Migratory connectivity of Palaearctic–African migratory birds and their responses to environmental change: the serial residency hypothesis

In most long‐distance migratory birds, juveniles migrate without their parents and so are likely to lack detailed knowledge of where to go. This suggests the potential for stochasticity to affect their choice of wintering area at a large scale (> 1000 km). Adults, in contrast, may re‐use non‐breeding sites that promote their survival, so removing uncertainty from their subsequent migrations. I review the evidence for large‐scale stochastic juvenile site selection followed by adult site fidelity, and then develop a ‘serial‐residency’ hypothesis based on these two traits as a framework to explain both the migratory connectivity and the population dynamics of migrant birds and how these are affected by environmental change. Juvenile stochasticity is apparent in the age‐dependent effects of weather or experimental displacement on the outcome of migration and in the very wide variation in the destinations of individuals originating from the same area. Adults have been shown to be very faithful to their wintering grounds and even to staging sites. The serial residency hypothesis predicts that migrants that show these two traits will rely on an individually unique but fixed series of temporally and spatially linked sites to complete their annual cycle. As a consequence, migratory connectivity will be apparent at a very small scale for individuals, but only at a large scale for a population, and juveniles are predicted to occur more often at less suitable sites than adults, so that survival will be lower for juveniles. Migratory connectivity will arise only through spatial and temporal autocorrelation with local environmental constraints, particularly on passage, and the distribution and age structure of the population may reflect past environmental constraints. At least some juveniles will discover suitable habitat that they may re‐use as adults, thus promoting overall population‐level resilience to environmental change, and suggesting value in site‐based conservation. However, because migratory connectivity only acts on a large scale, any population of migrants will contain individuals that encounter a change in suitability somewhere in their non‐breeding range, so affecting average survival. Differences in population trends will therefore reflect variation in local breeding output added to average survival from wintering and staging areas. The latter is likely to be declining given increasing levels of environmental degradation throughout Africa. Large‐scale migratory connectivity also has implications for the evolutionary ecology of migrants, generally because this is likely to lead to selection for generalist traits.     

High juvenile mortality during migration in a declining population of a long‐distance migratory raptor

Many populations of long‐distance migrants are declining and there is increasing evidence that declines may be caused by factors operating outside the breeding season. Among the four vulture species breeding in the western Palaearctic, the species showing the steepest population decline, the Egyptian Vulture Neophron percnopterus, is a long‐distance migrant wintering in Africa. However, the flyways and wintering areas of the species are only known for some populations, and without knowledge of where mortality occurs, effective conservation management is not possible. We tracked 19 juvenile Egyptian Vultures from the declining breeding population on the Balkan Peninsula between 2010 and 2014 to estimate survival and identify important migratory routes and wintering areas for this species. Mortality during the first autumn migration was high (monthly survival probability 0.75) but mortality during migration was exclusively associated with suboptimal navigation. All birds from western breeding areas and three birds from central and eastern breeding areas attempted to fly south over the Mediterranean Sea, but only one in 10 birds survived this route, probably due to stronger tailwind. All eight birds using the migratory route via Turkey and the Middle East successfully completed their first autumn migration. Of 14 individual and environmental variables examined to explain why juvenile birds did or did not successfully complete their first migration, the natal origin of the bird was the most influential. We speculate that in a declining population with fewer experienced adults, an increasing proportion of juvenile birds are forced to migrate without conspecific guidance, leading to high mortality as a consequence of following sub‐optimal migratory routes. Juvenile Egyptian Vultures wintered across a vast range of the Sahel and eastern Africa, and had large movement ranges with core use areas at intermediate elevations in savannah, cropland or desert. Two birds were shot in Africa, where several significant threats exist for vultures at continental scales. Given the broad distribution of the birds and threats, effective conservation in Africa will be challenging and will require long‐term investment. We recommend that in the short term, more efficient conservation could target narrow migration corridors in southern Turkey and the Middle East, and known congregation sites in African wintering areas.     

