds use photoperiod to control the time of breeding and moult. However, it is unclear whether responses are dependent on absolute photoperiod, the direction and rate of change in photoperiod, or if photoperiod entrains a circannual clock. If starlings (Sturnus vulgaris) are kept on a constant photoperiod of 12h light:12h darkness per day (12L:12D), then they can show repeated cycles of gonadal maturation, regression and moult, which is evidence for a circannual clock. In this study, starlings kept on constant 11.5L:12.5D for 4 years or 12.5L:11.5D for 3 years showed no circannual cycles in gonadal maturation or moult. So, if there is a circannual clock, it is overridden by a modest deviation in photoperiod from 12L:12D. The responses to 11.5L:12.5D and 12.5L:11.5D were very different, the former perceived as a short photoperiod (birds were photosensitive for most of the time) and the latter as a long photoperiod (birds remained permanently photorefractory). Starlings were then kept on a schedule which ranged from 11.5L:12.5D in mid-winter to 12.5L:11.5D in mid-summer (simulating the annual cycle at 9 degrees N) for 3 years. These birds entrained precisely to calendar time and changes in testicular size and moult were similar to those of birds under a simulated cycle at 52 degrees N. These data show that birds are very sensitive to changes in photoperiod but that they do not simply respond to absolute photoperiod nor can they rely on a circannual clock. Instead, birds appear to respond to the shape of the annual change in photoperiod. This proximate control could operate from near equatorial latitudes and would account for similar seasonal timing in individuals of a species over a wide range of latitudes.     

Mechanisms controling the timing of spring migration in birds

The question “Which factors govern the timing of migration in birds?” has fascinated researchers for a long time. It was initially assumed that avian migration is triggered by environmental factors, such as ambient temperature and food availability. Later laboratory experiments in various avian species convincingly showed that timing of spring migration is mainly governed by daylength (photoperiod) and is controlled by circannual rhythms. As a result, the concept that environmental factors (air temperature, precipitation, food availability) have no significant impact on timing of spring migration generally took hold. However, in recent decades more and more data has become available showing that the timing of spring migration in many bird species has significantly changed. These data allow the formulation of a novel concept of regulation mechanisms of timing of spring migration which accounts not only for photoperiodic and endogenous control, but also for the already mentioned extrinsic factors. Studies of endocrine control of spring migratory disposition showed that features of endocrine mechanisms governing the onset of spring migration depend on speciesspecific migratory strategies and the stability of environmental conditions in winter quarters and on migratory routes. It is becoming clear precisely which endocrine mechanisms are involved in adjusting migratory behaviour to variation of the local environment. In recent years, progress has also been made in finding genetic mechanisms controlling the timing of spring migration.     

Complex bird clocks.

The circadian pacemaking system of birds comprises three major components: (i) the pineal gland, which rhythmically synthesizes and secretes melatonin; (ii) a hypothalamic region, possibly equivalent to the mammalian suprachiasmatic nuclei; and (iii) the retinae of the eyes. These components jointly interact, stabilize and amplify each other to produce a highly self-sustained circadian output. Their relative contribution to overt rhythmicity appears to differ between species and the system may change its properties even within an individual depending, for example, on its state in the annual cycle or its photic environment. Changes in pacemaker properties are partly mediated by changes in certain features of the pineal melatonin rhythm. It is proposed that this variability is functionally important, for instance, for enabling high-Arctic birds to retain synchronized circadian rhythms during the low-amplitude zeitgeber conditions in midsummer or for allowing birds to adjust quickly their circadian system to changing environmental conditions during migratory seasons. The pineal melatonin rhythm, apart from being involved in generating the avian pacemaking oscillation, is also capable of retaining day length information after isolation from the animal. Hence, it appears to participate in photoperiodic after-effects. Our results suggest that complex circadian clocks have evolved to help birds cope with complex environments.     

