Migration phenology and winter habitat quality are related to circulating androgen in a long‐distance migratory bird

In migratory birds, the timing of departure from wintering grounds is often dependant on the quality of habitat on an individual's territory and may influence individual fitness, resulting in an interaction of life history stages across large geographical distances. American redstart Setophaga ruticilla males who overwinter in high quality habitats arrive early to breed and subsequently produce more offspring than late arrivers. Since many migratory species overlap vernal migration with the physiological transition to breeding, we examined if breeding preparation plays a role in this seasonal interaction. We tested the hypothesis that early arriving male redstarts from high quality winter habitats are in superior breeding condition by simultaneously measuring winter habitat quality (stable‐carbon isotopes) and breeding preparation (circulating androgen, cloacal protuberance (CP) diameter) upon arrival at breeding grounds. Compared with late arrivers, early arriving males were from higher quality winter habitats and had higher androgen, but smaller CPs. Males arriving with higher androgen were in more advanced physiological migratory condition, as measured by haematocrit. Early arrivers were more likely to successfully breed, but there was no significant relationship between androgen upon arrival and breeding success. One possible explanation for these relationships is that androgen measured during arrival is most relevant in a migratory context, such that birds with high androgen may benefit from effects on migratory condition, positively influencing fitness through earlier arrival.     

Life history varies with migratory distance in western sandpipers Calidris mauri

The propensity of migratory waders to remain on the non-breeding grounds during the arctic breeding season ("oversummer") in their first biological year of life ("juveniles") may be latitude, and thus migratory distance dependent. We compared the extent of preparation for northward migration of western sandpipers Calidris mauri spending the non-breeding season in México and Panamá during 1995–1998. During winter residency and premigratory periods, we measured body mass and scored the extent of dull basic versus bright alternate breeding plumage of captured juveniles and adults (second biological year or older), and obtained additional plumage scores from observations of uniquely colour banded birds. Nearly all western sandpipers in México prepared for northward migration by increasing body mass and moulting into breeding plumage. In Panamá, most adults prepared for migration, but few, if any, juveniles did so. Patterns of body mass and breeding plumage development do not generally support the hypothesis that oversummering by juveniles results directly from less efficient foraging or from resource competition with adults. We suggest instead that costs directly associated with migratory distance per se influence the life history strategies of sandpipers spending the non-breeding seasons at different latitudes. This latitudinal difference should interact with the well documented sex-ratio cline in non-breeding distribution (male western sandpipers predominating in northern parts of the range and females in southern parts). This suggests that females have more conservative life histories, prioritizing first year survivorship, relative to males that instead weight first-year breeding opportunities.     

Breeding phenology and winter activity predict subsequent breeding success in a trans-global migratory seabird

Inter-seasonal events are believed to connect and affect reproductive performance (RP) in animals. However, much remains unknown about such carry-over effects (COEs), in particular how behaviour patterns during highly mobile life-history stages, such as migration, affect RP. To address this question, we measured at-sea behaviour in a long-lived migratory seabird, the Manx shearwater (Puffinus puffinus) and obtained data for individual migration cycles over 5 years, by tracking with geolocator/immersion loggers, along with 6 years of RP data. We found that individual breeding and non-breeding phenology correlated with subsequent RP, with birds hyperactive during winter more likely to fail to reproduce. Furthermore, parental investment during one year influenced breeding success during the next, a COE reflecting the trade-off between current and future RP. Our results suggest that different life-history stages interact to influence RP in the next breeding season, so that behaviour patterns during winter may be important determinants of variation in subsequent fitness among individuals.     

