Moult Strategies Research Articles

Sex, age, molt strategy, and migration distance explain the phenology of songbirds at a stopover along the East Asian flyway

Sex- and age-specific differences in the timing of migration are widespread among animals. In birds, common patterns are protandry, the earlier arrival of males in spring, and age-differential migration during autumn. However, knowledge of these differences stems mainly from the Palearctic-African and Nearctic-Neotropical flyways, while detailed information about the phenology of migrant birds from the East Asian flyway is far scarcer. To help fill parts of this gap, we analyzed how migration distance, sex, age, and molt strategy affect the spring and autumn phenologies of 36 migrant songbirds (altogether 18,427 individuals) at a stopover site in the Russian Far East. Sex-differential migration was more pronounced in spring than in autumn, with half of the studied species (6 out of 12) showing a protandrous migration pattern. Age-differences in migration were rare in spring but found in nearly half of the studied species (11 out of 25) in autumn. These age effects were associated with the birds’ molt strategy and the mean latitudinal distances from the assumed breeding area to the study site. Adults performing a complete molt before the onset of autumn migration passed the study site later than first-year birds undergoing only a partial molt. This pattern, however, reversed with increasing migration distance to the study site. These sex-, age-, and molt-specific migration patterns agree with those found along other flyways and seem to be common features of land bird migration strategies.Significance statementThe timing of animal migration is shaped by the availability of resources and the organization of annual cycles. In migrant birds, sex- and age-differential migration is a common phenomenon. For the rarely studied East Asian flyway, we show for the first time and based on a large set of migrant songbirds that earlier migration of males is a common pattern there in spring. Further, the timing and extent of molt explained age-differential migration during autumn. Adults molting their complete plumage at the breeding area before migration showed delayed phenology in comparison to first-year birds, which perform only a partial molt. This pattern, however, reversed with increasing migration distance to the study site. Since our results agree with the general patterns from the other migration flyways, similar drivers for differential migration may act across different flyway systems, provoking a similar evolutionary response.

Molting strategies of Arctic seals drive annual patterns in metabolism

Arctic seals, including spotted (Phoca largha), ringed (Pusa hispida) and bearded (Erignathus barbatus) seals, are directly affected by sea ice loss. These species use sea ice as a haul-out substrate for various critical functions, including their annual molt. Continued environmental warming will inevitably alter the routine behavior and overall energy budgets of Arctic seals, but it is difficult to quantify these impacts as their metabolic requirements are not well known-due in part to the difficulty of studying wild individuals. Thus, data pertaining to species-specific energy demands are urgently needed to better understand the physiological consequences of rapid environmental change. We used open-flow respirometry over a four-year period to track fine-scale, longitudinal changes in the resting metabolic rate (RMR) of four spotted seals, three ringed seals and one bearded seal trained to participate in research. Simultaneously, we collected complementary physiological and environmental data. Species-specific metabolic demands followed expected patterns based on body size, with the largest species, the bearded seal, exhibiting the highest absolute RMR (0.48 ± 0.04L O2 min-1) and the lowest mass-specific RMR (4.10 ± 0.47ml O2 min-1kg-1), followed by spotted (absolute: 0.33 ± 0.07L O2 min-1; mass-specific: 6.13 ± 0.73ml O2 min-1kg-1) and ringed (absolute: 0.20 ± 0.04L O2 min-1; mass-specific: 7.01 ± 1.38ml O2 min-1kg-1) seals. Further, we observed clear and consistent annual patterns in RMR that related to the distinct molting strategies of each species. For species that molted over relatively short intervals-spotted (33 ± 4days) and ringed (28 ± 6days) seals-metabolic demands increased markedly in association with molt. In contrast, the bearded seal exhibited a prolonged molting strategy (119 ± 2days), which appeared to limit the overall cost of molting as indicated by a relatively stable annual RMR. These findings highlight energetic trade-offs associated with different molting strategies and provide quantitative data that can be used to assess species-specific vulnerabilities to changing conditions.

Complex postbreeding molt strategies in a songbird migrating along the East Asian Flyway, the Pallas's Grasshopper Warbler Locustella certhiola.

