Prealternate Molt Research Articles

Comparison of the Phenotypic Flexibility of Muscle and Body Condition of Migrant and Resident White-Crowned Sparrows.

AbstractSeasonally breeding birds express variations of traits (phenotypic flexibility) throughout their life history stages that represent adaptations to environmental conditions. Changes of body condition during migration have been well studied, whereas alterations of skeletal and cardiac muscles, body mass, and fat scores have yet to be characterized throughout the spring or fall migratory stages. Additionally, we examined flexible patterns of muscle, body mass, and fat score in migrant white-crowned sparrows (Zonotrichia leucophrys gambelii) in comparison with those in a resident subspecies (Zonotrichia leucophrys nuttalli) during the stages they share to evaluate the influence of different life histories. Migrants showed hypertrophy of the pectoralis muscle fiber area on the wintering grounds in late prealternate molt, yet increased pectoralis muscle mass was not detected until birds readied for spring departure. While pectoralis profile and fat scores enlarged at predeparture in spring and fall, pectoralis, cardiac, and body masses were greater only in spring stages, suggesting seasonal differences for migratory preparation. Gastrocnemius mass showed little change throughout all stages, whereas gastrocnemius fiber area declined steadily but rebounded in fall on the wintering grounds, where migrants become more sedentary. In general, residents are heavier birds with larger leg structures, while migrants sport longer wings and greater heart mass. Phenotypic flexibility was most prominent among residents with peaks of pectoralis, gastrocnemius, and body masses during the winter stage, when local weather is most severe. Thus, the subspecies express specific patterns of phenotypic flexibility with peaks coinciding with the stages of heightened energy demands: the winter stage for residents and the spring stages for migrants.

Extensive Prealternate Molts in Peruvian Kelp Gulls

According to current literature, the Kelp Gull (Larus dominicanus) matures at the same rate and molts according to the same patterns as most other large gulls, such as the American Herring (L. argentatus smithsonianus) and Western (L. occidentalis). The Kelp Gull, however, is widespread through the Southern Hemisphere, with separate populations occupying no fewer than four different climate zones and breeding at different times of the year; the molt of some of those populations appears to have not been studied yet. Here we demonstrate that many immature Kelp Gulls of the Peruvian population undergo much more extensive first and second prealternate molts than has been known so far and achieve an adult-like plumage aspect in as soon as 2.5 years rather than the usual 4. In these respects, these Kelp Gulls recall the Yellow-footed Gull (L. livens) or subspecies heuglini and fuscus of the Lesser Black-backed Gull (L. fuscus).

Sexual selection, feather wear, and time constraints on the pre-basic molt explain the acquisition of the pre-alternate molt in European passerines.

Avian feathers need to be replaced periodically to fulfill their functions, with natural, social, and sexual selection presumably driving the evolution of molting strategies. In temperate birds, a common pattern is to molt feathers immediately after the breeding season, the pre‐basic molt. However, some species undergo another molt in winter‐spring, the pre‐alternate molt. Using a sample of 188 European passerine species, Bayesian phylogenetic mixed models, and correlated evolution analyses, we tested whether the occurrence of the pre‐alternate molt was positively associated with proxies for sexual selection (sexual selection hypothesis) and nonsexual social selection (social selection hypothesis), and with factors related to feather wear (feather wear hypothesis) and time constraints on the pre‐basic molt (time constraints hypothesis). We found that the pre‐alternate molt was more frequent in migratory and less gregarious species inhabiting open/xeric habitats and feeding on the wing, and marginally more frequent in species with strong sexual selection and those showing a winter territorial behavior. Moreover, an increase in migratory behavior and sexual selection intensity preceded the acquisition of the pre‐alternate molt. These results provide support for the feather wear hypothesis, partial support for the sexual selection and time constraints hypotheses, and no support for the social selection hypothesis.