Timing of breeding carries over to influence migratory departure in a songbird: an automated radiotracking study

1. Determining how events interact across stages of the annual cycle is critical for understanding the factors that affect individual fitness. However, there is currently little information detailing how breeding events influence migratory behaviour. 2. Using an automated digital telemetry array and an isolated island-breeding population of Savannah sparrows Passerculus sandwichensis, we provide the first direct evidence that the timing of breeding events carries over to influence the timing of migration in a songbird and assess for the first time how weather conditions on the breeding grounds also affect departure dates. 3. Date of migratory departure between September and October was strongly influenced by date of breeding completion in adults and fledging date in juveniles from June to July. 4. With respect to weather, adults departed during the first half of high-pressure systems, while juveniles departed throughout the entirety of high-pressure systems (including rainy evenings on the western edge of systems). 5. By combining both ecological and weather data, we could explain almost all variation in departure date for adults (95%), but weather conditions were not a good predictor of departure date for juveniles. 6. Our results provide strong evidence that the timing of breeding events is an important driver of migration timing and that exact departure dates are fine-tuned according to local weather conditions in adults, but not in juveniles.     

Carry‐over effects and compensation: late arrival on non‐breeding grounds affects wing moult but not plumage or schedules of departing bar‐tailed godwits Limosa lapponica baueri

In the annual cycle of migratory birds, temporal and energetic constraints can lead to carry‐over effects, in which performance in one life history stage affects later stages. Bar‐tailed godwits Limosa lapponica baueri, which achieve remarkably high pre‐migratory fuel loads, undertake the longest non‐stop migratory flights yet recorded, and breed during brief high‐latitude summers, may be particularly vulnerable to persistent effects of disruptions to their rigidly‐timed annual routines. Using three years of non‐breeding data in New Zealand, we asked how arrival timing after a non‐stop flight from Alaska (>11 000 km) affected an individual godwit's performance in subsequent flight feather moult, contour feather moults, and migratory departure. Late arrival led to later wing moult, but godwits partially compensated for delayed moult initiation by increasing moult rate and decreasing the total duration of moult. Delays in arrival and wing moult up to 34–37 d had no apparent effect on an individual's migratory departure or extent of breeding plumage at departure, both of which were extraordinarily consistent between years. Thus, ‘errors’ in timing early in the non‐breeding season were essentially corrected in New Zealand prior to spring migration. Variation in migration timing also had no apparent effect on an individual's likelihood of returning the following season. The bar‐tailed godwits’ rigid maintenance of plumage and spring migration schedules, coupled with high annual survival, imply a surprising degree of flexibility to address unforeseen circumstances in the annual cycle.     

Winter rainfall predicts phenology in widely separated populations of a migrant songbird

Climate change is affecting behaviour and phenology in many animals. In migratory birds, weather patterns both at breeding and at non-breeding sites can influence the timing of spring migration and breeding. However, variation in responses to weather across a species range has rarely been studied, particularly among populations that may winter in different locations. We used prior knowledge of migratory connectivity to test the influence of weather from predicted non-breeding sites on bird phenology in two breeding populations of a long-distance migratory bird species separated by 3,000 km. We found that winter rainfall showed similar associations with arrival and egg-laying dates in separate breeding populations on an east–west axis: greater rainfall in Jamaica and eastern Mexico was generally associated with advanced American redstart (Setophaga ruticilla) phenology in Ontario and Alberta, respectively. In Ontario, these patterns of response could largely be explained by changes in the behaviour of individual birds, i.e., phenotypic plasticity. By explicitly incorporating migratory connectivity into responses to climate, our data suggest that widely separated breeding populations can show independent and geographically specific associations with changing weather conditions. The tendency of individuals to delay migration and breeding following dry winters could result in population declines due to predicted drying trends in tropical areas and the tight linkage between early arrival/breeding and reproductive success in long-distance migrants.     

