Avian migration: temporal multitasking and a case study of melatonin cycles in waders.

Timing "in the real world" must cope with the temporal complexity of natural environments. Extreme examples for the resultant "multitasking" are migratory birds, which precisely time movements to remote areas. New field technologies highlight temporal accuracy, while captivity studies emphasize underlying programs and plasticity of schedules. After reviewing these findings, we focus on waders, which undertake spectacular long-distance migrations, have robust circannual clocks, and cope with diel, tidal, and polar environments. To explore features that may facilitate such multitasking, we speculated that melatonin amplitudes are low and damped during seasons when entrainment to subtle Zeitgebers occurs. We measured melatonin profiles under European daylength in two species with different ecologies and found low-amplitude melatonin cycles that changed over the year. Annual patterns neither fully supported our hypothesis, nor simply reflected daylight availability. While migratory birds are inspiring models for chronobiology, mechanistic understanding of their multitasking is still poor.

Persistent diel melatonin rhythmicity during the Arctic summer in free-living willow warblers.

Arctic environments are challenging for circadian systems. Around the solstices, the most important zeitgeber, the change between night and day, is reduced to minor fluctuations in light intensities. However, many species including songbirds nonetheless show clear diel activity patterns. Here we examine the possible physiological basis underlying diel rhythmicity under continuous Arctic summer light. Rhythmic secretion of the hormone melatonin constitutes an important part of the songbird circadian system and its experimental suppression, e.g., by constant light, usually leads to behavioral arrhythmia. We therefore studied melatonin patterns in a free-living migratory songbird, the willow warbler (Phylloscopus trochilus), that maintains diel activity during the Arctic summer. We compared melatonin profiles during late spring and summer solstice in two Swedish populations from the south (58 degrees N) and near the Arctic circle (66 degrees N). We found the northern Swedish population maintained clear diel changes in melatonin secretion during the summer solstice, although peak concentrations were lower than in southern Sweden. Melatonin levels were highest before midnight and in good accordance with periods of reduced activity. The maintenance of diel melatonin rhythmicity under conditions of continuous light may be one of the physiological mechanisms that enables continued functioning of the circadian system.

Circannual basis of geographically distinct bird schedules.

To anticipate seasonal change, organisms schedule their annual activities by using calendrical cues like photoperiod. The use of cues must be fitted to local conditions because schedules differ between species and habitats. In complete absence of temporal information, many species show persistent circannual cycles that are synchronised, but not driven, by photoperiod. The contribution of circannual rhythms to timing under natural photoperiodic conditions is still unclear. In a suite of experiments, we examined timing in two closely related songbirds (Siberian and European stonechats) that inhabit similar latitudes but differ in seasonal behaviour. Under a more continental climate, Siberian stonechats breed later, moult faster and migrate further than European stonechats. We tested hypotheses for seasonal timing mechanisms by comparing the birds under constant and naturally changing daylengths. The taxa retained characteristic reproductive and moult schedules and hybrids behaved roughly intermediately. Based on their distinct circannual cycles, we expected European and Siberian stonechats to differ in photoperiodic responses at a given time of year. We found that the taxa responded, as predicted, in opposite ways to photoperiodic simulations as experienced on different migration routes. The findings indicate that circannual rhythms reflect geographically distinct periodic changes in seasonal disposition and cue-response mechanisms. Under natural daylengths, the phase relationship of the underlying circannual rhythm to the external year determines the action of photoperiod. Circannual rhythms are widespread among long-lived species. Accordingly, responses to environmental change, range expansion and novel migration patterns may depend on the particulars of a species' underlying circannual programming.

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.

Circadian behavioral and melatonin rhythms in the European starling under light-dark cycles with steadily changing periods: evidence for close mutual coupling?

