Onset Of Moult Research Articles

When slow explorers are fast: Personality‐related differences in timing of migration in Red Knots (Calidris canutus)

The timing of migration varies significantly among individuals, even within populations sharing breeding sites. Consistent individual behavioural differences, known as personality traits, have been linked to variation in movement behaviour. However, little attention has been given to investigating whether personality traits can explain such variation in the timing of migration. We studied post‐breeding migration of Red Knots Calidris canutus islandica breeding in the High Arctic and wintering in European coastal areas. We captured Red Knots in the Dutch Wadden Sea, a main non‐breeding and moulting site, and assessed their exploration behaviour, a personality trait, before releasing them with colour‐rings. We first investigated whether arrival timing in marine areas was associated with exploration speed. Secondly, we asked whether slow explorers were more likely to fly non‐stop to the Wadden Sea compared with fast explorers, as faster explorers were expected to move more and visit more staging sites. To determine arrival timing in marine non‐breeding areas and non‐stop flights to the Wadden Sea, we analysed isotope signatures in blood samples collected after capture, as these differ between the terrestrial breeding grounds (arthropod diet) and marine non‐breeding and moulting sites (benthic invertebrate diet). Thirdly, we estimated arrival time in the Wadden Sea based on primary moult progress, allowing us to examine the relationship between arrival timing estimated from isotope values and the onset of moult. Our findings revealed that slower exploring Red Knots departed earlier from the breeding site and were more likely to fly non‐stop to the Wadden Sea than were faster exploring individuals. Arrival timing to marine areas as estimated from isotope analyses predicted arrival timing in the Wadden Sea (via moult progress) but this relationship exhibited significant variation, possibly due to individual differences in the use of staging sites en route. By migrating early, slower explorers can better outpace the aerial predators also arriving at the Wadden Sea moulting site and thereby reduce mortality risk due to predation.

The nuclear receptor gene E75 plays a key role in regulating the molting process of the spider mite, Tetranychus urticae.

The nuclear receptor gene Ecdysone-induced protein 75 (E75), as the component of ecdysone response genes in the ecdysone signaling pathway, has important regulatory function for insect molting. However, the regulatory function of E75 during the molting process of spider mites is not yet clear. In this study, the expression pattern of E75 in the molting process of the spider mite Tetranychus urticae was analyzed. The results showed that there was a peak at 8h post-molting, followed by a decline 8h after entering each respective quiescent stage across various developmental stages. During the deutonymph stage, the expression dynamics of E75, observed at 4-h intervals, indicated that the transcript levels of TuE75 peaked at 24h, coinciding with the onset of molting in the mites. To investigate the function of TuE75 during the molting process, silencing TuE75 through dsRNA injection into deutonymph mites at the age of 8h yielded a notable outcome: 78% of the deutonymph mites were unable to progress to the adult stage. Among these phenotypic mites, 37% were incapable of transitioning into the quiescent state and eventually succumbed after a certain period. An additional 41% of the mites successfully entered the quiescent state but encountered difficulties in shedding the old epidermis, leading to eventual mortality. In summary, these results suggested that TuE75 plays a key role in the molting process of T. urticae.

Using banding data to estimate primary feather molt and hatch chronology of white‐winged doves in Texas

AbstractBanding data are commonly used to estimate vital rates for migratory game bird management. We used white‐winged dove (Zenaida asiatica) banding data to estimate molt and hatch chronology in Texas. We used Texas Parks and Wildlife Department's long‐term, state‐wide banding data (71,675 banded individuals) from 1 June to 15 August 2007–2016 to investigate primary feather molt and hatching in white‐winged doves in Texas. We estimated primary feather molt and used individual recapture data to determine reliability of models predicting primary feather molt rates. For hatching, we used primary feather molt scores of captured hatch‐year doves to backdate to an estimated hatch date. Our modeling predicted mean after‐hatch‐year primary feather molt rate of 13.21 ± 0.93 days. We predicted 95% of adult white‐winged doves began molting between 7 April to 8 July and completed molt between 17 August to 17 November. Across all years, white‐winged doves hatched as early as 6 January and as late as 27 July, with 95% of all hatching occurring between 22 March and 18 June and peaking on 4 May. Primary feather molt initiation peaked 16 days after the peak of hatching, suggesting that white‐winged doves delay the onset of primary molt until reproductive activity slows. Secondary data collected during banding operations on migratory game birds may be used to understand additional life processes without the requirement to initiate additional survey efforts.