Weak migratory connectivity,loop migration and multiple non-breeding site use in British breeding Whinchats Saxicola rubetra

Determining the links between breeding populations and the pressures, threats and conditions they experience presents a challenge for the conservation of migratory birds which can use multiple sites separated by hundreds to thousands of kilometres. Furthermore, migratory connectivity – the connections made by migrating individuals between networks of breeding and non-breeding sites – has important implications for population dynamics. The Whinchat Saxicola rubetra is declining across its range, and tracking data from a single African non-breeding site implies high migratory spread. We used geolocators to describe the migration routes and non-breeding areas of 20 Whinchats from three British breeding populations. As expected, migratory spread was high, with birds from the three populations overlapping across a wide area of West Africa. On average, in non-breeding areas, British breeding Whinchats were located 652 km apart from one another, with some likely to share non-breeding areas with individuals from breeding populations as far east as Russia. Four males made a direct non-breeding season movement to a second, more westerly, non-breeding location in January. Autumn migration was through Iberia and around the western edge of the Sahara Desert, whereas spring migration was more direct, indicating an anticlockwise loop migration. Weak migratory connectivity implies that Whinchat populations are somewhat buffered against local changes in non-breeding conditions. If non-breeding season processes have played a role in the species’ decline, then large-scale drivers are likely to be the cause, although processes operating on migration, or interactions between breeding and non-breeding processes, cannot be ruled out.     

Density-mediated carry-over effects explain variation in breeding output across time in a seasonal population

In seasonal environments, where density dependence can operate throughout the annual cycle, vital rates are typically considered to be a function of the number of individuals at the beginning of each season. However, variation in density in the previous season could also cause surviving individuals to be in poor physiological condition, which could carry over to influence individual success in the following season. We examine this hypothesis using replicated populations of Drosophila melanogaster, the common fruitfly, over 23 non-overlapping generations with distinct breeding and non-breeding seasons. We found that the density at the beginning of the non-breeding season negatively affected the fresh weight of individuals that survived the non-breeding season and resulted in a 25% decrease in per capita breeding output among those that survived to the next season to breed. At the population level, per capita breeding output was best explained by a model that incorporated density at the beginning of the previous non-breeding season (carry-over effect, COE) and density at the beginning of the breeding season. Our results support the idea that density-mediated COEs are critical for understanding population dynamics in seasonal environments.     

Consistent annual schedules in a migratory shorebird

Many migratory birds start prebreeding moult and premigratory fuelling some months before the breeding season and face severe time constraints, while travelling up to 15,000 km between non-breeding and breeding grounds. Shorebirds typically leave Southern Hemisphere non-breeding areas over a 3-4 week period, but whether they benefit from interannually consistent timing of departure is unknown. Here, I show that individual bar-tailed godwits (Limosa limosa baueri) from New Zealand are highly consistent in their migratory scheduling. Most birds left within the same week each year (between-year repeatability, r, of 0.83) and adult males, which moult into a bright breeding plumage, were also highly repeatable in the extent of their prebreeding moult (r=0.86). This is consistent with the hypothesis that birds have individually optimized migration schedules. Within adult males, but not females, smaller birds tended to migrate earlier than large birds. Whether this reflects differences in size-related migration speed, optimal breeding time at different sites or size-related natural or sexual selection pressures, remains unknown.     

The evolution of migration in a seasonal environment

Despite the fact that migration occurs in a wide variety of taxa worldwide, little is known about the conditions under which migration is expected to evolve from an ancestral resident population. We develop a model that focuses on ecological factors affecting the evolution of migration in a seasonal environment within a genetically explicit framework. We model the evolution of migration for two common types of migration: ‘shared breeding’ where migrants share a breeding ground with residents and migrate to a separate non-breeding area, versus ‘shared non-breeding’, where migrants share a non-breeding ground with residents and migrate to a separate breeding area. Ecologically, migration is more easily established in the shared-breeding case versus the shared-non-breeding case. Genetically, the additive effect of a migratory allele affects its establishment more in the shared-non-breeding case versus the shared-breeding case, whereas the dominance effect of the allele affects its establishment more in the shared-breeding case versus the shared-non-breeding case. Generally, migratory alleles can invade even when residents are competitively superior to migrants during the shared season. Partial migration occurs when the population is polymorphic for migratory and non-migratory alleles, and is dependent upon which season is shared and the additive and dominance behaviour of the migratory allele.     