Identifying critical migratory bottlenecks and high‐use areas for an endangered migratory soaring bird across three continents

Migrant birds face a number of threats throughout their annual cycle, including persecution, collision with energy infrastructure, and habitat and climate change. A key challenge for the conservation of migrants is the identification of important habitat, including migratory concentration areas, because species survival rates may be determined by events in geographically very limited areas. Remote‐tracking technology is facilitating the identification of such critical habitat, although the strategic identification of important sites and incorporation of such knowledge in conservation planning remains limited. We tracked 45 individuals of an endangered, soaring migrant (Egyptian vulture Neophron percnopterus), over 75 complete migrations that traversed three continents along the Red Sea Flyway. We summarize and contextualize migration statistics by season and age class, including migration start, midpoint, and end dates, as well as linear and cumulative migration distance, migration duration and speed, and route straightness. Then, using dynamic Brownian bridge movement models, we quantified space use to identify the most important migratory bottlenecks and high‐use areas on the flyway. These areas each accounted for < 5% of the overall movement range of the tracked birds, yet > 20% of all tracks passed through bottlenecks, and > 50% of the overall vulture time spent on migration fell within high‐use areas. The most important sites were located at the southeastern Red Sea coast and Bab‐el‐Mandeb Strait (Saudi Arabia, Yemen, Djibouti), the Suez Canal zone (Egypt), and the Gulf of Iskenderun (Turkey). Discouragingly however, none of the area within the major migratory bottlenecks was protected and < 13% of the high‐use areas were protected. This demonstrates a very concerning gap in the protected area network for migratory soaring birds along the Red Sea Flyway. Because reducing threats at migratory concentrations can be a very efficient approach to protect populations, our work provides clear guidelines where conservation investment is urgently needed to benefit as many as 35 migratory soaring‐bird species that regularly use the Red Sea Flyway.     

Importance of artificial stopover sites through avian migration flyways: a landfill‐based assessment with the White Stork Ciconia ciconia

Open landfills seem to be playing an increasing role as target feeding areas for several species, not only in their breeding areas or during the winter, but also during the migration period. Evaluating the extent to which landfill sites are used by migrants is crucial to understanding their role in driving stopover decisions during migration, and in the potential health risks linked to feeding on refuse. The aim of this study was to evaluate the role of two open landfills located just before (France) and after (Spain) the East‐Atlantic flyway enters Iberia through the western Pyrenees as potentially important stopover sites for the White Stork populations moving along this route. Overall, we detected that these sites were used by storks that had been ringed from many western European breeding populations, mainly during the migration period, but also in winter. The mean distance between the stork breeding/ringing origin and the landfill sites increased from summer to winter, suggesting that storks breeding further away pass through Iberia later in the season, reflecting population‐specific timing of migration. During the autumn migration period (August–September), the first encountered landfill in France was estimated to be used by c. 1200 storks, and the other in Spain by 4000 storks. Our study hence contributes to a better understanding of the current and potentially hazardous role played by landfill sites in White Stork ecology, which is essential in order to provide management recommendations, and to evaluate the consequences of proposed open landfill closures in Europe.     

Size‐dependent costs of migration: Migrant bird species are subordinate to residents,but only at small body sizes

Migrant species are commonly thought to be poor competitors in aggressive interactions with resident species. However, no studies have tested whether this relationship is widespread. Here, we compare the behavioural dominance of closely related species of migratory and nonmigratory birds, testing whether migrants are consistently subordinate to resident species in aggressive contests. We compiled published behavioural dominance data involving migrant and resident congeners, gathering additional data on the body mass and migratory distance of each species. Focal species included a diverse array of birds (28 taxonomic families, 12 orders) from around the world. We found that migrant species are usually subordinate to resident species, but that this relationship disappears at larger body sizes. For smaller birds (<500 g), resident species were behaviourally dominant in 83%–88% of comparisons; for larger birds (>500 g), resident species were dominant in only 25%–30% of comparisons. The relative difference in body mass best predicted dominance relationships among species, with larger species dominant in 80%–84% of comparisons. When migrant and resident masses were equal, however, resident species were still more likely to be dominant in smaller birds, suggesting that other factors may also contribute to the subordinate status of migrants. Overall, our results suggest that in smaller species, the evolution of migration is associated with lighter weights and other traits that compromise the competitive abilities of migrants relative to residents. In contrast, larger species appear able to evolve migration without compromising their size or competitive abilities in aggressive contests, suggesting size‐dependent constraints on the evolution of migration.     