Molt Strategies by Age and Subspecies in the Willow Flycatcher

We clarify the molt strategies of the Willow Flycatcher (Empidonax traillii) by subspecies through the examination of 455 museum specimens. Most and typically all juvenile primary coverts are retained during the preformative molt in fall and winter, allowing yearlings to be distinguished from older birds from their first spring through their second fall. In addition to replacing the body feathers and some to (usually) all secondary coverts and tertials, the preformative molt can include no other remiges or rectrices (46% of specimens), all remiges and rectrices (33%), or some remiges in an “eccentric” sequence (21%). During the prealternate molt, replacement of upper wing coverts and tertials is much less extensive than in the preformative molt, varying from replacement of no feathers in 27% of specimens to replacing more than half of the median and greater coverts and all three tertials in other specimens. Both the preformative and the first prealternate molts are significantly more extensive in eastern E. t. traillii than in the three western subspecies (brewsteri, adastus, and extimus). The definitive prealternate molt is also significantly more extensive than the first prealternate molt in the western subspecies but not in traillii. These differences between traillii and the other subspecies may result from longer migration distances, hence more solar exposure on an annual basis, and different molt-strategy dynamics between eastern and western North American passerines, perhaps relating to preferences for moister habitats. Further investigation is needed on the timing of molts on the winter grounds and the extent of body-feather replacement during the prealternate molt.

Molt patterns and age and sex determination criteria for selected landbirds in an urban area of the Peruvian Central Coast

Despite molting is an important and expensive activity in the life cycle of birds, this process is poorly known for neotropical species and even less so for species inhabiting urban zones. We studied molt patterns, aging and sexing criteria, and morphometrics for seven common resident species in an urban area of ​​Lima city, Peru. All species evidenced complete prebasic molts. Partial to eccentric incomplete preformative molts were evidenced for six species (Camptostoma obsoletum, Pyrocephalus rubinus, Catamenia analis, Poospiza hispaniolensis, Coereba flaveola, and Thraupis episcopus); whereas Columbina cruziana exhibited a complete molt. All thraupid species exhibited partial prealternate molts. Preformative and prebasic molts were recorded mostly during the dry season, and the few records of prealternate molts might suggest its occurrence soon after the wet season. Our species also exhibited low molt synchrony, with some of them (C. cruziana, P. rubinus, and C. flaveola) exhibiting extensive molt periods likely explained by its potential to reproduce throughout the year and/or the existence of suspended/ protracted molts. Morphometric analysis evidenced significant differences according to sex for C. cruziana, P. rubinus, and C. analis, with the wing chord being the most common characteristic for distinguishing between sexes. Our study covers an important gap in the knowledge of the natural history of urban birds and provides information to test hypotheses of molt pattern variation on species with widespread distribution throughout the Americas.

Clarification of molt strategies in three Empidonax flycatchers

We studied plumages and molt patterns of Yellow-bellied (Empidonax flaviventris), Least (E. minimus), and Alder (E. alnorum) flycatchers, primarily on the breeding grounds but with additional data from wintering grounds and a migration stopover site, in order to clarify our knowledge of molt strategies in these species. We determined that these 3 species have a similar overall molt strategy: (1) a partial-to-incomplete preformative molt begun on the breeding grounds, suspended over migration, and completed on the wintering grounds; (2) a limited-to-partial first prealternate molt on the wintering grounds that can overlap the final stages of the preformative molt; (3) a complete definitive prebasic molt begun on summer grounds and completed on wintering grounds; and (4) a limited-to-partial definitive prealternate molt that may overlap with the final stages of the prebasic molt. Our study clarifies previous terminologies and allows these molts to be interpreted in an evolutionary framework. We suggest that observed differences in molt extents among species may be linked to differences in wintering latitude and habitat. The broadleaf forest preferred by Yellow-bellied Flycatcher likely retains insect biomass later into the winter season than the scrub habitat preferred by Least Flycatcher, and this may promote more extensive preformative feather replacement in that species. The much more extensive preformative molt of Alder Flycatcher may be due to its wintering farther south, where there is greater annual solar exposure, perhaps resulting in higher rates of feather degradation. Our data also suggest that the extent of prealternate molt in these species may vary with proximate factors on the winter grounds affecting individuals, rather than body condition at the beginning of winter.

Evolution of winter molting strategies in European and North American migratory passerines.

