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This work was funded in part by The Leon Levy Foundation, The Wolf Creek Foundation and the National Science Foundation (ABI sustaining: DBI-1939187; MSA: DEB-2017817; computing support from CNS-1059284 and CCF-1522054). Aim Animal migration is often explained as the result of resource tracking in seasonally dynamic environments. Therefore, resource availability should influence both the distributions of migratory animals and their seasonal abundance. We examined the relationship between primary productivity and the spatio-temporal distributions of migratory birds to assess the role of energy availability in avian migration. Location North America. Time period Full annual cycle, 2011-2016. Major taxa studied Nocturnally migrating landbirds. Methods We used observations of nocturnally migrating landbirds from the eBird community-science programme to estimate weekly spatial distributions of total biomass, abundance and species richness. We related these patterns to primary productivity and seasonal productivity surplus estimated using a remotely sensed measure of vegetation greenness. Results All three avian metrics showed positive spatial associations with primary productivity, and this was more pronounced with seasonal productivity surplus. Surprisingly, biomass showed a weaker association than did abundance and richness, despite being a better indicator of energetic requirements. The strength of associations varied across seasons, being the weakest during migration. During spring migration, avian biomass increased ahead of vegetation green-up in temperate regions, a pattern also previously described for herbivorous waterfowl. In the south-eastern USA, spring green-up was instead associated with a net decrease in biomass, and winter biomass greatly exceeded that of summer, highlighting the region as a winter refuge for short-distance migrants. Main conclusions Although instantaneous energy availability is important in shaping the distribution of migratory birds, the stronger association of productivity with abundance and richness than with biomass suggests the role of additional drivers unrelated to energetic requirements that are nonetheless correlated with productivity. Given recent reports of widespread North American avifaunal declines, including many common species that winter in the south-eastern USA, understanding how anthropogenic activities are impacting winter bird populations in the region should be a research priority. Postprint Postprint

1.Aircraft collisions with birds span the entire history of human aviation, including fatal collisions during some of the first powered human flights. Much effort has been expended to reduce such collisions, but increased knowledge about bird movements and species occurrence could dramatically improve decision support and proactive measures to reduce them. Migratory movements of birds pose a unique, often overlooked, threat to aviation that is particularly difficult for individual airports to monitor and predict the occurrence of birds vary extensively in space and time at the local scales of airport responses. 2. We use two publicly available datasets, radar data from the US NEXRAD network characterizing migration movements and eBird data collected by citizen scientists to map bird movements and species composition with low human effort expenditures but high temporal and spatial resolution relative to other large-scale bird survey methods. As a test case, we compare results from weather radar distributions and eBird species composition with detailed bird strike records from three major New York airports. 3. We show that weather radar-based estimates of migration intensity can accurately predict the probability of bird strikes, with 80% of the variation in bird strikes across the year explained by the average amount of migratory movements captured on weather radar. We also show that eBird-based estimates of species occurrence can, using species’ body mass and flocking propensity, accurately predict when most damaging strikes occur.4. Synthesis and applications. By better understanding when and where different bird species occur, airports across the world can predict seasonal periods of collision risks with greater temporal and spatial resolution; such predictions include potential to predict when the most severe and damaging strikes may occur. Our results highlight the power of federating datasets with bird movement and distribution data for developing better and more taxonomically and ecologically tuned models of likelihood of strikes occurring and severity of strikes.

Ontogenetic niche shifts have helped to understand population dynamics. Here we show that ontogenetic niche shifts also offer an explanation, complementary to traditional concepts, as to why certain species show seasonal migration. We describe how demographic processes (survival, reproduction and migration) and associated ecological requirements of species may change with ontogenetic stage (juvenile, adult) and across the migratory range (breeding, non-breeding). We apply this concept to widely different species (dark-bellied brent geese (Branta b. bernicla), humpback whales (Megaptera novaeangliae) and migratory Pacific salmon (Oncorhynchus gorbuscha) to check the generality of this hypothesis. Consistent with the idea that ontogenetic niche shifts are an important driver of seasonal migration, we find that growth and survival of juvenile life stages profit most from ecological conditions that are specific to breeding areas. We suggest that matrix population modelling techniques are promising to detect the importance of the ontogenetic niche shifts in maintaining migratory strategies. As a proof of concept, we applied a first analysis to resident, partial migratory and fully migratory populations of barnacle geese (Branta leucopsis). We argue that recognition of the costs and benefits of migration, and how these vary with life stages, is important to understand and conserve migration under global environmental change. Electronic supplementary material The online version of this article (10.1007/s00442-020-04682-0) contains supplementary material, which is available to authorized users.