Molt strategies have received relatively little attention in current ornithology, and knowledge concerning the evolution, variability and extent of molt is sparse in many bird species. This is especially true for East Asian Locustella species where assumptions on molt patterns are based on incomplete information. We provide evidence indicating a complex postbreeding molt strategy and variable molt extent among the Pallas's Grasshopper Warbler Locustella certhiola, based on data from six ringing sites situated along its flyway from the breeding grounds to the wintering areas. Detailed study revealed for the first time that in most individuals wing feather molt proceeds from the center both toward the body and the wing‐tip, a molt pattern known as divergent molt (which is rare among Palearctic passerines). In the Russian Far East, where both breeding birds and passage migrants occur, a third of the adult birds were molting in late summer. In Central Siberia, at the northwestern limit of its distribution, adult individuals commenced their primary molt partly divergently and partly with unknown sequence. During migration in Mongolia, only descendantly (i.e., from the body toward the wing‐tip) molting birds were observed, while further south in Korea, Hong Kong, and Thailand the proportion of potential eccentric and divergent feather renewal was not identifiable since the renewed feathers were already fully grown as expected. We found an increase in the mean number of molted primaries during the progress of the autumn migration. Moderate body mass levels and low‐fat and muscle scores were observed in molting adult birds, without any remarkable increase in the later season. According to optimality models, we suggest that an extremely short season of high food abundance in tall grass habitats and a largely overland route allow autumn migration with low fuel loads combined with molt migration in at least a part of the population. This study highlights the importance of further studying molt strategy as well as stopover behavior decisions and the trade‐offs among migratory birds that are now facing a panoply of anthropogenic threats along their flyways.

Archaeopteryx feather sheaths reveal sequential center-out flight-related molting strategy

Modern flying birds molt to replace old and worn feathers that inhibit flight performance, but its origins are unclear. We address this by presenting and evaluating a ~150 million year old record of molting in a feathered dinosaur from the early bird Archaeopteryx. Laser-Stimulated Fluorescence revealed feather sheaths that are otherwise invisible under white light. These are separated by one feather and are not in numerical sequential order and are mirrored in both wings. This indicates that a sequential center-out molting strategy was already present at the origins of flight, which is used in living falcons to preserve maximum flight performance. This strategy would have been a welcome advantage for early theropod flyers that had poor flight capabilities. This discovery provides important insights into how birds refined their early flight capabilities before the appearance of the keeled sternum, pygostyle and triosseal canal.

Split moult: time constraints or endogenous strategy?

Plumage renewal is an important process in the avian yearly cycle, yet knowledge about the factors that shape moult strategies is far from complete. A varying proportion of individuals of several Palaearctic passerine species divide their flight‐feather moult between breeding and wintering areas. Here we focus on split moult, which includes retaining some or all the secondary feathers during the main moult period (interrupted moult) and later renewal of these feathers (complementary moult), after migration or breeding. We built a large moult database (213 species, 64 076 individuals) to describe split moult pattern in Palaearctic passerines, identify its frequency and test whether it is more common in long‐distance migrants than in short‐distance migrants and sedentary species. We found that first‐year individuals, tested shortly after the main moult period, exhibited interrupted moult, yet individuals tested later in the annual cycle also retained non‐moulted secondaries. In contrast, in most species, adults tested later in the annual cycle had fully renewed flight‐feathers. This would suggest that age may be a factor associated with the probability of exhibiting split moult, as this strategy was more common in adults than in first‐year birds. Furthermore, we found that interrupted and split moults occur more often among long‐distance migrants than among sedentary or short‐distance migrants. Our results probably imply that the split flight‐feather moult is not initiated in the first winter of the bird's life, as has been suggested in previous studies. In addition, the occurrence of split moult among long‐distance migrants in which the main moult takes place during the winter and among sedentary or short‐distance migratory species, suggests that split moult probably does not represent an evolutionary transition between summer and winter moult; in both species groups, an evolutionary transition towards winter moult is probably impossible. Based on our findings, we suggest that split moult may be a moult strategy that allows birds to cope with time and energy constraints during the main moult period by retaining part of their flight‐feathers and resuming their moult later in the yearly cycle.