Molt is critical for birds as it replaces damaged feathers and worn plumage, enhancing flight performance, thermoregulation, and communication. In passerines, molt generally occurs on the breeding grounds during the postbreeding period once a year. However, some species of migrant passerines that breed in the Nearctic and Western Palearctic regions have evolved different molting strategies that involve molting on the overwintering grounds. Some species forego molt on the breeding grounds and instead complete their prebasic molt on the overwintering grounds. Other species molt some or all feathers a second time (prealternate molt) during the overwintering period. Using phylogenetic analyses, we explored the potential drivers of the evolution of winter molts in Nearctic and Western Palearctic breeding passerines. Our results indicate an association between longer photoperiods and the presence of prebasic and prealternate molts on the overwintering grounds for both Nearctic and Western Palearctic species. We also found a relationship between prealternate molt and generalist and water habitats for Western Palearctic species. Finally, the complete prealternate molt in Western Palearctic passerines was linked to longer days on the overwintering grounds and longer migration distance. Longer days may favor the evolution of winter prebasic molt by increasing the time window when birds can absorb essential nutrients for molt. Alternatively, for birds undertaking a prealternate molt at the end of the overwintering period, longer days may increase exposure to feather‐degrading ultra‐violet radiation, necessitating the replacement of feathers. Our study underlines the importance of the overwintering grounds in the critical process of molt for many passerines that breed in the Nearctic and Western Palearctic regions.

Moult nestedness and its imperfections: insights to unravel the nature of passerine wing‐feather moult rules

Passerine moult shows two well‐known although intriguing characteristics: 1) a varying subset of wing feathers are retained during certain moult episodes, and 2) the identity of the replaced feathers is not random. However, the rules that underlie these moult features are largely unknown. Non‐randomness could arise under time and energy constraints if feathers differed in their importance, such that they could be ordered following a hypothetical moult rank. This rule predicts a nested relationship among wing‐feather moult phenotypes, where phenotypes of smaller extent would consist of subsets of the wing feathers replaced in phenotypes of larger extent. Here, we analysed a dataset of 3909 preformative and prealternate moult phenotypes from 434 Holarctic and Neotropical passerine species. We found that passerine wing‐feather moult was nested (nodf = 84.31). However, systematic imperfections in the nestedness pattern led us to test whether the existence of different feather moult ranks may increase nestedness in homogeneous subsets of moult phenotypes. We tested this hypothesis between moult episodes and found that nestedness increased for the preformative moult (nodf = 90.99) and decreased for the prealternate moult (nodf = 67.28). We further analysed the adaptive meaning of different moult ranks associated with feather mass, feather exposure and feather contribution to flight. Overall, these three feather traits explained 53.74% of the variation in the priority of wing‐feather replacement, with exposure having a stronger explanatory power than the other two traits. Our findings provide a first model to interpret variation of wing‐feather moult phenotypes in passerines, and outlines the relevance of season for passerine moult, suggesting a dynamic link between moult expression and plumage requirements.

Evolution of breeding plumages in birds: A multiple-step pathway to seasonal dichromatism in New World warblers (Aves: Parulidae).

Many species of birds show distinctive seasonal breeding and nonbreeding plumages. A number of hypotheses have been proposed for the evolution of this seasonal dichromatism, specifically related to the idea that birds may experience variable levels of sexual selection relative to natural selection throughout the year. However, these hypotheses have not addressed the selective forces that have shaped molt, the underlying mechanism of plumage change. Here, we examined relationships between life‐history variation, the evolution of a seasonal molt, and seasonal plumage dichromatism in the New World warblers (Aves: Parulidae), a family with a remarkable diversity of plumage, molt, and life‐history strategies. We used phylogenetic comparative methods and path analysis to understand how and why distinctive breeding and nonbreeding plumages evolve in this family. We found that color change alone poorly explains the evolution of patterns of biannual molt evolution in warblers. Instead, molt evolution is better explained by a combination of other life‐history factors, especially migration distance and foraging stratum. We found that the evolution of biannual molt and seasonal dichromatism is decoupled, with a biannual molt appearing earlier on the tree, more dispersed across taxa and body regions, and correlating with separate life‐history factors than seasonal dichromatism. This result helps explain the apparent paradox of birds that molt biannually but show breeding plumages that are identical to the nonbreeding plumage. We find support for a two‐step process for the evolution of distinctive breeding and nonbreeding plumages: That prealternate molt evolves primarily under selection for feather renewal, with seasonal color change sometimes following later. These results reveal how life‐history strategies and a birds' environment act upon multiple and separate feather functions to drive the evolution of feather replacement patterns and bird coloration.