Regulation of vernal migration in Gambel's white-crowned sparrows: Role of thyroxine and triiodothyronine

Appropriate timing of migratory behavior is critical for migrant species. For many temperate zone birds in the spring, lengthening photoperiod is the initial cue leading to morphological, physiological and behavior changes that are necessary for vernal migration and breeding. Strong evidence has emerged in recent years linking thyroid hormone signaling to the photoinduction of breeding in birds while more limited information suggest a potential role in the regulation of vernal migration in photoperiodic songbirds. Here we investigate the development and expression of the vernal migratory life history stage in captive Gambel's white-crowned sparrows (Zonotrichia leucophrys gambelii) in a hypothyroidic state, induced by chemical inhibition of thyroid hormone production. To explore possible variations in the effects of the two thyroid hormones, triiodothyronine and thyroxine, we subsequently performed a thyroid inhibition coupled with replacement therapy. We found that chemical inhibition of thyroid hormones resulted in complete abolishment of mass gain, fattening, and muscle hypertrophy associated with migratory preparation as well as resulting in failure to display nocturnal restlessness behavior. Replacement of thyroxine rescued all of these elements to near control levels while triiodothyronine replacement displayed partial or delayed rescue. Our findings support thyroid hormones as being necessary for the expression of changes in morphology and physiology associated with migration as well as migratory behavior itself.     

Carry-Over Effects of Nonbreeding Habitat on Start-to-Finish Spring Migration Performance of a Songbird

For migratory animals, conditions during the nonbreeding period may carry-over to influence spring migration performance. Animals in low-quality habitats are predicted to be in poorer condition, show later migration timing, and travel at slower speeds. This can result in subsequent negative effects on fitness. We tested the hypothesis that nonbreeding season body condition and habitat quality carry-over to affect spring migration performance of a long-distance migratory songbird, the Wood Thrush (Hylocichla mustelina). We tracked individual birds between multiple breeding sites in North America and nonbreeding sites in Central America. First, we compared body condition of nonbreeding birds migrating to the same general region of the breeding range with spring migration performance (timing, speed, and duration) obtained from light-level geolocators. Second, we assessed the Normalized Difference Vegetation Index (NDVI) as a proxy for nonbreeding habitat quality, and predicted that birds from wetter habitat or in wetter years (higher NDVI) would show improved migration performance relative to birds from drier sites. We found no evidence of individual-level carry-over effects of nonbreeding season body condition on spring migration performance. Lower NDVI of nonbreeding habitat resulted in delayed spring migration departure, but this effect disappeared by arrival at breeding sites. Birds occupying drier nonbreeding sites migrated faster and for fewer days, compensating for their relatively late departure. We also documented a broader pattern in NDVI and migration timing and distance, in that birds that occupied the wettest areas in the southern part of the nonbreeding range departed significantly later and migrated farther. Our results suggest that individual carry-over effects of nonbreeding habitat quality may be compensated for by a faster and shorter migration strategy. At a broad scale, consistently later spring timing and longer migration distances were associated with the wettest areas (the highest quality habitats) of the Wood Thrush non-breeding range. This supports the theory that high-quality habitats offset the costs of farther migration, resulting in a leap-frog migration pattern.     