Simultaneous Wing Molt as a Catalyst for the Evolution of Flightlessness in Birds.

AbstractComplex features, such as vision, limbs, and flight, have been lost by many groups of animals. Some groups of birds are more prone to loss of flight than others, but few studies have investigated possible reasons for this variation. I tested the hypothesis that a rare strategy of flight feather replacement is involved in rate variation in the evolution of flightlessness in birds. This strategy involves a simultaneous molt of the flight feathers of the wing, resulting in a temporary flightless condition during molt. I hypothesized that adaptations for this flightless period may serve as preadaptations for permanent flightlessness under conditions that favor permanent loss of flight. I found an elevated rate of loss of flight in lineages with simultaneous wing molt compared with loss of flight in lineages without simultaneous wing molt. This may indicate that birds with simultaneous molt are more prepared to adjust quickly to open niches that do not require flight, such as terrestrial niches on island habitats. These results illustrate how molt strategies can influence the long-term evolutionary trajectories of birds and provide insight into how phenotypic precursors may act as a mechanism of rate variation in the loss of complex traits.

Partial or complete? The evolution of post-juvenile moult strategies in passerine birds.

Moulting strategies in birds have evolved to avoid overlap with, or prepare for, other demanding parts of the annual cycle, such as reproduction or migration. When moulting for the first time after leaving the nest, young birds replace their typically poor-quality plumage during the post-juvenile moult. The extent of this moult varies between species from partial to complete. Earlier studies, restricted to Western Palearctic birds, suggest that in most species a complete post-juvenile moult may not be possible simply because young birds are constrained by not having the same access to resources as adults, unless environmental conditions are favourable. These studies also show that complete post-juvenile moult is more common in species with poor-quality nest-grown plumage. We expanded the spatial and taxonomic scope of previous studies to 1,315 species of passerines from across the world and considered both the role of constraints, plumage quality and other selective pressures favouring a complete post-juvenile moult. Thus, we test whether complete moult is more prevalent in species where nest-grown feathers are presumably of poor quality (shorter nestling period), that live in environments that foster quick plumage degradation (open habitats, high insolation and humidity), and where males are under strong sexual selection. Our data reveal that 24% of species carry out a complete post-juvenile moult, and that this trait has a strong phylogenetic signal. Complete moult is more common in species that inhabit warmer regions and open habitats, show no delayed plumage maturation and have higher levels of sexual dichromatism (indicative of strong sexual selection). Neither the presumed quality of the nest-grown plumage nor living in regions with high insolation correlates with complete moult. In conclusion, the evolution of complete post-juvenile moult not only depends on whether birds can perform a complete moult (i.e. suitable environmental conditions) but also on the strength of selection associated with the need of a complete moult. In particular, the necessity to keep the plumage in good condition in challenging environments and the benefits associated with producing adult-like plumage colours to attract mates or deter rivals seem to play an important role.

Factors controlling the migration phenology of Siberian Phylloscopus species

The timing of annual events in migratory species is determined by the position of breeding and wintering grounds, length of the breeding season, the occurrence of their preferred diet or the timing of moult in birds. Previous studies found significant differences in the migration phenology of Asian Phylloscopus warbler species, a group of long-distance migratory passerines. However, the factors that influence the observed differences in migration phenology have not been studied yet. Here, we analyze the effect of the above-mentioned factors on the migration timing of seven Phylloscopus species at a stopover site in the Russian Far East. We found that spring migration phenology was best explained by moult strategy and the preferred prey size, while autumn migration phenology was linked to the southernmost wintering latitude of the species. Interspecific differences in migration timing were much higher in autumn than in spring. The duration of the species-specific migration periods was also longer in autumn than in spring, most likely caused by higher competition during spring to arrive early at the breeding grounds. Our results contribute to the understanding of migration ecology in songbirds moving along the little-studied East Asian flyway.

Evolution of moult-migration is directly linked to aridity of the breeding grounds in North American passerines.