Unraveling a complex alternate molt strategy: more evidence for prealternate molts and alternate plumages in the Blue-and-yellow Tanager (Pipraeidea bonariensis darwinii)

Some birds replace feathers after the formative plumage of the first cycle and after basic plumages in subsequent cycles as a consequence of prealternate molts. The presence of prealternate molts has been suggested to be due to prolonged exposure to sunlight, especially for birds inhabiting exposed and scrubby areas. Although prealternate molts appear to be common in many Nearctic-Neotropic passerines, these can be also present in resident Neotropical species. For example, prealternate molt has been reported in 14 species among nine genera in thraupids. Here, using wild individuals and museum specimens, we describe the presence of a Complex Alternate Strategy in the molt cycle of the Peru’s Blue-and-yellow Tanager (Pipraeidea bonariensis darwinii). We categorized seven molt cycle–based age groups in 75 wild-captured birds and thirteen molt cycle–based age categories across 167 museum specimens. Our results confirm the existence of limited to partial prealternate molts in Blue-and-yellow Tanager and demonstrate the importance of investigating the presence of these types of molts in wild bird populations.

Wing‐feather moult phenotypes differ between the preformative and prealternate episodes and along passerine phylogeny

Passerine wing‐feather moult has been studied historically in terms of its intensity, duration, timing and extent. However, little is known about variation of wing‐moult phenotypes (i.e. the identity of moulted wing feathers in a given individual) within species, among moult episodes and in relation to passerine phylogeny. Here we studied 5373 wing‐moult cards from 285 Holarctic and 155 Neotropical passerine species. Variation of moult phenotypes, although high, was far below that expected by chance, and the same phenotypes were repeated among individuals within and among species, suggesting the existence of shared mechanisms of moult control. We successfully classified moult phenotypes according to nine moult patterns described in the literature and found an uneven distribution between the preformative and the prealternate moult. Moult patterns were phylogenetically conserved for the preformative but not for the prealternate moult. Our results suggest differential seasonal control of bird moult with preformative moults being associated with (evolutionarily conserved) somatic demands, whereas prealternate moults are probably associated with (rapidly evolving) signalling functions.

Prealternate molt-migration in Rusty Blackbirds and its implications for stopover biology

ABSTRACT To achieve greater understanding of the full annual cycles of birds, it is critical to describe the spatial nature of little-understood phases. One of the least understood aspects of avian annual cycles is the ecology of molt: the periodic replacement of feathers. While work on the spatial nature of molt in migratory passerines has increasingly found incidences of species and populations completing molt during migration, this work has been limited entirely to prebasic flight feather molt. We examined the prevalence and progression of contour feather molt in a migratory songbird, the Rusty Blackbird (Euphagus carolinus), during spring stopover. We found that 98% of birds exhibited a partial prealternate molt during stopover, primarily in the head region. Furthermore, molt intensity peaked in the middle of the migration period and was negatively associated with fat score. This is the first evidence in the passerine literature of an obligate prealternate molt completed during migration, which is in ...

A rare dark morph in the Canadian Arctic raises questions about molting and polymorphism in Long-tailed Jaeger (Stercorarius longicaudus)

ABSTRACT The existence of a dark morph in adult Long-tailed Jaeger (Stercorarius longicaudus) is still unclear and debated. For this species, dark plumages occur in juveniles and immatures, but no dark morph adults have yet been unequivocally confirmed. Ambiguous reports of presumed dark morph adults and lack of information on molt progression of immatures generate confusion regarding this issue. Here we describe observations conducted in the Canadian Arctic (Bylot Island) and show that Long-tailed Jaegers can apparently exhibit dark morph adult-like plumage (i.e., individuals showing some obvious criteria associated with definitive alternate ‘adult breeding' plumage). Careful examination of those sightings, however, also revealed immature characteristics, indicating the high variability of the first prealternate molt in this species. Well-documented observations of rare plumages like the one reported in this study are needed to elucidate molting strategies and polymorphism in Long-tailed Jaeger. Received...