Seasonal differences in behavior patterns of the migratory white-throated sparrow

Avian migration and associated changes in behavior patterns occur on a seasonal basis and are regulated by both circadian and circannual clocks. It is well known that seasonally migrating birds must alter the temporal partitioning of behaviors in order to successfully complete nocturnal migration, although the precise changes in allocation have not previously been examined. Because of the difficulties related to the study of bird migration in the wild, many studies regarding the physiology of migration have been performed in captivity, where nocturnal migratory restlessness is readily observable at the appropriate times of the year. Even so, quantifying the amount and timing of daily behavioral rhythms in migrating and non-migrating individuals has not previously been performed. We used video recordings of a common nocturnal migrant, the white-throated sparrow (Zonotrichia albicollis), to generate a basic ethogram and to chart the timing and frequency of occurrence of each behavior in order to compare migrant individuals with non-migrating birds. Our study suggests that the specific allocation of behaviors are altered during migration and that these changes might contribute to the development of the “migratory syndrome”.     

Control of annual gonadal cycles in Indian songbirds

Abstract

Reproduction is a part of life cycle with great environmental dependence. In contrast to temperate avian species, which mostly breed during summer, the Indian songbirds have more flexible breeding programs and exhibit a spectrum of reproductive strategies with the breeding season scattered all over the year. Control of breeding cycles in the Indian songbirds, therefore, are broadly viewed in light of two strategies (i) birds showing strong photoperiodic component in regulation of reproductive and post-reproductive events (ii) birds that do not exhibit typical photoperiodic regulation indicating the involvement of an inherent rhythm of reproduction. Both circadian and circannual rhythms have been demonstrated to regulate annual gonadal cycles of Indian songbirds. While photoperiod continues to be a predominant proximate factor for timing of breeding in majority of Indian songbirds investigated so far, some studies reveal the role of non photoperiodic cues such as the food availability, temperature, rainfall, etc. in timing/modulating the timing of breeding. The conversion or non-conversion of thyroxine to triiodothyronine may act as a long or short photoperiod signal and may up or downregulate the synthesis and release of GnRH-I in hypothalamus, FSH and LH in anterior pituitary and gonadal steroids in gonads causing gonadal growth or regression, respectively.     

Daily increments of light hours near vernal equinox synchronize circannual testicular cycle of tropical spotted munia

In some long-lived organisms, particularly in tropical birds and migrants that spend part of the year close to the equator, endogenous circannual rhythms have been demonstrated in seasonal events like reproduction, molt, and migration. These, like the circadian rhythms, are expressed only in constant conditions of illumination with a periodicity deviating from 1 yr. If birds followed this periodicity, they would soon be out of phase with the annual calendar and perish and, therefore, they would need to be synchronized. However, almost nothing is known as to how synchronization is achieved in birds. Herein, with the help of a suitable model, viz., the tropical spotted munia and long-term experiments conducted in series over a 5-yr period, we provide direct evidence for the first time indicating that the segment of annual photocycle with maximal rate of increase prior to vernal equinox (approximately between mid-February and mid-March) synchronizes the circannual reproductive cycle with the monsoon period of ample food supply through a phase delay. Data also indicate, contrary to the prevalent view, that birds in the tropics can perceive minor changes in day-length, that birds respond to progressive changes in day-length as distinct from responding to fixed photoperiods of particular durations, and that birds can actually distinguish the quality of the environmental signal, viz., vernal equinox from early spring, or increasing days of spring from decreasing days of autumn. The underlying mechanisms, although yet to be identified, appear to involve a gonado-inhibitory rather than the conventional gonado-stimulatory response to increasing day-length. The photoperiodic responses of spotted munia are distinctly different from that of any "long-day" birds described thus far and do not conform to the prevalent "circadian coincidence" hypothesis of photoperiodism.     

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

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 (approximately 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 degrees N and those immediately arrived from their breeding grounds (approximately 40 degrees 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 (initiation-termination-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.     

Daily Increments of Light Hours Near Vernal Equinox Synchronize Circannual Testicular Cycle of Tropical Spotted Munia