To avoid energy allocation conflicts, birds generally separate breeding, migration and moult during the annual cycle. North American passerines typically moult on the breeding grounds prior to autumn migration. However, some have evolved a moult-migration strategy in which they delay moult until stopping over during autumn migration. Rohwer et al. (2005) proposed the 'push-pull hypothesis' as an explanation for the evolution of this moult strategy, but it has not been empirically tested. Poor conditions on the breeding grounds at the end of the summer would push birds to depart prior to moult, while productive stopover locations would pull them. We tested for a relationship between moult-migration and breeding grounds aridity as measured by the normalized difference vegetation index. Our results strongly support the 'push' aspect of the push-pull hypothesis and indicate that arid breeding grounds, primarily in western North America, would drive species to evolve stopover moult-migration, although this relationship may depend upon migration distance.

Moult and plumage patterns of the critically endangered Yellow-breasted Bunting (Emberiza aureola) at a stopover site in Korea

Travelling from across its breeding range in northern Eurasia, Yellow-breasted Buntings (Emberiza aureola) stopover in China before migrating to their wintering areas in South-East Asia and the Indian subcontinent. This species is now considered critically endangered because its global population has declined dramatically; moreover, it is overexploited by illegal trapping at its stopover sites during the moult. Currently, two subspecies of the Yellow-breasted Bunting are recognized: aureola and ornata. However, the moult strategies of these subspecies at their stopover sites remain unknown. In the present study, we analysed the moult and plumage patterns of E. a. ornata in Korea (n = 262) between 2005 and 2018 as part of a long-term bird ringing project at a key migrant stopover site. We observed significant sexual dimorphism in the morphological characteristics of both adults and immatures: the males had longer wings, tails, and tarsi, and weighed more than the females. Moult patterns were distinct in relation to age. Adults underwent a complete post-breeding moult, entirely replacing their body and flight feathers before the onset of autumn migration over a breeding range. Remarkably, 93.4% of first-winter birds underwent an eccentric primary moult, having moulted the 3–4 outermost primaries (P6–P9 or P7–P9) before arriving at the stopover site. Males achieve full adult breeding plumage in their second summer. More than three types of median wing coverts were identified in the first-winter males, with various plumage colours and patterns. To the best of our knowledge, this is the first report on the moulting and plumage patterns in the subspecies ornata as it passes through an East Asian migratory flyway stopover site in Korea. Our results will contribute to an understanding of the migration behaviour and subspecies populations that are at risk in their specific environments. This is essential for ecological monitoring to facilitate the conservation of this critically endangered species.

Effects of body size, sex, parental care and moult strategies on auk diving behaviour outside the breeding season

Information on seabird foraging behaviour outside the breeding season is currently limited. This knowledge gap is critical as this period is energetically demanding due to post‐fledging parental care, feather moult and changing environmental conditions. Based on species’ body size, post‐fledging parental strategy and primary moult schedule we tested predictions for key aspects of foraging behaviour (maximum dive depth (MDD), daily time submerged (DTS) and diurnal dive activity (DDA)) using dive depth data collected from three seabird species (common guillemot Uria aalge, razorbill Alca torda and Atlantic puffin Fratercula arctica) from the end of the breeding season (July) to mid‐winter (January). We found partial support for predictions associated with body size; guillemots had greater MDD than razorbills but MDD did not differ between razorbills and puffins, despite the former being 35% heavier. In accordance with sexual monomorphism in all three species, MDD did not differ overall between the sexes. However, in guillemots and razorbills there were sex‐specific differences, such that male guillemots made deeper dives than females, and males of both species had higher DTS. In contrast, there were no marked sex differences in dive behaviour of puffins in July and August in accordance with their lack of post‐fledging parental care and variable moult schedule. We found support for the prediction that diving effort would be greater in mid‐winter compared to the period after the breeding season. Despite reduced daylight in mid‐winter, this increase in DTS occurred predominantly during the day and only guillemots appeared to dive nocturnally to any great extent. In comparison to diving behaviour of these species recorded during the breeding season, MDD was shallower and DTS was greater during the non‐breeding period. Such differences in diving behaviour during the post‐breeding period are relevant when identifying potential energetic bottlenecks, known to be key drivers of seabird population dynamics.