An experimental study of trade‐offs between brood size and moult in the blue tit (Cyanistes caeruleus)

Life‐history theory predicts that whenever expensive activities overlap, there will be a trade‐off between the activities. In a field study of Blue TitsCyanistes caeruleusin Norway, we tested whether parental energy expenditure during reproduction affected timing of moult of primary feathers. Five days after hatching, we manipulated broods to contain either two (n = 10 broods) or 9–12 nestlings (n = 13). In contrast to previous studies, we measured stage of moult at two time points during the nestling period (7 and 14 days after hatching). The onset of moult in the male but not in the female was strongly affected by a reduction in brood size. Stage of moult did not differ between the experimental groups 7 days after hatching, but did after a further week. This difference was also evident when considering only males that had started moulting on day 7. Male moult score was not related to male age (yearling or older), body size (wing length, tarsus length) or hatching time (calendar date) of the brood. However, body condition, expressed as residuals of the regression of body mass on wing length, tended to be greater for parents of both sexes attending small broods than large broods. None of 23 females had initiated moult 7 days after hatching, and only two had done so by day 14. Surprisingly, moult did not seem to affect food provisioning by males relative to females. These results confirm previous studies of passerine birds, namely, that males may start moulting before females and that the moult can quickly be adjusted to the demands of brood‐rearing. The novel result in this case was that such demands affected not only the timing of moult onset but also its speed once it had started.

Evolution of some Haematological Parameters during the Process of Spontaneous Moult in Hy-Line Brown Hens

The purpose of this paper was to find out the evolution of the main haematological parameters in Hy-Line var. Brown hens raised in industrial system, during a natural moult process. The process of spontaneous moult started towards the end of the laying cycle (63 weeks of age) and lasted about seven weeks. Two groups were set up for monitoring: a control group of hens that did not show signs of moult and an experimental group of spontaneously moulted hens. Blood was sampled weekly, from the moment of observing the onset of moult, to the complete re-dressing with feathers, respectively a period of seven weeks. Haematological parameters showed a specific dynamic during this process. This dynamic is dominated by a decrease in red blood cells, packed cell volume and haemoglobin (P<0.05), on the one hand, and a decrease in the white cell number and in the percentage of lymphocytes (P<0.05) on the other. These changes take place predominantly in the first three weeks of the moult period. In the next period of the process, the modified haematological parameters return to the values comparable to those of the control (P > 0.05).

Characteristics of Hunter Harvested Montezuma Quail Wings and Implications for Molt Phenology

We obtained 1,899 hunter-harvested Montezuma quail (Cyrtonyx montezumae) wings from southeastern Arizona, USA, from the 2008–2009 hunting season. We determined age and sex based on plumage characteristics for 98.2% (1,864) of the original sample. One-way analysis of variance (ANOVA) of wing-chord length found differences (P < 0.001) based on sex, but not age, with mean (± standard error) male wing chord (113.76 ± 0.15 mm) longer than mean female wing chord (111.03 ± 0.13 mm). Mean male and female wing-chord lengths from our study population were 6.8% and 7.7% shorter, respectively, than previously reported in the literature. We additionally calculated a complete prebasic molt cycle of 177 days based on previously reported preformative molt patterns. The primary benefits of our results are: 1) a more accurate sex-based wing-chord length based on a large sample size, 2) a method to back-calculate molt onset dates for hunter harvested after hatch year Montezuma quail, and 3) a potential means to model the influence of precipitation on population dynamics of Montezuma quail.