Staggering decline of bird populations Because birds are conspicuous and easy to identify and count, reliable records of their occurrence have been gathered over many decades in many parts of the world. Drawing on such data for North America, Rosenberg et al. report wide-spread population declines of birds over the past half-century, resulting in the cumulative loss of billions of breeding individuals across a wide range of species and habitats. They show that declines are not restricted to rare and threatened species—those once considered common and wide-spread are also diminished. These results have major implications for ecosystem integrity, the conservation of wildlife more broadly, and policies associated with the protection of birds and native ecosystems on which they depend. Science , this issue p. 120

Weather surveillance radars are increasingly used for monitoring the movements and abundances of animals in the airspace. However, analysis of weather radar data remains a specialised task that can be technically challenging. Major hurdles are the difficulty of accessing and visualising radar data on a software platform familiar to ecologists and biologists, processing the low‐level data into products that are biologically meaningful, and summarizing these results in standardized measures. To overcome these hurdles, we developed the open source R package bioRad, which provides a toolbox for accessing, visualizing and analyzing weather radar data for biological studies. It provides functionality to access low‐level radar data, process these data into meaningful biological information on animal speeds and directions at different altitudes in the atmosphere, visualize these biological extractions, and calculate further summary statistics. The package aims to standardize methods for extracting and reporting biological signals from weather radars. Here we describe a roadmap for analyzing weather radar data using bioRad. We also define weather radar equivalents for familiar measures used in the field of migration ecology, such as migration traffic rates, and recommend several good practices for reporting these measures. The bioRad package integrates with low‐level data from both the European radar network (OPERA) and the radar network of the United States (NEXRAD). bioRad aims to make weather radar studies in ecology easier and more reproducible, allowing for better inter‐comparability of studies. Weather surveillance radars are increasingly used for monitoring the movements and abundances of animals in the airspace. However, analysis of weather radar data remains a specialised task that can be technically challenging. Major hurdles are the difficulty of accessing and visualising radar data on a software platform familiar to ecologists and biologists, processing the low-level data into products that are biologically meaningful, and summarizing these results in standardized measures. To overcome these hurdles, we developed the open source R package bioRad, which provides a toolbox for accessing, visualizing and analyzing weather radar data for biological studies. It provides functionality to access low-level radar data, process these data into meaningful biological information on animal speeds and directions at different altitudes in the atmosphere, visualize these biological extractions, and calculate further summary statistics. The package aims to standardize methods for extracting and reporting biological signals from weather radars. Here we describe a roadmap for analyzing weather radar data using bioRad. We also define weather radar equivalents for familiar measures used in the field of migration ecology, such as migration traffic rates, and recommend several good practices for reporting these measures. The bioRad package integrates with low-level data from both the European radar network (OPERA) and the radar network of the United States (NEXRAD). bioRad aims to make weather radar studies in ecology easier and more reproducible, allowing for better inter-comparability of studies.

Nocturnal avian migration flyways remain an elusive concept, as we have largely lacked methods to map their full extent. We used the network of European weather radars to investigate nocturnal bird movements at the scale of the European flyway. We mapped the main migration directions and showed the intensity of movement across part of Europe by extracting biological information from 70 weather radar stations from northern Scandinavia to Portugal, during the autumn migration season of 2016. On average, over the 20 nights and all sites, 389 birds passed per 1 km transect per hour. The night with highest migration intensity showed an average of 1621 birds km–1 h–1 passing the radar stations, but there was considerable geographical and temporal variation in migration intensity. The highest intensity of migration was seen in central France. The overall migration directions showed strong southwest components. Migration dynamics were strongly related to synoptic wind conditions. A wind‐related mass migration event occurred immediately after a change in wind conditions, but quickly diminished even when supporting winds continued to prevail. This first continental‐scale study using the European network of weather radars demonstrates the wealth of information available and its potential for investigating large‐scale bird movements, with consequences for ecosystem function, nutrient transfer, human and livestock health, and civil and military aviation.