Ecdysis in a stem-group euarthropod from the early Cambrian of China

Moulting is a fundamental component of the ecdysozoan life cycle, but the fossil record of this strategy is susceptible to preservation biases, making evidence of ecdysis in soft-bodied organisms extremely rare. Here, we report an exceptional specimen of the fuxianhuiid Alacaris mirabilis preserved in the act of moulting from the Cambrian (Stage 3) Xiaoshiba Lagerstätte, South China. The specimen displays a flattened and wrinkled head shield, inverted overlap of the trunk tergites over the head shield, and duplication of exoskeletal elements including the posterior body margins and telson. We interpret this fossil as a discarded exoskeleton overlying the carcass of an emerging individual. The moulting behaviour of A. mirabilis evokes that of decapods, in which the carapace is separated posteriorly and rotated forward from the body, forming a wide gape for the emerging individual. A. mirabilis illuminates the moult strategy of stem-group Euarthropoda, offers the stratigraphically and phylogenetically earliest direct evidence of ecdysis within total-group Euarthropoda, and represents one of the oldest examples of this growth strategy in the evolution of Ecdysozoa.

Convergence of biannual moulting strategies across birds and mammals.

Birds and mammals have developed numerous strategies for replacing worn feathers and hair. Moulting usually occurs on an annual basis; however, moults that take place twice per year (biannual moults) also occur. Here, we review the forces driving the evolution of various moult strategies, focusing on the special case of the complete biannual moult as a convergence of selection pressures across birds and mammals. Current evidence suggests that harsh environmental conditions or seasonality (e.g. larger variation in temperatures) drive evolution of a biannual moult. In turn, the biannual moult can respond to secondary selection that results in phenotypic alteration such as colour changes for mate choice dynamics (sexual selection) or camouflage requirements (natural selection). We discuss the contributions of natural and sexual selection to the evolution of biannual moulting strategies in the contexts of energetics, niche selection, functionality and physiological mechanisms. Finally, we suggest that moult strategies are directly related to species niche because environmental attributes drive the utility (e.g. thermoregulation, camouflage, social dynamics) of the hair or feathers. Functional efficiency of moult may be undermined if the pace of evolution fails to match that of the changing climate. Thus, future research should seek to understand the plasticity of moult duration and phenology, especially in the context of annual cycles.

Primary moult of continental Black-tailed Godwits Limosa limosa limosa in the Doñana wetlands, Spain

ABSTRACTCapsule: Most Continental Black-tailed Godwits Limosa limosa limosa using the Doñana wetlands during post-breeding migration appear to begin moult before they arrive and suspend moult before they migrate onwards to West Africa.Aims: We aim to describe the primary moult strategies and patterns in the Continental Black-tailed Godwits using the Doñana wetlands, a major passage and wintering area for waterbirds in southern Spain.Methods: Individual godwits were captured, marked and their primary moult was scored in Doñana during the non-breeding season (June–March) in 2011 and 2012. Data from resightings of colour-marked godwits and birds equipped with satellite transmitters were used to estimate stopover duration during post-breeding migration (June–September) to determine if godwits move to West Africa before completing their primary moult.Results: Average primary moult duration was estimated to be 84 days ± 9 se, during 29 June–21 September and did not differ between sexes. Only 2% of individuals were observed with suspended moult. We estimated stopover duration in Doñana to be 13 days ± 2 se before migrating to West Africa.Conclusions: Most godwits stage for about two weeks in the Doñana wetlands during southward migration, moult their primaries and appear to suspend moult before crossing the Sahara. Others may complete their primary moult in Doñana, or elsewhere in Europe and overwinter in Doñana where increasing numbers of godwits have been detected in recent years. A few individuals may finish the moult in Doñana and migrate to West Africa late in the post-breeding season.