Onset of primary moult in immature and breeding adult Common Swifts Apus apus

ABSTRACT We studied the primary moult of immature and adult breeding Common Swift Apus apus in Italy. Birds were sampled at breeding colonies, or by attracting them to mist nets with the playback of recorded calls. Our sample of 590 immatures (2cy) assessed that about a quarter (25.6%) of these birds start moulting the inner primaries during their stay in Europe. We find that this moult starts usually in the second half of June, earlier than was observed in other European countries, where individuals begin moulting in July. Interestingly, and in contrast to previous knowledge on the moulting schedules of breeding Common Swifts, we found that more than half of breeding adults (56%, 15 of 27) started their primary moult in July. Moult before the swifts left our study area was limited to the first two inner primaries. Our study showed that breeding Commons Swifts are able to regulate their moult when already engaged in rearing young, and both immature and adult Swifts start their moult before leaving Europe towards their African winter quarters.

Effect of age at the onset of natural molting on carcass traits, muscle oxidative stability, and amino acid and fatty acid profiles in commercial laying hens

The aim was to evaluate the effect of age at the onset of natural molting on carcass traits, muscle oxidative stability, amino acid and fatty acid profiles in laying hens. According to the first signs of molting, 96 ISA Brown laying hens were selected to construct 3 equal groups, where birds molting at weeks 66–70, 71 to 75 and 76 to 80 were identified as “early molting” EM, “mid-molting” MM and “late molting” LM, respectively. There were no significant differences in the dressing percentage between the experimental groups. Meanwhile, the EM group showed greater abdominal fat percentage (4.82%). The EM group exhibited higher breast n3-PUFA contents (α-Linolenic, eicosatetraenoic, eicosapentaenoic and docosahexaenoic) (0.953, 1.04, 0.77 and 0.58 g 100 g−1, respectively). Moreover, the EM group showed greater breast isoleucine, valine and arginine contents as compared to the LM group (3.52, 4.08 and 5.35 g 100 g−1, respectively). The EM group exhibited lower concentrations of malondialdehyde in the breast (14.9 nmol/g) and thigh (17.4 nmol/g) muscles. In conclusion, the EM laying hens showed higher n3-PUFA and better oxidative stability of breast and thigh muscles. Hence, the EM laying hens may be used as an attractive source of chicken meat.

Testosterone Increase in Free-Living Young Blackcaps Sylvia atricapilla and Wood Warblers Phylloscopus sibilatrix During Post-Juvenile Molt with Possible Implications for Juvenile Dispersal

Testosterone may affect many aspects of passerine maturation including the expression of plumage coloration and spring migration in adult birds. However, how the testosterone level changes during bird development is not well known. We compared the testosterone profile during juvenile development of a sexually dimorphic species, the Blackcap Sylvia atricapilla, and a monomorphic species, the Wood Warbler Phylloscopus sibilatrix. We first tested whether testosterone influences plumage coloration and increases at the onset of molt in males of the dimorphic but not the monomorphic species. We found that a testosterone level increase occurred in both sexes and species during later stages of molt, and thus was not related to male plumage coloration. We also investigated whether the increase in testosterone level coincides with juvenile dispersal. If testosterone affects dispersal behavior in these species, both sexes should show an elevated testosterone level during dispersal, but this increase should occur earlier in the Wood Warbler, which disperses earlier than the Blackcap. In juvenile Blackcaps, the increase in testosterone level occurred on the 43-68th day after hatching, while in Wood Warblers it occurred on the 32-36th day (i.e. 11-32 days earlier). The increase in testosterone level coincided with the onset of the post-juvenile dispersal in both species. This study provides the first direct evidence of a testosterone level increase during juvenile development in two free-living migratory birds. While not correlated with species plumage coloration, this increase might affect other aspects of behavior, e.g. juvenile dispersal.

Urban links to molt schedule, body condition and carotenoid‐based coloration in the house finch Haemorhous mexicanus