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.

Effects of a social cue on reproductive development and pre-alternate molt in seasonally breeding migrant and resident female songbirds (Zonotrichia leucophrys).

ABSTRACTTo time reproduction optimally, birds have evolved diverse mechanisms by which they respond to environmental changes that help them anticipate and prepare for the breeding season. While residents initiate reproductive preparation and breed in the same geographic location, migrant birds simultaneously prepare for breeding and migration far from their breeding grounds. As a result, it is hypothesized that migrant and resident birds use environmental cues differently to prepare to breed and that there is adaptive specialization in mechanisms regulating reproductive preparation. Specifically, residents are expected to rely more on non-photic cues (e.g. food, temperature, social cues) than migrants. We tested this general prediction using a social cue manipulation. First, we compared the effects of subspecies-appropriate recorded male song on reproductive development in migrants and residents on a naturally increasing photoperiod. Second, we tested the sensitivity of migrant-specific life history events (fattening and pre-alternate molt) to song treatment. After 82 days, residents had higher luteinizing hormone and greater ovarian development than migrants, but song treatment had no effect on these metrics in either subspecies. Song advanced pre-alternate molt but had no effect on fattening in migrants. While our study does not support specialization in social cue use in migrants and residents, it is consistent with findings in the literature of specialization in photoperiodic response. It also demonstrates for the first time that social cues can influence molt in a migrant species. Additional findings from a pilot study looking at responses to a live male suggest it is important to test other kinds of social cues.

Winter diet of Bobolink, a long-distance migratory grassland bird, inferred from feather isotopes

ABSTRACT Effective conservation of migratory bird populations depends on advancements in our understanding of processes throughout the life cycle. Fundamental information about wintering ecology (e.g., habitat use and diet composition) remains limited, which limits assessment of threats to populations during winter. Bobolink (Dolichonyx oryzivorus) is a year-round grassland obligate and Nearctic-Neotropical migrant that undergoes 2 complete molts each year, including a complete prealternate molt on the South American wintering grounds. This unusual winter molt provides a rare opportunity to examine, using stable isotope analysis, the timing and contribution of foraging resources in the Bobolink diet prior to northbound migration from disparate breeding populations. We compared winter diet composition among 3 breeding populations of Bobolinks and during 3 stages of winter molt using stable carbon isotope ratios. We used mixing models to compare the ratio of carbon-12 to carbon-13 isotope (δ13C value) in fe...

Conditions on the Mexican moulting grounds influence feather colour and carotenoids in Bullock's orioles (Icterus bullockii)

Carotenoid‐based plumage coloration plays a critical role for both inter‐ and intrasexual communication. Habitat and diet during molt can have important consequences for the development of the ornamental signals used in these contexts. When molt occurs away from the breeding grounds (e.g., pre‐alternate molt on the wintering grounds, or stopover molt), discerning the influence of habitat and diet can be particularly important, as these effects may result in important carryover effects that influence territory acquisition or mate choice in subsequent seasons. Several species of songbirds in western North America, including the Bullock's oriole (Icterus bullockii), migrate from the breeding grounds to undergo a complete prebasic (post‐breeding) molt at a stopover site in the region affected by the Mexican monsoon climate pattern. This strategy appears to have evolved several times independently in response to the harsh, food‐limited late‐summer conditions in the arid West, which contrast strongly with the high productivity driven by heavy rains that is characteristic of the Mexican monsoon region. Within this region, individuals may be able to optimize plumage coloration by molting in favourable areas characterized by high resource abundance. We used stable isotope analysis (δ13C, δ15N) to ask whether the diet and molt habitat/location of Bullock's orioles influenced their expression of carotenoid‐based plumage coloration as well as plumage carotenoid content and composition. Bullock's orioles with lower feather δ15N values acquired more colorful plumage (orange‐shifted hue) but had feathers with lower total carotenoid concentration, lower zeaxanthin concentration, and marginally lower canthaxanthin and lutein concentration. Examining factors occurring throughout the annual cycle are critical for understanding evolutionary and ecological processes. Here, we demonstrate that conditions experienced during a stopover molt, occurring hundreds to thousands of kilometers from the breeding grounds, influence the production of ornamental plumage coloration, which may carryover to influence inter‐ and intrasexual signaling in subsequent seasons.