Molt strategy and aging of Rufous-collared Sparrow (Zonotrichia capensis) in Bogotá, Colombia

La muda es un proceso importante de la historia natural de las aves y aprender sobre este proceso permite entender varios aspectos de su vida. En zonas templadas, el conocimiento sobre las estrategias de muda, las características de cada tipo de plumaje y su relación con la edad del ave, son bien conocidas. Por el contrario, en el neotrópico, este conocimiento no está bien desarrollado. Una especie con bastantes estudios sobre muda en varias localidades es el ampliamente distribuido Copetón (Zonotrichia capensis). Esos estudios cubren localidades de su ámbito geográfico facilitando comparaciones entre poblaciones y proveyendo un excelente modelo para estudiar la variación del plumaje y la muda. En este estudio, determiné el alcance y la estrategia de muda de Z. capensis costaricensis mediante muestreos durante un año en dos localidades de Bogotá. Capturé y anillé individuos de Z. capensis y tomé información de características como los límites de muda, osificación del cráneo y color del iris que me permitieron identificar el tipo de plumaje/muda y relacionarlo con una categoría de edad del sistema de clasificación de edad basado en los ciclos de muda. Encontré que Z. capensis tiene una estrategia de muda básica compleja, donde todas las mudas prebásicas, incluida la prejuvenil, son completas y la muda preformativa es parcial. Estos resultados son congruentes con los de otras regiones. Sin embargo, la muda de Z. capensis varía mucho, e individuos lejos de los trópicos tienen una muda alterna parcial-incompleta adicional y aún desconocemos por qué.

Moult in the Loggerhead Shrike Lanius ludovicianus is influenced by sex, latitude and migration

We investigated moult strategies in Loggerhead Shrikes by examining first prebasic or preformative moult patterns and by assessing the general location where individual feathers were grown using stable hydrogen isotope (δ2H) analysis. We tested the relative importance of factors known to impact moult timing and pattern, including age, sex, body size, food availability and migration. Migratory Shrikes showed evidence of suspended moult, in which feathers are moulted on both the breeding and the non‐breeding grounds with a suspension of moult during migration. Extent of moult was best explained by sex, longitude, migratory behaviour and breeding‐ground latitude. Male Hatch Year (HY) Shrikes replaced more feathers on the breeding grounds prior to migration than did HY females and moulted more extensively on the breeding grounds than did females. Non‐migratory HY Shrikes underwent a more extensive preformative moult than migratory HY Shrikes. Individuals in more southerly migratory populations moulted more extensively on the breeding grounds than did those breeding further north. Our data also indicate that individuals in the northeastern populations moulted more extensively on the breeding grounds than did those in the north and southwest. Our study underlines the complex structure and variation in moult possible within species, revealing surprising levels of differentiation between sexes and age cohorts, linked to environmental factors on the breeding grounds. Our study highlights the utility of an intrinsic marker, specifically δ2H analysis, to test hypotheses regarding the evolutionary and ecological forces driving moult. Although the methodology has not commonly been applied to this area of research, our results indicate that it can provide unprecedented insight into inter‐ and intra‐specific adaptive response to constraints, whereby individuals maximize fitness.

Wings of tropical finches: interspecific differences in shape are consistent with levels of mobility, but moult and feather fault patterns are more complex

ABSTRACTBirds’ wings reflect their life histories, suggesting evolutionary selection for wing shapes and moult strategies. Compared to sedentary species, long-distance migrants have narrower wings (for fast, efficient flight); they have fewer feather faults and avoid moulting flight feathers during migration (to optimise flight surface performance). It is unclear whether these patterns apply to species that fly short-intermediate distances, like tropical nomads. We compared wing shape, feather faulting, and flight-feather moult across five finch species from northern Australia with varying mobility: Pictorella Mannikins (Heteromunia pectoralis) and Gouldian Finches (Erythrura gouldiae) are highly mobile, nomadic at regional scales; Long-tailed Finches (Poephila acuticauda) and Double-barred Finches (Taeniopygia bichenovii) are sedentary with local nomadism; Crimson Finches (Neochmia phaeton) are sedentary. More mobile species had narrower wings and higher wing loading than relatively sedentary species, and less feather faulting. Variation in moult strategies was complex, but mobile species carried out moult fast, in a short time window, and moulted a single feather at a time. Unexpectedly, Gouldian Finch wings were more suited for fast efficient flight than Pictorella Mannikin wings, and yet they had more feather faulting. Differences in mobility may be a key dimension of niche separation that allows these species to co-exist.