Accipiter hawks of the Laurentian Upland and the Interior Plains undertake the longest migrations: insights from birds banded or recovered in Veracruz,Mexico

Bird banding has allowed us to understand diverse aspects of the life histories of migratory raptors. However, most banding stations are located at northern latitudes so what we know about the movements of these raptors is biased toward higher latitudes, primarily from Canada and the United States, leaving important gaps in our knowledge of their movements at lower latitudes. Our objective was to describe the migratory movements of Sharp-shinned (Accipiter striatus) and Cooper’s (A. cooperii) hawks based on banding and recapture records of birds that migrate through Veracruz, Mexico. More specifically, we sought to determine their breeding, migration, and non-breeding locations, estimate their migration distances, and contribute to a better understanding of their migration patterns. With a total of 80 records, we calculated migration distances and used Kernel Density Estimation analyses to identify where these hawks were recaptured or recovered by season. The distribution of recaptures and recoveries largely coincided with breeding locations in the Laurentian Upland and the Interior Plains physiographic regions. All migration records follow a trajectory that extends from the midwestern United States to the Gulf coastal plain of Mexico. The mean breeding season migration distance to Veracruz was 3374 km (a difference of 27 degrees of latitude) for Sharp-shinned Hawks and 2926 km (a difference of 25 degrees of latitude) for Cooper’s Hawks. Our non-breeding records indicate that populations of Accipiter hawks from these North American populations migrate the longest distances to reach Central America, the southernmost distribution of their migratory populations. Distances covered by both species represent round-trip migrations that may be as long as 10,000 km. Our results support those of previous studies and illustrate how continental physiography influences the migration routes, migratory behavior, and migratory connectivity of these hawks.     

A quantitative measure of migratory connectivity

The study of the extent of the connection between areas where populations of birds breed and areas where they winter has flourished in recent years mainly thanks to the development of new techniques, but also due to traditional ringing and recovery schemes, which allow tracking of individuals or populations linking wintering and breeding distributions. Currently, studies on migratory connectivity focus on retention of breeding population spatial structure on the non-breeding grounds and vice versa.Here we propose a method to quantify migratory connectivity based on Mantel correlation coefficients and to statistically test for deviations of the observed connectivity from a random mix of individuals. In addition, we propose a procedure, based on clustering algorithms, to identify whether observed connectivity depends on aggregation of individuals or on rigid transference of distribution patterns between areas.We applied this method to a large dataset of ringing recoveries of barn swallows (Hirundo rustica L) migrating from their Western Palearctic breeding areas to sub-Saharan winter quarters. We show that migration of barn swallow populations connects specific breeding and wintering areas, and that the “sub-populations” quantitatively identified by our method are consistent with qualitative patterns of migratory connectivity identified by studies of individual geographical populations based on other methods. Finally, we tested the performance of the method by running simulations under different scenarios. Such simulations showed that the method is robust and able to correctly detect migratory connectivity even with smaller datasets and when a strong geographical pattern is not present in the population. Our method provides a quantitative measure of migratory connectivity and allows for the identification of populations showing high connectivity between the breeding and wintering areas. This method is suitable for a generalized application to diverse animal taxa as well as to large scale analyses of connectivity for conservation purposes.     

Behavioral and physiological conflicts in migrants: the transition between migration and breeding

Progression of the vernal migratory life history stage to breeding presents a number of apparent behavioral and physiological conflicts. Features that characterize the migratory stage include: high mobility, sociality, repetitive cycles of feeding (hyperphagia and fattening) and migratory flight. Breeding comprises: sedentary, territorial and reproductive behaviors, an initial decline in hyperphagia and reduction of fuel stores. Because morphology, physiology and behavior change, the transition between stages cannot be instantaneous. In many species development of the reproductive system actually occurs during migration, but in others gonadal development may not commence until later. This variation in degree of overlap of migration and reproductive functions is not well understood, but may be related to migratory distance and length of the breeding season, which tends to be shorter at higher latitudes and altitudes. In these habitats, migrants may arrive at their breeding sites to find unpredictable conditions that cannot support breeding. At this juncture, migrants may retreat to refugia and prolong maintenance of facultative migratory functions, termed arrival biology, until conditions improve sufficiently to initiate breeding. In this review, we focus on the Pacific races of the white-crowned sparrow, Zonototrichia leucophyrs, in which the entire spectrum of migratory strategies are represented from resident to long distance migrants and about which much is known. This species presents a unique view of the appearance and variations in arrival biology. Focusing on the juncture between migration and breeding, we discuss the diversity of responses of congeners to a spectrum of environmental conditions that favor survival and reproductive success.     