Habitat‐mediated timing of migration in polar bears: an individual perspective

Migration phenology is largely determined by how animals respond to seasonal changes in environmental conditions. Our perception of the relationship between migratory behavior and environmental cues can vary depending on the spatial scale at which these interactions are measured. Understanding the behavioral mechanisms behind population‐scale movements requires knowledge of how individuals respond to local cues. We show how time‐to‐event models can be used to predict what factors are associated with the timing of an individual's migratory behavior using data from GPS collared polar bears (Ursus maritimus) that move seasonally between sea ice and terrestrial habitats. We found the concentration of sea ice that bears experience at a local level, along with the duration of exposure to these conditions, was most associated with individual migration timing. Our results corroborate studies that assume thresholds of >50% sea ice concentration are necessary for suitable polar bear habitat; however, continued periods (e.g., days to weeks) of exposure to suboptimal ice concentrations during seasonal melting were required before the proportion of bears migrating to land increased substantially. Time‐to‐event models are advantageous for examining individual movement patterns because they account for the idea that animals make decisions based on an accumulation of knowledge from the landscapes they move through and not simply the environment they are exposed to at the time of a decision. Understanding the migration behavior of polar bears moving between terrestrial and marine habitat, at multiple spatiotemporal scales, will be a major aspect of quantifying observed and potential demographic responses to climate‐induced environmental changes.     

Extreme genetic similarity does not predict non‐breeding distribution of two closely related warblers

Detailed knowledge of migratory connectivity can facilitate effective conservation of Neotropical migrants by helping biologists understand where and when populations may be most limited. We studied the migratory behavior and non‐breeding distribution of two closely related species of conservation concern, the Golden‐winged Warbler (Vermivora chrysoptera) and Blue‐winged Warbler (Vermivora cyanoptera). Although both species have undergone dynamic range shifts and population changes attributed to habitat loss and social interactions promoting competition and hybridization, full life‐cycle conservation planning has been limited by a lack of information about their non‐breeding ecology. Because recent work has demonstrated that the two species are nearly identical genetically, we predicted that individuals from a single breeding population would have similar migratory timing and overwintering locations. In 2015, we placed light‐level geolocators on 25 males of both species and hybrids in an area of breeding sympatry at the Fort Drum Military Installation in Jefferson and Lewis counties, New York. Despite extreme genetic similarity, non‐breeding locations and duration of migration differed among genotypes. Golden‐winged Warblers (N = 2) overwintered > 1900 km southeast of the nearest Blue‐winged Warbler (N = 3) and spent nearly twice as many days in migration; hybrids (N = 2) had intermediate wintering distributions and migratory timing. Spring migration departure dates were staggered based on distance from the breeding area, and all birds arrived at the breeding site within 8 days of each other. Our results show that Golden‐winged Warblers and Blue‐winged Warblers in our study area retain species‐specific non‐breeding locations despite extreme genetic similarity, and suggest that non‐breeding locations and migratory timing vary along a genetic gradient. If the migratory period is limiting for these species, our results also suggest that Golden‐winged Warblers in our study population may be more vulnerable to population decline than Blue‐winged Warblers because they spend almost twice as many days migrating.     

Sex and age-specific differences in wing pointedness and wing length in blackcaps Sylvia atricapilla migrating through the southern Baltic coast

The blackcap Sylvia atricapilla shows a complex migratory pattern and is a suitable species for the studies of morphological migratory syndrome, including adaptations of wing shape to different migratory performance. Obligate migrants of this species that breed in northern, central, and Eastern Europe differ by migration distance and some cover shorter distance to the wintering grounds in the southern part of Europe/North Africa or the British Isles, although others migrate to sub-Saharan Africa. Based on ˃40 years of ringing data on blackcaps captured during autumn migration in the Southern Baltic region, we studied age- and sex-related correlations in wing pointedness and wing length of obligate blackcap migrants to understand the differences in migratory behavior of this species. Even though the recoveries of blackcaps were scarce, we reported some evidence that individuals which differ in migration distance differed also in wing length. We found that wing pointedness significantly increased with an increasing wing length of migrating birds, and adults had longer and more pointed wings than juvenile birds. This indicates stronger antipredator adaptation in juvenile blackcaps than selection on flight efficiency, which is particularly important during migration. Moreover, we documented more pronounced differences in wing length between adult and juvenile males and females. Such differences in wing length may enhance a faster speed of adult male blackcaps along the spring migration route and may be adaptive when taking into account climatic effects, which favor earlier arrival from migration to the breeding grounds.     