In European starlings exposed to constant conditions, circadian rhythms in locomotion and feeding can occasionally exhibit complete dissociation from each other. Whether such occasional dissociation between two behavioral rhythms reflects on the strength of the mutual coupling of their internal oscillators has not been investigated. To examine this, as well as to elucidate the role of melatonin in this system, we simultaneously measured the rhythms of locomotion, feeding and melatonin secretion in starlings exposed to light-dark (LD) cycles of low intensity with steadily changing periods (T). In birds initially entrained to T 24 LD cycles (12L:12D, 10:0.2 lx), beginning on day 15, T was either lengthened to 26.5 h (experiment 1) or shortened to T 21.5 h (experiment 2) by changing the daily dark period 4 min each day. After 18 and 19 cycles of T 26.5 and T 21.5, respectively, birds were released into constant dim light conditions (LL(dim); 0.2 lx) for about 2 weeks. Locomotor and feeding rhythms were continuously recorded. Plasma melatonin levels were measured at three times: in T 24, when T equaled 26 or 22 h and at the end of T 26.5 or T 21.5 exposure. The results show that, contrary to our expectations, the three rhythms were not dissociated. Rather they remained synchronized and changed their phase angle difference with the light zeitgeber concomitantly and at the same rate. The melatonin rhythm stayed in synchrony with the behavioral rhythms and as a consequence, peaked either during day or at night, depending on the phase relationship between the activity rhythm and the zeitgeber cycle.

Increased sedentariness in European Blackbirds following urbanization: a consequence of local adaptation?

Urbanization changes local environmental conditions and may lead to altered selection regimes for life history traits of organisms thriving in cities. Previous studies have reported changes in breeding phenology and even trends toward increased sedentariness in migratory bird species colonizing urban areas. However, does the change in migratory propensity simply represent a phenotypic adjustment to local urban environment, or is it genetically based and hence the result of local adaptation? To test this, we hand-raised European Blackbirds (Turdus merula) from urban and forest populations, quantified their nocturnal activity and fat deposition covering two complete migratory cycles and examined the consequences of a reduced migratory propensity for the timing of gonadal development (a physiological measure of the seasonal timing of reproduction). Although nocturnal activities differed strikingly between fall and spring seasons, with low activities during the fall and high activities during the spring seasons, our data confirm, even in birds kept from early life under common-garden conditions, a change toward reduced migratoriness in urban blackbirds. The first score of a principal component analysis including amount of nocturnal activity and fat deposition, defined as migratory disposition, was lower in urban than in forest males particularly during their first year, whereas females did not differ. The results suggest that the intrinsic but male-biased difference is genetically determined, although early developmental effects cannot be excluded. Moreover, individuals with low migratory disposition developed their gonads earlier, resulting in longer reproductive seasons. Since urban conditions facilitate earlier breeding, intrinsic shifts to sedentariness thus seem to be adaptive in urban habitats. These results corroborate the idea that urbanization has evolutionary consequences for life history traits such as migratory behavior.

Daytime light intensity affects seasonal timing via changes in the nocturnal melatonin levels.

Daytime light intensity can affect the photoperiodic regulation of the reproductive cycle in birds. The actual way by which light intensity information is transduced is, however, unknown. We postulate that transduction of the light intensity information is mediated by changes in the pattern of melatonin secretion. This study, therefore, investigated the effects of high and low daytime light intensities on the daily melatonin rhythm of Afro-tropical stonechats (Saxicola torquata axillaris) in which seasonal changes in daytime light intensity act as a zeitgeber of the circannual rhythms controlling annual reproduction and molt. Stonechats were subjected to light conditions simulated as closely as possible to native conditions near the equator. Photoperiod was held constant at 12.25 h of light and 11.75 h of darkness per day. At intervals of 2.5 to 3.5 weeks, daytime light intensity was changed from bright (12,000 lux at one and 2,000 lux at the other perch) to dim (1,600 lux at one and 250 lux at the other perch) and back to the original bright light. Daily plasma melatonin profiles showed that they were linked with changes in daytime light intensity: Nighttime peak and total nocturnal levels were altered when transitions between light conditions were made, and these changes were significant when light intensity was changed from dim to bright. We suggest that daytime light intensity could affect seasonal timing via changes in melatonin profiles.

Testosterone and corticosterone during the breeding cycle of equatorial and European stonechats (Saxicola torquata axillaris and S. t. rubicola).