Strong migratory connectivity and seasonally shifting isotopic niches in geographically separated populations of a long-distance migrating songbird

Whether migratory animals use similar resources during continental-scale movements that characterize their annual cycles is highly relevant to both individual performances and population dynamics. Direct knowledge of the locations and resources used by migrants during non-breeding is generally scarce. Our goal was to estimate migratory connectivity of a small Palaearctic long-distance migrant, the common nightingale Luscinia megarhynchos, and to compare resources used in non-breeding areas with resources used at the breeding grounds. We tracked individuals of three geographically separated populations and characterised their stable isotope niches during breeding and non-breeding over 2 years. Individuals spent the non-breeding period in population-specific clusters from west to central Africa, indicating strong migratory connectivity at the population level. Irrespective of origin, their isotopic niches were surprisingly similar within a particular period, although sites of residence were distant. However, niche characteristics differed markedly between breeding and non-breeding periods, indicating a consistent seasonal isotopic niche shift in the sampled populations. Although nightingales of distinct breeding populations migrated to different non-breeding areas, they chose similar foraging conditions within specific periods. However, nightingales clearly changed resource use between breeding and non-breeding periods, indicating adaptations to changes in food availability.     

Tracking through Life Stages: Adult,Immature and Juvenile Autumn Migration in a Long-Lived Seabird

Seasonal long-distance migration is likely to be experienced in a contrasted manner by juvenile, immature and adult birds, leading to variations in migratory routes, timing and behaviour. We provide the first analysis of late summer movements and autumn migration in these three life stages, which were tracked concurrently using satellite tags, geolocators or GPS recorders in a long-ranging migratory seabird, the Scopoli’s shearwater (formerly named Cory’s shearwater, Calonectris diomedea ) breeding on two French Mediterranean islands. During the late breeding season, immatures foraged around their colony like breeding adults, but they were the only group showing potential prospecting movements around non-natal colonies. Global migration routes were broadly comparable between the two populations and the three life stages, with all individuals heading towards the Atlantic Ocean through the strait of Gibraltar and travelling along the West African coast, up to 8000 km from their colony. However, detailed comparison of timing, trajectory and oceanographic conditions experienced by the birds revealed remarkable age-related differences. Compared to adults and immatures, juveniles made a longer stop-over in the Balearic Sea (10 days vs 4 days in average), showed lower synchrony in crossing the Gibraltar strait, had more sinuous pathways and covered longer daily distances (240 km.d^-1 vs 170 km.d^-1). Analysis of oceanographic habitats along migratory routes revealed funnelling selection of habitat towards coastal and more productive waters with increasing age. Younger birds may have reduced navigational ability and learn progressively fine-scale migration routes towards the more profitable travelling and wintering areas. Our study demonstrates the importance of tracking long-lived species through the stages, to better understand migratory behavior and assess differential exposure to at-sea threats. Shared distribution between life stages and populations make Scopoli’s shearwaters particularly vulnerable to extreme mortality events in autumn and winter. Such knowledge is key for the conservation of critical marine habitats.     