Many migratory species have experienced substantial declines that resulted from rapid and massive expansions of human structures and activities, habitat alterations and climate change. Migrants are also recognized as an integral component of biodiversity and provide a multitude of services and disservices that are relevant to human agri-culture, economy and health. The plethora of recently published studies reflects the need for better fundamental knowledge on migrations and for better management of their ecological and human-relevant effects. Yet, where are we in providing answers to fundamental questions and societal challenges?Engaging a broad network of researchers worldwide, we used a horizon-scan approach to identify the most important challenges which need to be overcome in order to gain a fuller understanding of migration ecology, and which could be addressed using radar aeroecological and macroecological approaches. The top challenges include both long-standing and novel topics, ranging from fundamental information on migration routes and phenology, orientation and navigation strategies, and the multitude of effects migrants may have on resident communities, to societal challenges, such as protecting or preventing migrant services and disservices, and the conservation of migrants in the face of environmental changes. We outline these challenges, identify the urgency of addressing them and the primary stakeholders – researchers, policy makers and practitioners, or funders of research

Current climate models and observations indicate that atmospheric circulation is being affected by global climate change. To assess how these changes may affect nocturnally migrating bird populations, we need to determine how current patterns of wind assistance at migration altitudes will be enhanced or reduced under future atmospheric conditions. Here, we use information compiled from 143 weather surveillance radars stations within the contiguous United States to estimate the daily altitude, density, and direction of nocturnal migration during the spring and autumn. We intersected this information with wind projections to estimate how wind assistance is expected to change during this century at current migration altitudes. The prevailing westerlies at midlatitudes are projected to increase in strength during spring migration and decrease in strength to a lesser degree during autumn migration. Southerly winds will increase in strength across the continent during both spring and autumn migration, with the strongest gains occurring in the center of the continent. Wind assistance is projected to increase across the central (0.44 m/s; 10.1%) and eastern portions of the continent (0.32 m/s; 9.6%) during spring migration, and wind assistance is projected to decrease within the central (0.32 m/s; 19.3%) and eastern portions of the continent (0.17 m/s; 6.6%) during autumn migration. Thus, across a broad portion of the continent where migration intensity is greatest, the efficiency of nocturnal migration is projected to increase in the spring and decrease in the autumn, potentially affecting time and energy expenditures for many migratory bird species. These findings highlight the importance of placing climate change projections within a relevant ecological context informed through empirical observations, and the need to consider the possibility that climate change may generate both positive and negative implications for natural systems.

1. Large-scale environmental forces can influence biodiversity at different levels of biological organization. Climate, in particular, is often associated with species distributions and diversity gradients. However, its mechanistic link to population dynamics is still poorly understood. 2. Here, we unravelled the full mechanistic path by which a climatic driver, the Atlantic trade winds, determines the viability of a bird population. 3. We monitored the breeding population of Eleonora's falcons in the Canary Islands for over a decade (2007–2017) and integrated different methods and data to reconstruct how the availability of their prey (migratory birds) is regulated by trade winds. We tracked foraging movements of breeding adults using GPS, monitored departure of migratory birds using weather radar and simulated their migration trajectories using an individual-based, spatially explicit model. 4. We demonstrate that regional easterly winds regulate the flux of migratory birds that is available to hunting falcons, determining food availability for their chicks and consequent breeding success. By reconstructing how migratory birds are pushed towards the Canary Islands by trade winds, we explain most of the variation (up to 86%) in annual productivity for over a decade. 5. This study unequivocally illustrates how a climatic driver can influence local-scale demographic processes while providing novel evidence of wind as a major determinant of population fitness in a top predator