Northern-temperate male birds show seasonal changes in testosterone concentrations with a peak during the breeding season. Many tropical birds express much lower concentrations of testosterone with slight elevations during breeding. Here we describe testosterone and corticosterone concentrations of male stonechats from equatorial Kenya during different substages of breeding and molt. This tropical species has a short breeding season of approximately 3 months. We compare their hormone concentrations to previously published data of males of a northern-temperate relative, the European stonechat, also a seasonal breeder but with a breeding season of approximately 5 months. Equatorial stonechats show a pronounced peak of testosterone during the nest-building and laying stage. During all other stages, testosterone concentrations are low, similar to other year-round territorial tropical bird species. Corticosterone concentrations peak also during the nest-building and laying stage suggesting that this period of maximum female fecundity is a demanding period for the male. Equatorial stonechats have significantly lower concentrations of testosterone than European stonechats during all stages, except during the nest-building and laying stage. During this stage of maximum female fertility, testosterone levels tend to be higher in equatorial than in European stonechats. Our results suggest that equatorial stonechats belong to a group of tropical bird species that are characterized by a short breeding season and a brief high peak of testosterone during the female's fertile period. Such brief, but substantial peaks of testosterone may be common in tropical birds, but they may easily be missed if the exact breeding stage of individual birds is not known.

Stress and the city: urbanization and its effects on the stress physiology in European blackbirds.

Animals colonizing cities are exposed to many novel and potentially stressful situations. There is evidence that chronic stress can cause deleterious effects. Hence, wild animals would suffer from city life unless they adjusted their stress response to the conditions in a city. Here we show that European Blackbirds born in a city have a lower stress response than their forest conspecifics. We hand-raised urban and forest-living individuals of that species under identical conditions and tested their corticosterone stress response at an age of 5, 8, and 11 months. The results suggest that the difference is genetically determined, although early developmental effects cannot be excluded. Either way, the results support the idea that urbanization creates a shift in coping styles by changing the stress physiology of animals. The reduced stress response could be ubiquitous and, presumably, necessary for all animals that thrive in ecosystems exposed to frequent anthropogenic disturbances, such as those in urban areas.

Migratory restlessness in an equatorial nonmigratory bird.

The urge of captive birds to migrate manifests itself in seasonally occurring restlessness, termed "Zugunruhe." Key insights into migration and an endogenous basis of behavior are based on Zugunruhe of migrants but have scarcely been tested in nonmigratory birds. We recorded Zugunruhe of African stonechats, small passerine birds that defend year-round territories and have diverged from northern migrants at least 1 million years ago. We demonstrate that Zugunruhe is a regular feature of their endogenous program, one that is precisely timed by photoperiod. These results extend ideas of programs for periodic movement to include nonmigratory birds. Such programs could be activated when movements become necessary, in line with observed fast changes and high flexibility of migration. Attention to Zugunruhe of resident birds promises new insights into diverse and dynamic migration systems and enhances predictions of avian responses to global change.

A separate circadian oscillator controls nocturnal migratory restlessness in the songbird Sylvia borin.

When confined to a cage, migratory songbirds exhibit nocturnal migratory restlessness (also called Zugunruhe) during the spring and autumn migratory periods, even though these birds are exclusively diurnal during the remainder of the year. Zugunruhe, which has been demonstrated to be under the direct control of a circannual timer, is characterized by a stereotypic "wing-whirring" behavior while the bird is perched. To elucidate the role played by the circadian system in the regulation of Zugunruhe, the authors studied the activity of garden warblers (Sylvia borin), long-distance nocturnal migrants, under skeleton photoperiods of different lengths and under constant dim light. In 11.5D:1L:10.5D:1L skeleton photoperiods, the authors found that Zugunruhe free-ran in a substantial proportion of birds, while their normal daily activities (e.g., feeding and preening) remained synchronized to 24 h. Some birds expressing Zugunruhe under constant dim light continued to show 2 distinct bouts of activity: one corresponding to daily activities, the other to wing-whirring. In some cases, these 2 bouts crossed while free-running with different periods. Birds expressing Zugunruhe also had significantly longer free-running periods than birds that did not. The study data suggest that the seasonal appearance of Zugunruhe is the result of the interactions of at least 2 circadian oscillators and that it is the phase relationship of these 2 oscillators that determines when nocturnal migratory restlessness is expressed. Furthermore, these data are consistent with the previously proposed internal coincidence hypothesis as a model for the ontogeny of circannual rhythms.