Postponed moult of primary coverts untangles the ageing of Wrynecks Jynx torquilla

ABSTRACTIn most bird species, all feathers are moulted at least once per year, usually after breeding (complete post-breeding moult). Woodpeckers (Picidae) often retain some primary coverts and secondaries during their otherwise complete post-breeding moult. To date, this phenomenon has received little attention and has not been studied thoroughly in European woodpeckers, even though it could prove very helpful for ageing and therefore for demographic studies. We studied the moult patterns of breeding adult Wrynecks Jynx torquilla in a Swiss Alpine valley between 2013 and 2016. Due to intensive ringing in the study area prior to the current study, our data set included birds of known age. Some individuals were captured and their moult documented in up to four years in a row. We found that Wrynecks present an extreme case of delayed primary-covert moult. No primary covert (PC) was renewed during the post-juvenile moult. At most three PCs were renewed by the second post-breeding moult and three to six at more advanced ages. That some juvenile PCs were retained until the sixth calendar year has not previously been documented for a species with such a relatively short lifespan. This principle of ‘postponed moult’ in Wrynecks affects the appearance of the PCs differently in each age class and allows us to age birds more accurately than previous methods. Here we describe how to interpret the moult pattern of PCs to age Wrynecks properly and argue that the moult strategy of Wrynecks was hitherto incompletely and in many cases mistakenly understood, leading to confusion in the ageing of Wrynecks.

Body Mass and Pectoral Muscle Size Changes in African Waterfowl During Moult

Body mass and organ size dynamics during flight-feather moult vary among waterfowl species. To better understand adaptations of the African waterfowl, we measured how body masses of Spur-winged Goose (Plectropterus gambensis),South African Shelduck (Tadorna cana), Yellow-billed Duck (Anas undulata), Red-billed Teal (Anas erythrorhyncha) and Southern Pochard (Netta erythrophthalma) change during flight-feather moult. We further assessed how pectoral muscle size of Spur-winged Goose and South African Shelduck varied during the same period of flight-feather moult. Our results indicate that Spur-winged Goose and South African Shelduck underwent atrophy and subsequent regeneration of the pectoral muscles, while their body mass decreased at the onset of moult and later stabilized from the time when flight feathers were two-thirds grown until moult was completed. Body mass of Yellow-billed Duck and Red-billed Teal decreased from the onset of moult until the mid-point but thereafter increased rapidly, returning to pre-moult levels by the time moult was completed. Southern Pochard gradually lost mass from the start of moult almost until moult completion, at which time mass increased slightly.We conclude that African waterfowl exhibit different fluctuations in body mass and pectoral muscle size during flight-feather moult. Taken together, these findings suggest that no single hypothesis can fully explain the interspecific differences in the moult strategies of African waterfowl as reflected in changes in body and pectoral muscle conditions.

Non-moulted primary coverts correlate with rapid primary moulting

Time constraint is a main factor which affects the moult strategies in passerines, mainly during the first year of life. The variability of moult strategies between species is associated with the extent of the moult. In the first year of life, the extent of the moult is highly variable between species and individuals. In most passerine species, juveniles only renew some of their feathers, but the factors that govern which feathers are renewed and which are retained have been largely overlooked. Here we examine the common pattern of non-moulted primary coverts (PC) in passerines during the first-year moult cycle (post-juvenile and first-year pre-breeding moults). On the interspecific level we found that among 63 species of passerines, PCs are the least commonly moulted feather tract. For five species (Hirundo rustica, Pycnonotus xanthopygos, Prinia gracilis, Acrocephalus stentoreus and Passer moabiticus) which perform a complete post-juvenile moult, we found that the PC moult occurs over a longer period than greater coverts (GCs) and is sequential (non-simultaneous). At the intraspecific level, we found that the main difference between a partial and complete moult in Prinia gracilis is the moulting or non-moulting of the PCs. We also demonstrate that for Prinia gracilis 1) juveniles which do not moult their PCs, moult their primaries at a higher speed than those which moult their PCs and 2) area/mass ratio of PCs is lower than of GCs. These two findings may explain why many passerines skip PC renewal during the first year of life. Because the PC moult lasts a long time, forgoing this moult enables long term resource savings that allow for dealing with time constraints. Our results highlight the adaptive advantages of non-moulted PCs in cases of time constraints.