Animals in urban environments are exposed to novel conditions, such as habitat and dietary alterations, and night‐time light pollution, that can shift the timing and expression of life‐history traits. Birds are popular urban‐ecological study subjects, and in these feathered animals regular plumage molt is a critical process for maintenance of feather quality and their associated functions (e.g. thermoregulation, aerodynamics and communicative coloration). We hypothesized that environmental changes associated with urbanization might affect the timing and progression of molt. As molt is energetically and nutritionally costly, we also tested whether putative urban–rural molt differences might be explained by indirect effects of urban environments on aspects of health (i.e. body condition, sexually selected plumage color). We tracked body molt intensity during molt onset, peak and completion in both sexes of urban and rural house finches Haemorhous mexicanus, and related this to body condition and sexually selected carotenoid‐based plumage coloration. We found that urban birds began molting earlier, peaked at a lower molt intensity and ended molt later than rural conspecifics. We also find that both body condition and plumage hue in males explain variation in body molt intensity differently for urban and rural populations at various points during the molting period. Overall, we demonstrate that songbird molt patterns and the development of carotenoid‐based sexual ornaments can be altered in urban environments, and that rapid urbanization may have broader implications for molt dynamics, annual life‐history traits and sexual selection.

Moult of overwintering Wood Warblers Phylloscopus sibilatrix in an annual-cycle perspective

Wood Warblers, an Afro-Palearctic migrant species, are declining steadily in Europe likely due to mortality outside their breeding grounds. However, little is known about their overwintering, and records about the sensitive life-cycle stage of moult in Africa are practically absent. To fill this gap, we report on moult of Wood Warblers captured over two winters (January–February) in 2019–2020 in Cameroon. We caught 14 individuals, of which 12 were monitored for flight feather moult. All inspected individuals showed advanced stages of flight feather renewal. Despite low sample sizes, Underhill-Zucchini moult models aptly explained variation in primary and secondary moult (R2 = 0.61). Estimated moult onset date was 26 December, completion date was 25 February, and moult duration was 61 days. These findings fit well with experimental data on the annual cycle and the timing of recently published migration tracks of Wood Warblers. Jointly, the data suggest that moult timing is set by an internal programme, which enables Wood Warblers to organise their multi-stage migration such that they reach suitable moulting habitat in time, and can depart in time with a fresh plumage for the breeding grounds. In our study, moult occurred during the peak of the dry season, which in Cameroon nonetheless shows high relative humidity. During our mist-netting on 28 cocoa plantations of varying shade cover, Wood Warblers were caught on 6 farms whose canopies were comparatively open. These data suggest that the birds encounter in Cameroon relatively stable climatic conditions for moult, and do not measurably prefer closed-canopy forests. Our findings are important, because successful moult increases survival prospects and because moult needs to be safely embedded in a migratory life cycle. Hence, information on moult timing and location is essential for identifying year-round vulnerabilities of Wood Warblers.

High-intensity flight feather molt and comparative molt ecology of warblers of eastern North America

Abstract Rapid high-intensity molt of flight feathers occurs in many bird species and can have several detrimental consequences, including reductions in flight capabilities, foraging performance, parental care, and plumage quality. Many migratory New World warblers (family Parulidae) are known to have intense remigial molt, and recent work has suggested that simultaneous replacement of the rectrices may be widespread in the family as well. However, the phylogenetic distribution of simultaneous rectrix molt, and high-intensity flight feather molt more generally, has not been systematically investigated in warblers. We addressed this issue by examining flight feather molt in 13 species, representing 7 different warbler genera, at Powdermill Avian Research Center in southwestern Pennsylvania, USA. All 13 species replaced their 12 rectrices simultaneously, with the onset of rectrix molt occurring in the early-middle stages of high-intensity primary molt. As expected, single-brooded early migrants molted earlier than double-brooded species whose nesting activities extend into late summer. However, our finding that late-molting species replaced their primaries more slowly and less intensively than early molting species was unexpected, as late-molting species are widely hypothesized to be under stronger migration-related time constraints. This surprising result appears to be at least partially explained by a positive association between the pace of molt and daylength; shorter late-summer days may mandate reduced daily food intake, lower molt intensity, and a slower pace of molt. In comparison to other passerines, flight feather molt in warblers of eastern North America is extraordinarily intense; at its peak, individuals are simultaneously replacing 50–67% of their 48 flight feathers (all 12 rectrices and 6–10 remiges on each wing) for 2–3 weeks or more. Because molt of this intensity is likely to present numerous challenges for flight, avoiding predators, foraging, and parental care, the period of flight feather molt for warblers constitutes a highly demanding phase of their annual cycle.