1. Advances in information technology are increasing the use of radar as a tool to investigate and monitor bird migration movements. We set up a field campaign to compare and validate outputs from different radar systems.2. Here we compare the pattern of nocturnal bird migration movements recorded by four different radar systems at a site in southern Sweden. Within the range of the weather radar (WR) Ängelholm, we operated a “BirdScan” (BS) dedicated bird radar, a standard marine radar (MR), and a tracking radar (TR).3. The measures of nightly migration intensities, provided by three of the radars (WR, BS, MR), corresponded well with respect to the relative seasonal course of migration, while absolute migration intensity agreed reasonably only between WR and BS. Flight directions derived from WR, BS and TR corresponded very well, despite very different sample sizes. Estimated mean ground speeds differed among all four systems. The correspondence among systems was highest under clear sky conditions and at high altitudes.4. Synthesis and applications. While different radar systems can provide useful information on nocturnal bird migration, they have distinct strengths and weaknesses, and all require supporting data to allow for species level inference. Weather radars continuously detect avian biomass flows across a wide altitude band, making them a useful tool for monitoring and predictive applications at regional to continental scales that do not rely on resolving individuals. BirdScan and marine radar’s strengths are in local and low altitude applications, such as collision risks with man‐made structures and airport safety, although marine radars should not be trusted for absolute intensities of movement. In quantifying flight behaviour of individuals, tracking radars are the most informative.

Billions of nocturnally migrating birds move through increasingly photopolluted skies, relying on cues for navigation and orientation that artificial light at night (ALAN) can impair. However, no studies have quantified avian responses to powerful ground-based light sources in urban areas. We studied effects of ALAN on migrating birds by monitoring the beams of the National September 11 Memorial & Museum’s “Tribute in Light” in New York, quantifying behavioral responses with radar and acoustic sensors and modeling disorientation and attraction with simulations. This single light source induced significant behavioral alterations in birds, even in good visibility conditions, in this heavily photopolluted environment, and to altitudes up to 4 km. We estimate that the installation influenced ≈1.1 million birds during our study period of 7 d over 7 y. When the installation was illuminated, birds aggregated in high densities, decreased flight speeds, followed circular flight paths, and vocalized frequently. Simulations revealed a high probability of disorientation and subsequent attraction for nearby birds, and bird densities near the installation exceeded magnitudes 20 times greater than surrounding baseline densities during each year’s observations. However, behavioral disruptions disappeared when lights were extinguished, suggesting that selective removal of light during nights with substantial bird migration is a viable strategy for minimizing potentially fatal interactions among ALAN, structures, and birds. Our results also highlight the value of additional studies describing behavioral patterns of nocturnally migrating birds in powerful lights in urban areas as well as conservation implications for such lighting installations.

Many species of migratory birds have evolved the ability to migrate at night, and the recent and rapid expansion of artificial light at night has markedly altered the nighttime sky through which they travel. Migrating birds regularly pass through heavily illuminated landscapes, and bright lights affect avian orientation. But risks to migrating birds from artificial light are not spatially or temporally uniform, representing a challenge for mitigating potential hazards and developing action plans to catalog risks at continental scales. We leveraged over two decades of remote-sensing data collected by weather surveillance radar and satellite-based sensors to identify locations and times of year when the highest numbers of migrating birds are exposed to light pollution in the contiguous US. Our continental-scale quantification of light exposure provides a novel opportunity for dynamic and targeted conservation strategies to address the hazards posed by light pollution to nocturnally migrating birds.

Large parts of the continents are continuously scanned by terrestrial weather radars to monitor precipitation and wind conditions. These systems also monitor the mass movements of bird, bat, and insect migration, but it is still unknown how many of these systems perform with regard to detection and quantification of migration intensities of the different groups. In this study that was undertaken within five regions across Europe and the Middle East we examined to what extent bird migration intensities derived from different weather radars are comparable between each other and relate to intensities measured by local small‐scaled radars, some of them specifically developed to monitor birds. Good correspondence was found for the relative day‐to‐day pattern in migration intensities among most radar systems that were compared. Absolute intensities varied between different systems and regions. The findings of this study can be used to infer about absolute bird migration intensities measured by different radar systems and consequently help resolving methodological issues regarding the estimation of migrant numbers in the Western‐Palearctic region. It further depicts a scientific basis for the future monitoring of migratory bird populations across a large spatio‐temporal scale, predicting their movements and studying its consequences on ecological systems and human lives. We thank, Klaus Stephan (‘Deutscher Wetterdienst’ DWD), Cédric Legal (Météo France) and Günther Haase (BALTRAD, SMHI) for supporting the access to the WR data. We acknowledge the European Operational Program for Exchange of Weather Radar Information (EUMETNET/OPERA) for providing access to European radar data, faciliated through a research-only license agreement between EUMETNET/OPERA members and ENRAM. We thank Femern A/S for allowing us to publish data from the EIA (northern Germany). We thank Asaf Mayrose for facilitating radar data processing and analysis from northern Israel. We thank two reviewers for their helpful comments.Funding – The study was financially support by COST – European Cooperation in Science and Technology – through the Action ES1305 ‘European Network for the Radar Surveillance of Animal Movement’ (ENRAM) for facilitating international collaboration. The Belgian case-study was part of the project RAVEN, which was funded by the Belgian Federal Science Policy office (BELSPO) under the BRAIN-be programme.