Effects of migration distance on life history strategies of Western and Semipalmated sandpipers in Perú

Migration distances of shorebird species correlate with life history strategies. To assess age‐specific migratory preparation and adult wing‐molt strategies, we studied Western Sandpipers (Calidris mauri) and Semipalmated Sandpipers (C. pusilla) with different migration routes at the Paracas National Reserve in Perú, one of the most austral non‐breeding areas for these sandpipers, from 2012 to 2015. Western Sandpipers breed near the Bering Sea, ~11,000 km from Paracas. Semipalmated Sandpiper populations at Paracas are a mixture of short‐billed birds from western Arctic breeding sites, plus long‐billed birds from eastern sites, ~8000 km distant. Adults of both species arrive in October with primary feathers already partially renewed so wing molt starts at sites further north. Semipalmated Sandpipers with longer bills completed wing molt later than shorter billed birds. Adults of both species prepared for migration in February and March. No juvenile Western Sandpipers prepared for migration, confirming the “slow” over‐summering life history strategy of more southerly non‐breeding populations. Juvenile Semipalmated Sandpipers showed bimodality in strategies. Most showed no migratory preparation, but, during three non‐breeding periods, from 27% to 31% fattened, molted, and partially replaced outer primaries during the pre‐migratory period. Juveniles with longer culmens were heavier and tended to have more alternate plumage. Juveniles that were partially molting primaries had longer culmens and more alternate plumage. Juvenile Semipalmated Sandpipers from eastern‐breeding populations thus have a higher propensity for a fast life history strategy, and western birds a slow one, at this non‐breeding site in Peru. Western‐breeding Semipalmated Sandpiper populations thus resemble Western Sandpipers, suggesting a common, possibly distance‐related, effect on life history strategy.     

Range-wide migration corridors and non-breeding areas of a northward expanding Afro-Palaearctic migrant,the European Bee-eater Merops apiaster

Across their ranges, different populations of migratory species often use separate routes to migrate between breeding and non-breeding grounds. Recent changes in climate and land-use have led to breeding range expansions in many species but it is unclear whether these populations also establish new migratory routes, non-breeding sites and migration phenology. Thus, we compared the migration patterns of European Bee-eaters Merops apiaster from two established western (n = 5) and eastern (n = 6) breeding populations in Europe, with those from a newly founded northern population (n = 19). We aimed to relate the breeding populations to the two known non-breeding clusters in Africa, and to test for similarities of migration routes and timing between the old and new populations. Western Bee-eaters used the western flyway to destinations in West Africa; the eastern birds uniformly headed south to southern African non-breeding sites, confirming a complete separation in time and space between these long-established populations. The recently founded northern population, however, also used a western corridor, but crossed the Mediterranean further east than the western population and overwintered mainly in a new non-breeding area in southern Congo/northern Angola. The migration routes and the new non-breeding range overlapped only slightly with the western, but not with the eastern, population. In contrast, migration phenology appeared to differ between the western and both the northern and the eastern populations, with tracked birds from the western population migrating 2–4 weeks earlier. The northern population thus shares some spatial traits with western Bee-eaters, but similar phenology only with eastern population. This divergence highlights the adjustments in the timing of migration to local environmental conditions in newly founded populations, and a parallel establishment of new breeding and non-breeding sites.     

The scheduling of molt in migratory birds

Summary We model the yearly cycle of small migratory birds to explain the variation in scheduling of complete molt, in particular why some birds molt immediately after breeding on the breeding grounds (summer molt) whereas others migrate to their wintering grounds before molt is initiated (winter molt). We employ the method of dynamic programming, because of its suitability for modelling life history traits. Feather quality and latitude entered the model as state variables and were assumed to affect survival rate and reproductive success. Migration and molt were assumed to be associated with increased mortality risks. By changing the parameters in the model we were able to generate most existing molt patterns, including summer and winter molt, biannual (summer and winter) molt, and molt migration. Our model suggests that the scheduling of molt is basically a result of a trade-off between having a high feather quality during breeding versus during the non-breeding period. A high impact of feather quality on survival rate in combination with low costs of molt resulted in biannual molt. Winter molt became more likely as the survival rateper se increased. A low seasonal amplitude in survival rate is a prerequisite for the occurrence of molt migration. Molt duration, migration costs and reproductive successper se were found to have no impact on the timing of molt. We also investigated the effect of benefits from prior occupancy at breeding and winter grounds.