Pinealectomy shortens resynchronisation times of house sparrow (Passer domesticus) circadian rhythms.

In many birds periodic melatonin secretion by the pineal organ is essential for the high-amplitude self-sustained output of the circadian pacemaker, and thus for the persistence of rhythmicity in 24 h oscillations controlled by it. The elimination of the pineal melatonin rhythm, or a reduction of its amplitude, renders the circadian pacemaker a less self-sustained, often highly damped, oscillatory system. A reduction in the degree of self-sustainment of a rhythm should not only increase its range of entrainment but also shorten the resynchronization times following phase-shifts of the zeitgeber. This hypothesis has not yet been directly tested. We therefore carried out the present study in which house sparrows (Passer domesticus) were subjected to both 6-h advance and 6-h delay phase-shifts of the light-dark cycle before and after the pinealectomy, and the rhythms in locomotion and feeding were recorded. The results indicate that following the delay, but not the advance, phase shift, resynchronization times were significantly shorter after pinealectomy. The dependence of resynchronization times on the presence or absence of the pineal organ is not only of theoretical interest but might also be of functional significance in the natural life of birds. A reduction or elimination of the amplitude of the melatonin secretion rhythm by the pineal organ might be responsible for faster adjustment to changes in zeitgeber conditions in nature.

Flexible seasonal timing and migratory behavior: results from stonechat breeding programs.

Rigid schedules of long-distance migrants could be among candidate traits for adaptive migratory syndromes. This prediction was tested on stonechats, passerines that differ widely in migratory behavior and seasonal schedules. Stonechats in Europe are short-distance migrants and multiclutched, whereas African residents and Siberian long-distance migrants usually raise single broods. In captivity, all subspecies displayed endogenous cycles of reproductive development and molt. The subspecies differed in time afforded to life cycle stages. Under conducive aviary conditions, African stonechats were multibrooded, whereas Siberian stonechats did not add clutches. This difference in flexibility was exclusively related to the length of breeding windows. Stonechats also differed in premigratory preparations. Postjuvenile molt started early in Siberian stonechats, but in European and African stonechats, depended strongly on hatching date. In contrast, all subspecies shortened molt duration at the same rate when hatched from late broods. Plasticity of Zugunruhe timing was identical in Siberian and European subspecies and nearly compensated for hatching late. The stonechat data suggest a refined understanding of temporal plasticity in long-distance migrants. Overall, plasticity was not reduced, but was differently organized. Apparently rigid migrant schedules were related to short breeding cycles and inflexible molt onset. Short windows for breeding and juvenile development could provide safety measures for timely departure. Once molt was initiated, temporal plasticity of long-distance migrants matched that of less migratory conspecifics. In addition to adjusting endogenous programs, stonechats differed in implementing them in the field. Modifying the conditions under which programs are expressed may be an efficient way to enhance seasonal plasticity.

Melatonin and nocturnal migration.

Many species of diurnal birds migrate nocturnally. Here, a series of studies of the blackcap (Sylvia atricapilla) on the relationship between nocturnal restlessness and melatonin, a hormone that in birds modulates day-night rhythms, are reviewed. Migratory populations from Sweden and Kenya were compared with resident populations from Cape Verde. In blackcaps of migratory populations, night levels of melatonin were lower during the migratory period, when birds showed nocturnal activity, than before and after this period, when birds did not show nocturnal activity. On the contrary, the occurrence of periodic or irregular phases of nocturnal activity in some nonmigratory birds from Cape Verde was not accompanied by a reduction in melatonin levels. In a second series of experiments, it was studied whether melatonin levels change when nocturnally active blackcaps are experimentally transferred from a migratory to a nonmigratory state. A long migratory flight and a refueling stopover were simulated by depriving birds of food for 2 days, subsequently readministering food. The experiments were done in autumn with birds collected in Sweden, and repeated in spring with birds collected in Kenya. In autumn, there was a suppression of nocturnal activity and an increase in melatonin in the night following food reintroduction. In spring, the effects were qualitatively similar, but their extent depended on the amount of body fat reserves. Taken together, the studies demonstrate the existence of a functional relationship between melatonin and nocturnal restlessness and of seasonal differences in the response of the migratory program to food availability.