Avian migration is one of Earth's largest processes of biomass transport, involving billions of birds. We estimated continental biomass flows of nocturnal avian migrants across the contiguous United States using a network of 143 weather radars. We show that, relative to biomass leaving in autumn, proportionally more biomass returned in spring across the southern United States than across the northern United States. Neotropical migrants apparently achieved higher survival during the combined migration and non-breeding period, despite an average three- to fourfold longer migration distance, compared with a more northern assemblage of mostly temperate-wintering migrants. Additional mortality expected with longer migration distances was probably offset by high survival in the (sub)tropics. Nearctic-Neotropical migrants relying on a 'higher survivorship' life-history strategy may be particularly sensitive to variations in survival on the overwintering grounds, highlighting the need to identify and conserve important non-breeding habitats.

Summary Climate warming challenges animals to advance their timing of reproduction [ 1 ], but many animals appear to be unable to advance at the same rate as their food species [ 2 , 3 ]. As a result, mismatches can arise between the moment of largest food requirements for their offspring and peak food availability [ 4 , 5 , 6 ], with important fitness consequences [ 7 ]. For long-distance migrants, adjustment of phenology to climate warming may be hampered by their inability to predict the optimal timing of arrival at the breeding grounds from their wintering grounds [ 8 ]. Arrival can be advanced if birds accelerate migration by reducing time on stopover sites [ 9 , 10 ], but a recent study suggests that most long-distance migrants are on too tight a schedule to do so [ 11 ]. This may be different for capital-breeding migrants, which use stopovers not only to fuel migration but also to acquire body stores needed for reproduction [ 12 , 13 , 14 ]. By combining multiple years of tracking and reproduction data, we show that a long-distance migratory bird (the barnacle goose, Branta leucopsis) accelerates its 3,000 km spring migration to advance arrival on its rapidly warming Arctic breeding grounds. As egg laying has advanced much less than arrival, they still encounter a phenological mismatch that reduces offspring survival. A shift toward using more local resources for reproduction suggests that geese first need to refuel body stores at the breeding grounds after accelerated migration. Although flexibility in body store use allows migrants to accelerate migration, this cannot solve the time constraint they are facing under climate warming.

1. Broad-scale land conversions and fertilizer use have dramatically altered the available staging area for herbivorous long-distance migrants. Instead of natural land, these birds rely increasingly on pastures for migratory fuelling and stopover, often conflicting with farming practices. To predict and manage birds' future habitat use, the relative advantages and disadvantages of natural (e.g. saltmarsh, intertidal) versus anthropogenic staging sites for foraging need to be understood.2. We compared the migratory staging of brent geese on saltmarsh and pasture sites in spring. Food quality (nitrogen and fibre content), antagonistic behaviour, and body weight were quantified at nearby sites in simultaneous seasons. Individuals were tracked with high-resolution GPS and accelerometers to compare timing of migration and time budgets during fuelling.3. On pastures, birds rested more and experienced higher ingestion rates, similar or superior food quality and reduced antagonistic interactions than on saltmarsh.4. Brent geese using fertilized grasslands advanced their fuelling and migration schedules compared to those using saltmarsh. Pasture birds reached heavy weights earlier, departed sooner, and arrived in the Arctic earlier.5. Intertidal mudflats were frequently visited by saltmarsh birds during the day, and available food there (algae, some seagrass) was of higher quality than terrestrial resources. Availability of intertidal resources was an important factor balancing the otherwise more favourable conditions on pastures relative to saltmarsh.6. Synthesis and applications. Disadvantages of longer foraging effort, more antagonistic interactions and delayed fuelling schedules on traditional saltmarshes may cause geese to exchange this traditional niche in favour of pastures, especially in a warming climate that requires advancement of migratory schedules. However, due to its high quality, intertidal forage can complement terrestrial foraging, potentially removing the incentive for habitat switches to pastures. The relatively high quality of green algae and seagrass, and birds' remarkable preference for these resources when available, provides a key for managers to create landscapes that can sustain this specialist's intertidal lifestyle. To keep natural habitats attractive to staging geese with the purpose of preventing conflicts with farming practices, management actions should focus on conservation and restoration of saltmarsh and especially intertidal habitat.