In some long-lived organisms, particularly in tropical birds and migrants that spend part of the year close to the equator, endogenous circannual rhythms have been demonstrated in seasonal events like reproduction, molt, and migration. These, like the circadian rhythms, are expressed only in constant conditions of illumination with a periodicity deviating from 1 yr. If birds followed this periodicity, they would soon be out of phase with the annual calendar and perish and, therefore, they would need to be synchronized. However, almost nothing is known as to how synchronization is achieved in birds. Herein, with the help of a suitable model, viz., the tropical spotted munia and long-term experiments conducted in series over a 5-yr period, we provide direct evidence for the first time indicating that the segment of annual photocycle with maximal rate of increase prior to vernal equinox (approximately between mid-February and mid-March) synchronizes the circannual reproductive cycle with the monsoon period of ample food supply through a phase delay. Data also indicate, contrary to the prevalent view, that birds in the tropics can perceive minor changes in day-length, that birds respond to progressive changes in day-length as distinct from responding to fixed photoperiods of particular durations, and that birds can actually distinguish the quality of the environmental signal, viz., vernal equinox from early spring, or increasing days of spring from decreasing days of autumn. The underlying mechanisms, although yet to be identified, appear to involve a gonado-inhibitory rather than the conventional gonado-stimulatory response to increasing day-length. The photoperiodic responses of spotted munia are distinctly different from that of any “long-day” birds described thus far and do not conform to the prevalent “circadian coincidence” hypothesis of photoperiodism.     

Daily Increments of Light Hours Near Vernal Equinox Synchronize Circannual Testicular Cycle of Tropical Spotted Munia

In some long-lived organisms, particularly in tropical birds and migrants that spend part of the year close to the equator, endogenous circannual rhythms have been demonstrated in seasonal events like reproduction, molt, and migration. These, like the circadian rhythms, are expressed only in constant conditions of illumination with a periodicity deviating from 1 yr. If birds followed this periodicity, they would soon be out of phase with the annual calendar and perish and, therefore, they would need to be synchronized. However, almost nothing is known as to how synchronization is achieved in birds. Herein, with the help of a suitable model, viz., the tropical spotted munia and long-term experiments conducted in series over a 5-yr period, we provide direct evidence for the first time indicating that the segment of annual photocycle with maximal rate of increase prior to vernal equinox (approximately between mid-February and mid-March) synchronizes the circannual reproductive cycle with the monsoon period of ample food supply through a phase delay. Data also indicate, contrary to the prevalent view, that birds in the tropics can perceive minor changes in day-length, that birds respond to progressive changes in day-length as distinct from responding to fixed photoperiods of particular durations, and that birds can actually distinguish the quality of the environmental signal, viz., vernal equinox from early spring, or increasing days of spring from decreasing days of autumn. The underlying mechanisms, although yet to be identified, appear to involve a gonado-inhibitory rather than the conventional gonado-stimulatory response to increasing day-length. The photoperiodic responses of spotted munia are distinctly different from that of any “long-day” birds described thus far and do not conform to the prevalent “circadian coincidence” hypothesis of photoperiodism.     

Conquering the night: understanding nocturnal migration in birds

Abstract

Migration is a biologically distinct and unique phenomenon that enables the birds to migrate twice-a-year between the breeding and wintering grounds. These movements are known as spring and autumn migration, respectively. Depending on their inherent programming, the migratory birds may fly during day or night or both. Different environmental factors such as, temperature, food, predator pressure and physiological demands of energy storage and expenditure, contribute to the pattern of migrations, day or nighttime. Since, most of them are nighttime migrants they have to make dramatic changes in their physiology and behavior to transform them from being diurnal to predominantly nocturnal. These changes result in different life history stages (LHSs) such as migration, reproduction and molt, in their annual cycle, which are regulated by endogenous circadian and circannual clocks. As a result, the birds start preparing well in advance for the approaching LHS. The present review focuses on behavioral strategies of a nocturnal migrant and understanding of the possible physiological responses to ensure successful migration.     

Persistence of a circannual rhythm of plasma prolactin concentrations in ewes exposed to a constant equatorial photoperiod

Circulating concentrations of prolactin were monitored for 3 yr in intact ewes kept either outdoors or indoors in a fixed equatorial photoperiod (12L:12D) and restricted range of environmental temperatures. Prolactin data were analyzed by spectral analysis. In all ewes kept outdoors, concentrations of prolactin showed robust circannual rhythms with a single predominant period of 359 days. In ewes kept indoors, the range of significant periods varied from 35 to 532 days. Although all ewes kept indoors showed a significant rhythm with a period of 354 days, this clearly was not the predominant period in all. The amplitude of the rhythm in ewes kept indoors was significantly lower (p less than 0.01) than that of ewes kept outdoors. Although the annual rhythm of circulating prolactin typical of ewes kept outdoors was significantly compromised in animals kept under a constant 12L:12D photoperiod and restricted environmental temperature range, there was evidence of an endogenous circannual rhythm.