Testosterone, reproductive stage, and territorial behavior of male and female European stonechats Saxicola torquata.

We investigated territorial behavior and circulating testosterone (T) levels in a multiple-brooded population of the European stonechat, a socially monogamous passerine bird with biparental care. Between arrival at and departure from the breeding territories, we (1) quantified behavior of both sexes in response to a simulated territorial intrusion (STI) of a male conspecific and (2) measured plasma T concentrations in males and females. Male response scores to a STI and male T concentrations varied with phase, but there was no temporal association between plasma T levels and the intensity of territorial behavior. During both two sexual and two parental phases, at least half of the tested males showed aggressive responses. About 20% of the tested males responded with courtship prior to laying of the first clutch, but none of the males courted during later phases. Age had a positive overall effect on male plasma T. Females also reacted to the STI of a male, but their responses did not vary with breeding phase. Female plasma T varied with phase, being elevated during production of the first but not of the second clutch. As with males, female responses to the STI were not correlated with T levels. Responses of pair partners were positively correlated with each other. We conclude that modulation of male territorial aggression with breeding phase is not regulated by changes of plasma T concentrations. In light of other studies showing reduced male aggression by pharmacological inhibition of cellular actions of T, we propose that T is permissive for male territorial aggression, but does not mediate short-term changes associated with breeding phase. The function of the high female plasma T concentrations during formation of first clutches could be related to the production of eggs with high concentrations of androgens.

The hormonal response of female European Stonechats to a territorial intrusion: the role of the male partner.

In many bird species, the female participates in defending a pair's breeding territory, however, the endocrine control mechanism of female aggressive behavior is largely unknown. The general statement that androgens are involved in the regulation of aggressive behavior is based on studies conducted only in males. Here, we tested whether paired female stonechats show a hormonal response to a simulated male territorial intruder. Since in males of territorial bird species androgen levels usually increase following a male-male encounter, we measured androgen-levels before and after a simulated male intrusion. In addition, we measured estradiol, the main gonadal hormone in females, and corticosterone, a stress hormone. The results show that a male intruder does not affect any of the measured hormones in females. In a second experiment, we also tested whether the endocrine state of the male partner affects the hormonal response of females to a male intruder by comparing the hormonal response of females paired with pharmacologically castrated males and females paired with control males. Females paired with pharmacologically castrated males had lower corticosterone levels both before and after the intrusion than females paired with control males. Additionally, in both groups, female corticosterone levels were increased following a male intrusion. We suggest that the differences found between females paired with pharmacologically castrated males and females paired with control males are due to differences in intra-pair interactions.

Convergent evolution of strigiform and caprimulgiform dark-activity is supported by phylogenetic analysis using the arylalkylamine N-acetyltransferase (Aanat) gene.

Alternative hypotheses propose the sister order of owls (Strigiformes) to be either day-active raptors (Falconiformes) or dark-active nightjars and allies (Caprimulgiformes). In an effort to identify molecular characters distinguishing between these hypotheses we examined a gene, arylalkylamine N-acetyltransferase (Aanat), potentially associated with the evolution of avian dark-activity. Partial Aanat coding sequences, and two introns, were obtained from the genomic DNA of 16 species: Strigiformes (four species), Falconiformes (four species), Caprimulgiformes (five species), with outgroups: Ciconiiformes (one species), Passeriformes (one species), and Apterygiformes (one species). Phylogenetic trees derived from aligned, evolutionarily conserved Aanat regions did not consistently recover clades corresponding to orders Strigiformes and Falconiformes but did place a caprimulgiform clade more distant from the strigiform and falconiform species than the latter two groups are to each other. This finding was supported by spectral analysis. The taxonomic distribution of seven intronic indels is consistent with the Aanat derived phylogenetic trees and supports conventional family-level groupings within both Strigiformes and Caprimulgiformes. The phylogenetic analyses also indicate that Caprimulgiformes is a polyphyletic grouping. In conclusion the data support, but do not conclusively prove, the proposal that Falconiformes is the sister order to Strigiformes and therefore, that the dark-activity characteristic of Strigiformes and Caprimulgiformes arose by convergent evolution.