Long‐term changes in the migration phenology of UK breeding birds detected by large‐scale citizen science recording schemes

The timing of migration is one of the key life‐history parameters of migratory birds. It is expected to be under strong selection, to be sensitive to changing environmental conditions and to have implications for population dynamics. However, most phenological studies do not describe arrival and departure phenologies for a species in a way that is robust to potential biases, or that can be clearly related to breeding populations. This hampers our ability to understand more fully how climate change may affect species’ migratory strategies, their life histories and ultimately their population dynamics. Using generalized additive models (GAMs) and extensive large‐scale data collected in the UK over a 40‐year period, we present standardized measures of migration phenology for common migratory birds, and examine how the phenology of bird migration has changed in the UK since the 1960s. Arrival dates for 11 of 14 common migrants became significantly earlier, with six species advancing their arrival by more than 10 days. These comprised two species, Blackcap Sylvia atricapilla and Chiffchaff Phylloscopus collybita, which winter closest to Britain in southern Europe and the arid northern zone of Africa, Common Redstart Phoenicurus phoenicurus, which winters in the arid zone, and three hirundines (Sand Martin Riparia riparia, House Martin Delichon urbicum and Barn Swallow Hirundo rustica), which winter in different parts of Africa. Concurrently, departure dates became significantly later for four of the 14 species and included species that winter in southern Europe (Blackcap and Chiffchaff) and in humid zones of Africa (Garden Warbler Sylvia borin and Whinchat Saxicola rubetra). Common Swift Apus apus was the exception in departing significantly earlier. The net result of earlier arrival and later departure for most species was that length of stay has become significantly longer for nine of the 14 species. Species that have advanced their timing of arrival showed the most positive trends in abundance, in accordance with previous studies. Related in part to earlier arrival and the relationship above, we also show that species extending their stay in Great Britain have shown the most positive trends. Further applications of our modelling approach will provide opportunities for more robust tests of relationships between phenological change and population dynamics than have been possible previously.     

The effects of long‐distance migration on the evolution of moult strategies in Western‐Palearctic passerines

Although feathers are the unifying characteristic of all birds, our understanding of the causes, mechanisms, patterns and consequences of the feather moult process lags behind that of other major avian life‐history phenomena such as reproduction and long‐distance migration. Migration, which evolved in many species of the temperate and arctic zones, requires high energy expenditure to endure long‐distance journeys. About a third of Western‐Palearctic passerines perform long‐distance migrations of thousands of kilometres each year using various morphological, physiological, biomechanical, behavioural and life‐history adaptations. The need to include the largely non‐overlapping breeding, long‐distance migration and feather moult processes within the annual cycle imposes a substantial constraint on the time over which the moult process can take place. Here, we review four feather‐moult‐related adaptations which, likely due to time constraints, evolved among long‐distance Western‐Palearctic migrants: (i) increased moult speed; (ii) increased overlap between moult and breeding or migration; (iii) decreased extent of plumage moult; and (iv) moult of part or all of the plumage during the over‐wintering period in the tropics rather than in the breeding areas. We suggest that long‐distance migration shaped the evolution of moult strategies and increased the diversity of these strategies among migratory passerines. In contrast to this variation, all resident passerines in the Western Palearctic moult immediately after breeding by renewing the entire plumage of adults and in some species also juveniles, while in other species juvenile moult is partial. We identify important gaps in our current understanding of the moult process that should be addressed in the future. Notably, previous studies suggested that the ancestral moult strategy is a post‐breeding summer moult in the Western Palearctic breeding areas and that moult during the winter evolved due to the scheduling of long‐distance migration immediately after breeding. We offer an alternative hypothesis based on the notion of southern ancestry, proposing that the ancestral moult strategy was a complete moult during the ‘northern winter’ in the Afro‐tropical region in these species, for both adults and juveniles. An important aspect of the observed variation in moult strategies relates to their control mechanisms and we suggest that there is insufficient knowledge regarding the physiological mechanisms that are involved, and whether they are genetically fixed or shaped by environmental factors. Finally, research effort is needed on how global climate changes may influence avian annual routines by altering the scheduling of major processes such as long‐distance migration and feather moult.     