SUMMARYEconomic development and energy exploration are increasing in the Arctic. Important breeding habitats for many waterbird species, which have previously been relatively undisturbed, are now being subjected to these anthropogenic pressures. The conservation of the habitats and the species they support is a significant challenge for sustainable development. Even if governments and corporates operating in this fragile environment are committed to sustainable development, there is little information available to avoid, mitigate and manage environmental risk and impacts. Taking a risk management perspective, we followed the International Finance Corporations’ (IFC) Performance Standard 6 (PS6) criteria on Environmental and Social Sustainability and developed an approach to identify “critical habitat”, as defined in IFC PS6, for waterbird species breeding in the Arctic. While the range of these waterbirds is roughly known, more accuracy is needed for proper risk assessment.We have therefore gone a step further by modelling suitable habitat within these ranges. Depending on the relevance of the species for IFC PS6 and the level of certainty we separated the classes likely and potential critical habitat. We tested the approach for Russian breeding populations of five Anatidae species (White-fronted Goose Anser albifrons, Lesser White-fronted Goose Anser erythropus, Brent Goose Branta bernicla, Redbreasted Goose Branta ruficollis and Bewick’s Swan Cygnus columbianus bewicki). Likely critical habitats were identified through a review of literature and available data for these waterbird species and multi-species congregations. To address the information gap for most of the Russian Arctic a species distribution modelling approach was used. The outputs of this approach were labelled as potential critical habitat, indicating the lower level of certainty than likely critical habitat.Based on existing information the amount of likely critical habitat is estimated to be at least x,xxx,xxx km2. For the five Anatidae species, X,XXX,XXX km2 potential critical habitat was identified; 95% of these areas were outside of the area boundaries of likely critical habitat for the species.Insufficient data in the east of the study area did affect the results, as some areas known to support breeding populations were not identified as suitable. Conversely, species’ distributions may be overpredicted in other areas; It should also be recognized that the analyzed species currently have depressed populations and may therefore only utilize a proportion of suitable habitat available.For risk assessment purposes however, it is better to predict false positives, rather than false negatives. The study indicates that there are large areas in the Arctic that are potentially important for each of the Anatidae species modelled, but are not yet recognised as key important areas. The results confirm that there is still much to learn about waterbird distribution and abundance in the Russian Arctic.Synthesis and applications The critical habitat maps produced do not just provide a new source of information for the economic development sector, but provide it in a way that is relevant to the sector and directly applicable. The maps are useful for initial risk assessments of potential developments, to identify likely impacts and to consider mitigation options, in accordance with IFC PS6. Risk assessors should exercise caution and detailed surveys for any development in areas predicted to be suitable for each species should be carried out.

Foraging distributions are thought to be density‐dependent, because animals not only select for a high availability and quality of resources, but also avoid conspecific interference. Since these processes are confounded, their relative importance in shaping foraging distributions remains poorly understood. Here we aimed to rank the contribution of density‐dependent and density‐independent effects on the spatio‐temporal foraging patterns of eurasian oystercatchers. In our intertidal study area, tides caused continuous variation in oystercatcher density, providing an opportunity to disentangle conspecific interference and density‐independent interactions with the food landscape. Spatial distributions were quantified using high‐resolution individual tracking of foraging activity and location. In a model environment that included a realistic reconstruction of both the tides and the benthic food, we tested a family of behaviour‐based optimality models against these tracking data. Density‐independent interactions affected spatial distributions much more strongly than conspecific interference, even in an interference‐prone species like oystercatchers. Spatial distributions were governed by avoidance of bill injury costs, selection for high interference‐free intake rates and a decreasing availability of benthic bivalve prey after their exposure. These density‐independent interactions outweighed interference competition in terms of effect size. We suggest that the bottleneck in our mechanistic understanding of foraging distributions may be primarily the role of density‐independent prey attributes unrelated to intake rates, like damage costs in the case of oystercatchers foraging on perilous prey. At a landscape scale, above the finest inter‐individual distances, effects of conspecific interaction on spatial distributions may have been overemphasised.