Altered expression of pectoral myosin heavy chain isoforms corresponds to migration status in the white-crowned sparrow (Zonotrichia leucophrys gambelii).

Birds undergo numerous changes as they progress through life-history stages, yet relatively few studies have examined how birds adapt to both the dynamic energetic and mechanical demands associated with such transitions. Myosin heavy chain (MyHC) expression, often linked with muscle fibre type, is strongly correlated with a muscle's mechanical power-generating capability, thus we examined several morphological properties, including MyHC expression of the pectoralis, in a long-distance migrant, the white-crowned sparrow (Zonotrichia leucophrys gambelii) throughout the progression from winter, spring departure and arrival on breeding grounds. White-crowned sparrows demonstrated significant phenotypic flexibility throughout the seasonal transition, including changes in prealternate moult status, lipid fuelling, body condition and flight muscle morphology. Pectoral MyHC expression also varied significantly over the course of the study. Wintering birds expressed a single, newly classified adult fast 2 isoform. At spring departure, pectoral isoform expression included two MyHC isoforms: the adult fast 2 isoform along with a smaller proportion of a newly present adult fast 1 isoform. By spring arrival, both adult fast isoforms present at departure remained, yet expression had shifted to a greater relative proportion of the adult fast 1 isoform. Altering pectoral MyHC isoform expression in preparation for and during spring migration may represent an adaptation to modulate muscle mechanical output to support long-distance flight.

Molt strategies of ten neotropical forest passerine species

ABSTRACT We describe the number of inserted molts in the annual molt cycle and topographies resulting from the preformative molt in 10 Neotropical forest passerine species. Seven species showed two annual molt peaks, one in early spring (Mar–Apr) and one in late summer (Jul–Sept). The spring molt consisted of a light replacement of body feathers in <20% of individuals in our sample, and would be most consistent with a prealternate molt. We found two modes of preformative molt feather replacement where the “general mode” included all feathers of an anterior wing-tract being replaced before proceeding to the next posterior tract; the general mode was shown by all 10 species at moderate to high frequencies (69–99%). Seven species at low to mid frequencies (1–31%) exhibited “proximal mode” to include all wing feathers adjacent to the body replaced prior to distal feathers. Mean molt angle (a measure of direction of the replaced feathers when placed on a Cartesian plane) ranged from 12° to 24° and was higher i...

Seasonal Time Keeping in a Long-Distance Migrating Shorebird.

Because of the complications in achieving the necessary long-term observations and experiments, the nature and adaptive value of seasonal time-keeping mechanisms in long-lived organisms remain understudied. Here we present the results of a 20-year-long study of the repeated seasonal changes in body mass, plumage state, and primary molt of 45 captive red knots Calidris canutus islandica, a High Arctic breeding shorebird that spends the nonbreeding season in temperate coastal areas. Birds kept outdoors and experiencing the natural photoperiod of the nonbreeding area maintained sequences of life-cycle stages, roughly following the timing in nature. For 6 to 8 years, 14 of these birds were exposed to unvarying ambient temperature (12 °C) and photoperiodic conditions (12:12 LD). Under these conditions, for at least 5 years they expressed free-running circannual cycles of body mass, plumage state, and wing molt. The circannual cycles of the free-running traits gradually became longer than 12 months, but at different rates. The prebreeding events (onset and offset of prealternate molt and the onset of spring body mass increase) occurred at the same time of the year as in the wild population for 1 or several cycles. As a result, after 4 years in 12:12 LD, the circannual cycles of prealternate plumage state had drifted less than the cycles of prebasic plumage state and wing molt. As the onset of body mass gain drifted less than the offset, the period of high body mass became longer under unvarying conditions. We see these differences between the prebreeding and postbreeding life-cycle stages as evidence for adaptive seasonal time keeping in red knots: the life-cycle stages linked to the initiation of reproduction rely mostly on endogenous oscillators, whereas the later stages rather respond to environmental conditions. Postbreeding stages are also prone to carryover effects from the earlier stages.