Equal nonbreeding period survival in adults and juveniles of a long‐distant migrant bird

In migrant birds, survival estimates for the different life‐history stages between fledging and first breeding are scarce. First‐year survival is shown to be strongly reduced compared with annual survival of adult birds. However, it remains unclear whether the main bottleneck in juvenile long‐distant migrants occurs in the postfledging period within the breeding ranges or en route. Quantifying survival rates during different life‐history stages and during different periods of the migration cycle is crucial to understand forces driving the evolution of optimal life histories in migrant birds. Here, we estimate survival rates of adult and juvenile barn swallows (Hirundo rustica L.) in the breeding and nonbreeding areas using a population model integrating survival estimates in the breeding ranges based on a large radio‐telemetry data set and published estimates of demographic parameters from large‐scale population‐monitoring projects across Switzerland. Input parameters included the country‐wide population trend, annual productivity estimates of the double‐brooded species, and year‐to‐year survival corrected for breeding dispersal. Juvenile survival in the 3‐week postfledging period was low (S = 0.32; SE = 0.05), whereas in the rest of the annual cycle survival estimates of adults and juveniles were similarly high (S > 0.957). Thus, the postfledging period was the main survival bottleneck, revealing the striking result that nonbreeding period mortality (including migration) is not higher for juveniles than for adult birds. Therefore, focusing future research on sources of variation in postfledging mortality can provide new insights into determinants of population dynamics and life‐history evolution of migrant birds.     

Adverse wind conditions during northward Sahara crossings increase the in‐flight mortality of Black‐tailed Godwits

Long‐distance migratory flights are predicted to be associated with higher mortality rates when individuals encounter adverse weather conditions. However, directly connecting environmental conditions experienced in‐flight with the survival of migrants has proven difficult. We studied how the in‐flight mortality of 53 satellite‐tagged Black‐tailed Godwits (Limosa limosa limosa) during 132 crossings of the Sahara Desert, a major geographical barrier along their migration route between The Netherlands and sub‐Saharan Africa, is correlated with the experienced wind conditions and departure date during both southward and northward migration. We show that godwits experienced higher wind assistance during southward crossings, which seems to reflect local prevailing trade winds. Critically, we found that fatal northward crossings (15 deaths during 61 crossings) were associated with adverse wind conditions. Wind conditions during migration can thus directly influence vital rates. Changing wind conditions associated with global change may thus profoundly influence the costs of long‐distance migration in the future.     

Short‐ and long‐term costs of reproduction in a migratory songbird

Costs of reproduction represent a common life‐history trade‐off. Critical to understanding these costs in migratory species is the ability to track individuals across successive stages of the annual cycle. We assessed the effects of total number of offspring fledged and date of breeding completion on pre‐migratory body condition, the schedule of moult and annual survival in a migratory songbird, the Savannah Sparrow Passerculus sandwichensis. Between 2008 and 2010, moult was delayed for individuals that finished breeding later in the breeding period and resulted in reduced lean tissue mass during the pre‐migratory period, suggesting an indirect trade‐off between the timing of breeding completion and condition just prior to migration. Lean tissue mass decreased as the number of offspring fledged increased in 2009, a particularly cool and wet year, illustrating a direct trade‐off between reproductive effort and condition just prior to migration in years when weather is poor. However, using a 17‐year dataset from the same population, we found that parents that fledged young late in the breeding period had the highest survival and that number of offspring fledged did not affect survival, suggesting that individuals do not experience long‐term trade‐offs between reproduction and survival. Taken together, our results suggest that adult Savannah Sparrows pay short‐term costs of reproduction, but that longer‐term costs are mitigated by individual quality, perhaps through individual variation in resource acquisition.     

Is there a connection between weather at departure sites,onset of migration and timing of soaring‐bird autumn migration in Israel?

Aims Different aspects of soaring‐bird migration are influenced by weather. However, the relationship between weather and the onset of soaring‐bird migration, particularly in autumn, is not clear. Although long‐term migration counts are often unavailable near the breeding areas of many soaring birds in the western Palaearctic, soaring‐bird migration has been systematically monitored in Israel, a region where populations from large geographical areas converge. This study tests several fundamental hypotheses regarding the onset of migration and explores the connection between weather, migration onset and arrival at a distant site. Location Globally gridded meteorological data from the breeding areas in north‐eastern Europe were used as predictive variables in relation to the arrival of soaring migrants in Israel. Methods Inverse modelling was used to study the temporal and spatial influence of weather on initiation of migration based on autumn soaring‐bird migration counts in Israel. Numerous combinations of migration duration and temporal influence of meteorological variables (temperature, sea‐level pressure and precipitable water) were tested with different models for meteorological sensitivity. Results The day of arrival in Israel of white storks, honey buzzards, Levant sparrowhawks and lesser spotted eagles was significantly and strongly related to meteorological conditions in the breeding area days or even weeks before arrival in Israel. The cumulative number of days or cumulative value above or below a meteorological threshold performed significantly better than other models tested. Models provided reliable estimates of migration duration for each species. Main conclusions The meteorological triggers of migration at the breeding grounds differed between species and were related to deteriorating living conditions and deteriorating migratory flight conditions. Soaring birds are sensitive to meteorological triggers at the same period every year and their temporal response to weather appears to be constrained by their annual routine.     