Excremental androgen metabolite concentrations and gonad sizes in temperate zone vs. tropical Stonechats (Saxicola torquata ssp.).

Many songbirds in the temperate zones have comparatively short breeding seasons and are migratory. This often implies high breeding synchrony and competition for food, space, and mates. Tropical birds, in contrast, tend to be resident and react more flexibly to a more variable onset of environmental conditions conducive for breeding. Therefore, it can be expected that androgen metabolite concentrations in breeding males vary among birds from different latitudes. Studies on tropical birds, however, have revealed inconsistent results suggesting that additional factors not directly related to latitude, modify hormone levels. Furthermore, there is a potential for a phylogenetic bias when comparing birds of different taxonomic groups. In the present study, we compared androgen metabolite concentrations among closely related taxa of hand-raised male stonechats originating from tropical and temperate-zone populations in Kenya (0 degrees latitude), Kazakhstan (52 degrees N), and Ireland (52 degrees N), which differ partly in latitude of their provenance but also in the length of their breeding season (birds from Kazakhstan vs. Ireland). Hand-raised birds were housed in their natural photoperiod under otherwise identical environmental conditions. Androgen levels were determined from excrements to reduce disturbance. All three groups demonstrated a seasonal cycle of androgen metabolite concentrations concomitant with testis growth. Peak androgen metabolite concentrations were significantly lower in Kenyan stonechats compared to Kazakh stonechats. Irish birds had intermediate concentrations. Differences between Kazakh and Kenyan stonechats correlate with latitude, but data from the Irish population suggest the involvement of factors not directly related to latitude.

Differences in the timing of reproduction between urban and forest European blackbirds (Turdus merula): result of phenotypic flexibility or genetic differences?

Species which have settled in urban environments are exposed to different conditions from their wild conspecifics. A previous comparative study of an urban and a forest-living European blackbird population had revealed a three weeks earlier onset of gonadal growth in urban individuals. These physiological adjustments are either the result of genetic differences that have evolved during the urbanization process, or of phenotypic flexibility resulting from the bird's exposure to the different environmental conditions of town or forest. To identify which of these two mechanisms causes the differences in reproductive timing, hand-reared birds originating from the urban and the forest populations were kept in identical conditions. The substantial differences in the timing of reproduction between urban and forest birds known from the field did not persist under laboratory conditions, indicating that temporal differences in reproductive timing between these two populations are mainly a result of phenotypic flexibility. Nevertheless, urban males initiated plasma luteinizing hormone (LH) secretion and testicular development earlier than forest males in their first reproductive season. Moreover, plasma LH concentration and follicle size declined earlier in urban females than in forest females, suggesting that genetic differences are also involved and might contribute to the variations in the timing of reproduction in the wild.

Simulation of migratory flight and stopover affects night levels of melatonin in a nocturnal migrant.

Several species of diurnal birds are nocturnal migrants. The activation of nocturnal activity requires major physiological changes, which are essentially unknown. Previous work has shown that during migratory periods nocturnal migrants have reduced night-time levels of melatonin. Since this hormone is involved in the modulation of day-night rhythms, it is a good candidate regulator of nocturnal migratory activity. We studied whether melatonin levels change when nocturnally active blackcaps (Sylvia atricapilla) are experimentally transferred from a migratory to a non-migratory state. We simulated a long migratory flight by depriving birds of food for 2 days, and a refuelling stopover by subsequently re-administering food. Such a regimen is known to induce a reduction in migratory restlessness ('Zugunruhe') in the night following food reintroduction. The experiments were performed in both autumn and spring using blackcaps taken from their breeding grounds (Sweden) and their wintering areas (Kenya). In autumn, the food regimen induced a suppression of Zugunruhe and an increase in melatonin in the night following food reintroduction. In spring, the effects of the treatment were qualitatively similar but their extent depended on the amount of body-fat reserves. This work shows that the reduction of night-time melatonin during migratory periods is functionally related to nocturnal migration, and that fat reserves influence the response of the migratory programme to food deprivation.