Laboratory Growth of &amp;lt;i&amp;gt;Anopheles gambiae&amp;lt;/i&amp;gt; (Diptera: Culicidae) and Morphological Determinism of Moulting

Growth in arthropods in general and in insects in particular, is supposed to be discontinuous and occurs during moulting. In Culicidae in general and Anopheles in particular, the number of moults is five with the fourth which gives the pupae. It is known that moulting in insects is a genetic and physiological phenomenon. Most physiological reactions are triggered by chemical or physical stimuli. The pressure exerted by the growth of the larval body on the exocuticle is one of the triggers of moulting. The objective of this work was therefore to determine the exact timing of the first three moults that determine the growth of An. gambiae larvae from egg hatch to pupation to highlight the role of increased larval size in the stimulation of moulting. We therefore, undertook to rear larvae of this anopheline species in the laboratory under conditions close to their natural environment from hatching to pupation. The length and width of the head, thorax and abdomen were recorded daily. Data analysis showed that the size of the head and thorax remained constant for the first three days (D0 to D2) of development and abdomen’s length for the first two days and then increased daily until day seven (D7) when it stopped. These observations led us to say that the M1 moult occurs at end of the third day of development and the M3 moult at end of the eighth day; the M2 moult could not be determined. All these observations led to the conclusion that the larval growth of An. gambiae has a continuous regimen and the growth of the head and thorax of the larva plays a crucial role in the onset of moulting.

Using community photography to investigate phenology: A case study of coat molt in the mountain goat (Oreamnos americanus) with missing data.

Participatory approaches, such as community photography, can engage the public in questions of societal and scientific interest while helping advance understanding of ecological patterns and processes. We combined data extracted from community‐sourced, spatially explicit photographs with research findings from 2018 fieldwork in the Yukon, Canada, to evaluate winter coat molt patterns and phenology in mountain goats (Oreamnos americanus), a cold‐adapted, alpine mammal. Leveraging the community science portals iNaturalist and CitSci, in less than a year we amassed a database of almost seven hundred unique photographs spanning some 4,500 km between latitudes 37.6°N and 61.1°N from 0 to 4,333 m elevation. Using statistical methods accounting for incomplete data, a common issue in community science datasets, we identified the effects of intrinsic (sex and presence of offspring) and broad environmental (latitude and elevation) factors on molt onset and rate and compared our findings with published data. Shedding occurred over a 3‐month period between 29 May and 6 September. Effects of sex and offspring on the timing of molt were consistent between the community‐sourced and our Yukon data and with findings on wild mountain goats at a long‐term research site in west‐central Alberta, Canada. Males molted first, followed by females without offspring (4.4 days later in the coarse‐grained, geographically wide community science sample; 29.2 days later in our fine‐grained Yukon sample) and lastly females with new kids (6.2; 21.2 days later, respectively). Shedding was later at higher elevations and faster at northern latitudes. Our findings establish a basis for employing community photography to examine broad‐scale questions about the timing of ecological events, as well as sex differences in response to possible climate drivers. In addition, community photography can help inspire public participation in environmental and outdoor activities specifically with reference to iconic wildlife.

Local timing of rainfall predicts the timing of moult within a single locality and the progress of moult among localities that vary in the onset of the wet season in a year-round breeding tropical songbird

Rainfall seasonality is likely an important cue for timing key annual cycle events like moult in birds living in seasonally arid environments, but its precise effect is difficult to establish because seasonal rainfall may affect other covarying annual events such as breeding in the same way. In central Nigeria, however, Common Bulbuls Pycnonotus barbatus moult in the wet season but only show weak breeding seasonality. This suggests that moult is more sensitive to rainfall than breeding, but a similar outcome is possible if moult is simply periodic. We tested the relationship between rainfall and moult in Common Bulbuls at a single location over 18 years: on average moult started 5th May (± 41 days: 25th March–15th June), being on average later than the onset of the rains which is usually mid-April. The likelihood of finding a moulting Common bulbul was best predicted by rainfall 9–15 weeks before moult was scored. We then tested the generality of this across populations: the progress of moult should, therefore, correlate with the average timing of the wet season along a spatial environmental gradient where the rains start at different times each year south-to-north of Nigeria. To test this, we modelled moult progress just before the rains across 15 localities 6°–13° N as a function of the onset of the wet season among localities. As predicted, moult progressed further in localities with earlier wet seasons, confirming that the onset of moult is timed to the onset of the wet season in each locality despite weak breeding seasonality in the Common Bulbul. This strategy may evolve to maintain optimal annual cycle routine in seasonal environments where breeding is prone to unpredictable local perturbations like nest predation. It may, however, be less obvious in temperate systems where all annual cycle stages are seasonally constrained, but it may help with explaining the high frequency of breeding–moult overlaps in tropical birds.