Up‐and‐down shift in residence depth of slickheads (Alepocephalidae) revealed by otolith stable oxygen isotopic composition

Otolith δ¹⁸O profiles for four slickhead species (Alepocephalidae) suggested that Alepocephalus umbriceps, Talismania okinawensis and Rouleina watasei migrated hundreds of metres to shallower depths during the juvenile to young stages before returning to their original depth or even deeper waters. Xenodermichthys nodulosus gradually shifted residence depth from shallow to deeper water during their life. These migratory patterns indicated that the slickheads examined had allopatric residence depths at different life stages, which might enhance the pelagic survival and growth rates of the juvenile and young fishes.     

Estimability of migration survival rates from integrated breeding and winter capture–recapture data

Long‐distance migration is a common phenomenon across the animal kingdom but the scale of annual migratory movements has made it difficult for researchers to estimate survival rates during these periods of the annual cycle. Estimating migration survival is particularly challenging for small‐bodied species that cannot carry satellite tags, a group that includes the vast majority of migratory species. When capture–recapture data are available for linked breeding and non‐breeding populations, estimation of overall migration survival is possible but current methods do not allow separate estimation of spring and autumn survival rates. Recent development of a Bayesian integrated survival model has provided a method to separately estimate the latent spring and autumn survival rates using capture–recapture data, though the accuracy and precision of these estimates has not been formally tested. Here, I used simulated data to explore the estimability of migration survival rates using this model. Under a variety of biologically realistic scenarios, I demonstrate that spring and autumn migration survival can be estimated from the integrated survival model, though estimates are biased toward the overall migration survival probability. The direction and magnitude of this bias are influenced by the relative difference in spring and autumn survival rates as well as the degree of annual variation in these rates. The inclusion of covariates can improve the model's performance, especially when annual variation in migration survival rates is low. Migration survival rates can be estimated from relatively short time series (4–5 years), but bias and precision of estimates are improved when longer time series (10–12 years) are available. The ability to estimate seasonal survival rates of small, migratory organisms opens the door to advancing our understanding of the ecology and conservation of these species. Application of this method will enable researchers to better understand when mortality occurs across the annual cycle and how the migratory periods contribute to population dynamics. Integrating summer and winter capture data requires knowledge of the migratory connectivity of sampled populations and therefore efforts to simultaneously collect both survival and tracking data should be a high priority, especially for species of conservation concern.     

Green‐wave surfing increases fat gain in a migratory ungulate

Each spring, migratory herbivores around the world track or ‘surf’ green waves of newly emergent vegetation to distant summer or wet‐season ranges. This foraging tactic may help explain the great abundance of migratory herbivores on many seasonal landscapes. However, the underlying fitness benefits of this life‐history strategy remain poorly understood. A fundamental prediction of the green‐wave hypothesis is that migratory herbivores obtain fitness benefits from surfing waves of newly emergent vegetation more closely than their resident counterparts. Here we evaluate whether this behavior increases body‐fat levels – a critically important correlate of reproduction and survival for most ungulates – in elk Cervus elaphus of the Greater Yellowstone Ecosystem. Using satellite imagery and GPS tracking data, we found evidence that migrants (n = 23) indeed surfed the green wave, occupying sites 12.7 days closer to peak green‐up than residents (n = 16). Importantly, individual variation in surfing may help account for up to 6 kg of variation in autumn body‐fat levels. Our findings point to a pathway for anthropogenic changes to the green wave (e.g. climate change) or migrants’ ability to surf it (e.g. development) to impact migratory populations. To explore this possibility, we evaluated potential population‐level consequences of constrained surfing with a heuristic model. If green‐wave surfing deteriorates by 5–15 days from observed, our model predicts up to a 20% decrease in pregnancy rates, a 2.5% decrease in population growth, and a 30% decrease in abundance over 50 years. By linking green‐wave surfing to fitness and illustrating potential effects on population growth, our study provides new insights into the evolution of migratory behavior and the prospects for the persistence of migratory ungulate populations in a changing world.     