Migratory animals provide a multitude of services and disservices—with benefits or costs in the order of billions of dollars annually. Monitoring, quantifying, and forecasting migrations across continents could assist diverse stakeholders in utilizing migrant services, reducing disservices, or mitigating human–wildlife conflicts. Radars are powerful tools for such monitoring as they can assess directional intensities, such as migration traffic rates, and biomass transported. Currently, however, most radar applications are local or small scale and therefore substantially limited in their ability to address large-scale phenomena. As weather radars are organized into continent-wide networks and also detect “biological targets,” they could routinely monitor aerial migrations over the relevant spatial scales and over the timescales required for detecting responses to environmental perturbations. To tap these unexploited resources, a concerted effort is needed among diverse fields of expertise and among stakeholders to recognize the value of the existing infrastructure and data beyond weather forecasting.

The behavioural rhythms of organisms are thought to be under strong selection, influenced by the rhythmicity of the environment1,2,3,4. Such behavioural rhythms are well studied in isolated individuals under laboratory conditions1,5, but free-living individuals have to temporally synchronize their activities with those of others, including potential mates, competitors, prey and predators6,7,8,9,10. Individuals can temporally segregate their daily activities (for example, prey avoiding predators, subordinates avoiding dominants) or synchronize their activities (for example, group foraging, communal defence, pairs reproducing or caring for offspring)6,7,8,9,11. The behavioural rhythms that emerge from such social synchronization and the underlying evolutionary and ecological drivers that shape them remain poorly understood5,6,7,9. Here we investigate these rhythms in the context of biparental care, a particularly sensitive phase of social synchronization12 where pair members potentially compromise their individual rhythms. Using data from 729 nests of 91 populations of 32 biparentally incubating shorebird species, where parents synchronize to achieve continuous coverage of developing eggs, we report remarkable within- and between-species diversity in incubation rhythms. Between species, the median length of one parent’s incubation bout varied from 1–19 h, whereas period length—the time in which a parent’s probability to incubate cycles once between its highest and lowest value—varied from 6–43 h. The length of incubation bouts was unrelated to variables reflecting energetic demands, but species relying on crypsis (the ability to avoid detection by other animals) had longer incubation bouts than those that are readily visible or who actively protect their nest against predators. Rhythms entrainable to the 24-h light–dark cycle were less prevalent at high latitudes and absent in 18 species. Our results indicate that even under similar environmental conditions and despite 24-h environmental cues, social synchronization can generate far more diverse behavioural rhythms than expected from studies of individuals in captivity5,6,7,9. The risk of predation, not the risk of starvation, may be a key factor underlying the diversity in these rhythms., The study was supported by the Max Planck Society (to B.K.). M.B. is a PhD student in the International Max Planck Research School for Organismal Biology.

By altering its flight altitude, a bird can change the atmospheric conditions it experiences during migration. Although many factors may influence a bird's choice of altitude, wind is generally accepted as being the most influential. However, the influence of wind is not clearly understood, particularly outside the trade-wind zone, and other factors may play a role. We used operational weather radar to measure the flight altitudes of nocturnally migrating birds during spring and autumn in the Netherlands. We first assessed whether the nocturnal altitudinal distribution of proportional bird density could be explained by the vertical distribution of wind support using three different methods. We then used generalized additive models to assess which atmospheric variables, in addition to altitude, best explained variability in proportional bird density per altitudinal layer each night. Migrants generally remained at low altitudes, and flight altitude explained 52 and 73% of the observed variability in proportional bird density in spring and autumn, respectively. Overall, there were weak correlations between altitudinal distributions of wind support and proportional bird density. Improving tailwind support with height increased the probability of birds climbing to higher altitude, but when birds did fly higher than normal, they generally concentrated around the lowest altitude with acceptable wind conditions. The generalized additive model analysis also indicated an influence of temperature on flight altitudes, suggesting that birds avoided colder layers. These findings suggested that birds increased flight altitudes to seek out more supportive winds when wind conditions near the surface were prohibitive. Thus, birds did not select flight altitudes only to optimize wind support. Rather, they preferred to fly at low altitudes unless wind conditions there were unsupportive of migration. Overall, flight altitudes of birds in relation to environmental conditions appear to reflect a balance between different adaptive pressures.