Transcriptomic regulation of seasonal coat color change in hares.

Color molts from summer brown to winter white coats have evolved in several species to maintain camouflage year‐round in environments with seasonal snow. Despite the eco‐evolutionary relevance of this key phenological adaptation, its molecular regulation has only recently begun to be addressed. Here, we analyze skin transcription changes during the autumn molt of the mountain hare (Lepus timidus) and integrate the results with an established model of gene regulation across the spring molt of the closely related snowshoe hare (L. americanus). We quantified differences in gene expression among three stages of molt progression—“brown” (early molt), “intermediate,” and “white” (late molt). We found 632 differentially expressed genes, with a major pulse of expression early in the molt, followed by a milder one in late molt. The functional makeup of differentially expressed genes anchored the sampled molt stages to the developmental timeline of the hair growth cycle, associating anagen to early molt and the transition to catagen to late molt. The progression of color change was characterized by differential expression of genes involved in pigmentation, circadian, and behavioral regulation. We found significant overlap between differentially expressed genes across the seasonal molts of mountain and snowshoe hares, particularly at molt onset, suggesting conservatism of gene regulation across species and seasons. However, some discrepancies suggest seasonal differences in melanocyte differentiation and the integration of nutritional cues. Our established regulatory model of seasonal coat color molt provides an important mechanistic context to study the functional architecture and evolution of this crucial seasonal adaptation.

Unusual mortality of Tufted puffins (Fratercula cirrhata) in the eastern Bering Sea.

Mass mortality events are increasing in frequency and magnitude, potentially linked with ongoing climate change. In October 2016 through January 2017, St. Paul Island, Bering Sea, Alaska, experienced a mortality event of alcids (family: Alcidae), with over 350 carcasses recovered. Almost three-quarters of the carcasses were unscavenged, a rate much higher than in baseline surveys (17%), suggesting ongoing deposition and elevated mortality around St Paul over a 2–3 month period. Based on the observation that carcasses were not observed on the neighboring island of St. George, we bounded the at-sea distribution of moribund birds, and estimated all species mortality at 3,150 to 8,800 birds. The event was particularly anomalous given the late fall/winter timing when low numbers of beached birds are typical. In addition, the predominance of Tufted puffins (Fratercula cirrhata, 79% of carcass finds) and Crested auklets (Aethia cristatella, 11% of carcass finds) was unusual, as these species are nearly absent from long-term baseline surveys. Collected specimens were severely emaciated, suggesting starvation as the ultimate cause of mortality. The majority (95%, N = 245) of Tufted puffins were adults regrowing flight feathers, indicating a potential contribution of molt stress. Immediately prior to this event, shifts in zooplankton community composition and in forage fish distribution and energy density were documented in the eastern Bering Sea following a period of elevated sea surface temperatures, evidence cumulatively suggestive of a bottom-up shift in seabird prey availability. We posit that shifts in prey composition and/or distribution, combined with the onset of molt, resulted in this mortality event.

Timing manipulations reveal the lack of a causal link across timing of annual-cycle stages in a long-distance migrant.