Patterns of bird migration phenology in South Africa suggest northern hemisphere climate as the most consistent driver of change

Current knowledge of phenological shifts in Palearctic bird migration is largely based on data collected on migrants at their breeding grounds; little is known about the phenology of these birds at their nonbreeding grounds, and even less about that of intra‐African migrants. Because climate change patterns are not uniform across the globe, we can expect regional disparities in bird phenological responses. It is also likely that they vary across species, as species show differences in the strength of affinities they have with particular habitats and environments. Here, we examine the arrival and departure of nine Palearctic and seven intra‐African migratory species in the central Highveld of South Africa, where the former spend their nonbreeding season and the latter their breeding season. Using novel analytical methods based on bird atlas data, we show phenological shifts in migration of five species – red‐backed shrike, spotted flycatcher, common sandpiper, white‐winged tern (Palearctic migrants), and diederik cuckoo (intra‐African migrant) – between two atlas periods: 1987–1991 and 2007–2012. During this time period, Palearctic migrants advanced their departure from their South African nonbreeding grounds. This trend was mainly driven by waterbirds. No consistent changes were observed for intra‐African migrants. Our results suggest that the most consistent drivers of migration phenological shifts act in the northern hemisphere, probably at the breeding grounds.     

Conservation planning for freshwater–marine carryover effects on Chinook salmon survival

Experiences of migratory species in one habitat may affect their survival in the next habitat, in what is known as carryover effects. These effects are especially relevant for understanding how freshwater experience affects survival in anadromous fishes. Here, we study the carryover effects of juvenile salmon passage through a hydropower system (Snake and Columbia rivers, northwestern United States). To reduce the direct effect of hydrosystem passage on juveniles, some fishes are transported through the hydrosystem in barges, while the others are allowed to migrate in‐river. Although hydrosystem survival of transported fishes is greater than that of their run‐of‐river counterparts, their relative juvenile‐to‐adult survival (hereafter survival) can be less. We tested for carryover effects using generalized linear mixed effects models of survival with over 1 million tagged Chinook salmon, Oncorhynchus tshawytscha (Walbaum) (Salmonidae), migrating in 1999–2013. Carryover effects were identified with rear‐type (wild vs. hatchery), passage‐type (run‐of‐river vs. transported), and freshwater and marine covariates. Importantly, the Pacific Decadal Oscillation (PDO) index characterizing cool/warm (i.e., productive/nonproductive) ocean phases had a strong influence on the relative survival of rear‐ and passage‐types. Specifically, transportation benefited wild Chinook salmon more in cool PDO years, while hatchery counterparts benefited more in warm PDO years. Transportation was detrimental for wild Chinook salmon migrating early in the season, but beneficial for later season migrants. Hatchery counterparts benefited from transportation throughout the season. Altogether, wild fish could benefit from transportation approximately 2 weeks earlier during cool PDO years, with still a benefit to hatchery counterparts. Furthermore, we found some support for hypotheses related to higher survival with increased river flow, high predation in the estuary and plume areas, and faster migration and development‐related increased survival with temperature. Thus, pre‐ and within‐season information on local‐ and broad‐scale conditions across habitats can be useful for planning and implementing real‐time conservation programs.     

Predicting conditions for migration: effects of density dependence and habitat quality

Migration is widespread among animals, but the factors that influence the decision to migrate are poorly understood. Within a single species, populations may be completely migratory, completely sedentary or partially migratory. We use a population model to derive conditions for migration and demonstrate how migratory survival, habitat quality and density dependence on both the breeding and non-breeding grounds influence conditions for migration and the proportion of migrants within a population. Density dependence during the season in which migratory and sedentary individuals use separate sites is necessary for partial migration. High levels of density dependence at the non-shared sites widen the range of survival values within which we predict partial migration, whereas increasing the strength of density dependence at the shared sites narrows the range of survival values within which we predict partial migration. Our results have important implications for predicting how contemporary populations with variable migration strategies may respond to changes in the quality or quantity of habitat.