In different ecosystems herbivores highly prefer particular plant species. This is often explained in a stoichiometric framework of nutrient-based plant adaptations to herbivory. We hypothesize that such super-palatability can also arise as an evolutionary by-product of osmoregulatory adaptations of plants to stressful environmental conditions, as salinity, drought and cold. Here, we investigate in a coastal salt marsh why some plant species are highly preferred by migratory brent geese Branta bernicla bernicla in spring while others are avoided. This salt marsh is an important spring staging site for the geese. Sufficient energy storage in a short period is critical to enable their northward migration to Siberia and subsequent reproduction. We test if geese prefer plants that balance their internal osmotic potential with the saline environment through energy-rich soluble sugars over plant species that use (compartmentalized) salts for this. We find that plant nitrogen and acid detergent fiber content, classic predictors of herbivore preferences, poorly explain which plants the geese prefer. Instead, plant species that are highly preferred by the geese adapt to salinity by high soluble sugar concentrations while avoided species do this by high plant salt concentrations. Thus, the type of osmoregulatory adaptation to stress displayed by different plant species is a good predictor for the food preference of geese on this salt marsh. We suggest that variation in other types of osmoregulation-based stress adaptations, as plant cold adaptations in tundras and plant drought adaptations in savannas, have similar important consequences for trophic interactions. This article is protected by copyright. All rights reserved.

Anthropogenic disturbances of wildlife, such as noise, human presence, hunting activity, and motor vehicles, are becoming an increasing concern in conservation biology. Fireworks are an important part of celebrations worldwide, and although humans often find fireworks spectacular, fireworks are probably perceived quite differently by wild animals. Behavioral responses to fireworks are difficult to study at night, and little is known about the negative effects fireworks may have on wildlife. Every year, thousands of tons of fireworks are lit by civilians on New Year's Eve in the Netherlands. Using an operational weather radar, we quantified the reaction of birds to fireworks in 3 consecutive years. Thousands of birds took flight shortly after midnight, with high aerial movements lasting at least 45 min and peak densities measured at 500 m altitude. The highest densities were observed over grasslands and wetlands, including nature conservation sites, where thousands of waterfowl rest and feed. The Netherlands is the most important winter staging area for several species of waterfowl in Europe. We estimate that hundreds of thousands of birds in the Netherlands take flight due to fireworks. The spatial and temporal extent of disturbance is substantial, and potential consequences are discussed. Weather radar provides a unique opportunity to study the reaction of birds to fireworks, which has otherwise remained elusive. Copyright 2011, Oxford University Press.

At temperate latitudes the synoptic patterns of bird migration are strongly structured by the presence of cyclones and anticyclones, both in the horizontal and altitudinal dimensions. In certain synoptic conditions, birds may efficiently cross regions with opposing surface wind by choosing a higher flight altitude with more favourable wind. We observed migratory passerines at mid-latitudes that selected high altitude wind optima on particular nights, leading to the formation of structured migration layers at varying altitude up to 3 km. Using long-term vertical profiling of bird migration by C-band Doppler radar in the Netherlands, we find that such migration layers occur nearly exclusively during spring migration in the presence of a high-pressure system. A conceptual analytic framework providing insight into the synoptic patterns of wind assistance for migrants that includes the altitudinal dimension has so far been lacking. We present a simple model for a baroclinic atmosphere that relates vertical profiles of wind assistance to the pressure and temperature patterns occurring at temperate latitudes. We show how the magnitude and direction of the large scale horizontal temperature gradient affects the relative gain in wind assistance that migrants obtain through ascending. Temperature gradients typical for northerly high-pressure systems in spring are shown to cause high altitude wind optima in the easterly sectors of anticyclones, thereby explaining the frequent observations of high altitude migration in these synoptic conditions. Given the recurring synoptic arrangements of pressure systems across temperate continents, the opportunities for exploiting high altitude wind will differ between flyways, for example between easterly and westerly oceanic coasts.