Organisms need to time their annual-cycle stages, like breeding and migration, to occur at the right time of the year. Climate change has shifted the timing of annual-cycle stages at different rates, thereby tightening or lifting time constraints of these annual-cycle stages, a rarely studied consequence of climate change. The degree to which these constraints are affected by climate change depends on whether consecutive stages are causally linked (scenario I) or whether the timing of each stage is independent of other stages (scenario II). Under scenario I, a change in timing in one stage has knock-on timing effects on subsequent stages, whereas under scenario II, a shift in the timing of one stage affects the degree of overlap with previous and subsequent stages. To test this, we combined field manipulations, captivity measurements and geolocation data. We advanced and delayed hatching dates in pied flycatchers (Ficedula hypoleuca) and measured how the timing of subsequent stages (male moult and migration) were affected. There was no causal effect of manipulated hatching dates on the onset of moult and departure to Africa. Thus, advancing hatching dates reduced the male moult-breeding overlap with no effect on the moult-migration interval. Interestingly, the wintering location of delayed males was more westwards, suggesting that delaying the termination of breeding carries over to winter location. Because we found no causal linkage of the timing of annual-cycle stages, climate change could shift these stages at different rates, with the risk that the time available for some becomes so short that this will have major fitness consequences.

Annual Cycles of Wagtails (Passeriformes, Motacillidae) in Northwestern Siberia: Comparative Aspects

Abstract—The annual cycles of pipits and wagtails, which were studied during field and experimental observations in the Lower Ob River basin and Yamal Peninsula, have many common features. All species are similar in dates of arrival, nesting, molting, and departure onset; however, the timing of postnuptial migration and prenuptial molting differ somewhat. Positive air temperatures are an environmental factor that determines the beginning of arrival to the forest tundra and tundras. Only yellow wagtails arrive with developed gonads, while the other species require additional stimulation by a photoperiod of 24L : 0D. The reduction of some nesting stages is presumably an attempt of boreal populations to increase reproductive success. Differences in the annual cycle program are observed during the postnesting period. These variations include the timing of the onset and duration of postjuvenile and postnuptial molting, as well as the degree of overlap of postnuptial molting with nesting, the degree of overlap of the migration state with molting, photoperiodic molting conditions, and autumn migration. The intraspecific rhythm of migration in combination with environmental conditions along the migration track determines the timing of arrival to wintering sites. The threshold values of photoperiodic intervals of wintering and prenuptial molting in different species correspond to the latitude of hibernation and, possibly, determine this latitude.

Postjuvenile Moult in the Common Crossbill (Loxia curvirostra curvirostra): Photoperiodic Regulation and Its Role in Synchronization of the Annual Cycle

Captures of Common Crossbills on the southeastern shore of Lake Ladoga and in the St. Petersburg suburbs (1482 individuals from 1968 to 2015) show that the postjuvenile moult is preceded by migration and the overwhelming majority of migrating young birds have plumage that consists solely of juvenile feathers. The postjuvenile moult is recorded from the end of the first ten days of August to the middle of November, the timing of the onset of moult varies within one and a half months with different individuals being observed to have significant distinctions in the number of replaced feathers. The experiments with photoperiodic regulation of the postjuvenile moult were made on 48 individuals captured in nature in the second half of summer and six young birds from one pair of Crossbills, which bred in captivity. The experimental groups were kept in the conditions of a natural decrease in the day (60° N), decrease in the daylength after the transfer from a long day to a short daylength, increasing daylength, constant long daylength, and constant short daylength. The results of the experiments show that the species-specific features of moult in the Common Crossbill are its start being regulated in a wide age range (from 50 to 180 days) and a rather narrow photoperiodic interval of moult with short daylengths (14.5–9 h of daylight). As long as there is more than 14.5 h of daylight, moult begins under the influence of the endogenous rhythm, but not earlier than the age of 4–6 months. The individuals that moult within the photoperiodic interval replace plumage at high rates. After the daylength gets below the lower threshold of the photoperiodic interval, an inhibiting role of the photoperiodic response is activated, terminating moult in the birds that started moult too late, and their plumage mainly retains juvenile generation feathers. Due to the regulation of moult, intrapopulation differences in the timing of annual cycles that are more than seven months at the time of birth of young Common Crossbills decrease almost five times by its start and become even smaller as it ends. It is assumed that this not only makes it possible to implement moult under optimal conditions but also makes first-year birds ready to start breeding in a timely manner if they find themselves in an area with abundantly fruiting spruce trees.