Macro-scale relationship between body mass and timing of bird migration.

Clarifying migration timing and its link with underlying drivers is fundamental to understanding the evolution of bird migration. However, previous studies have focused mainly on environmental drivers such as the latitudes of seasonal distributions and migration distance, while the effect of intrinsic biological traits remains unclear. Here, we compile a global dataset on the annual cycle of migratory birds obtained by tracking 1531 individuals and 177 populations from 186 species, and investigate how body mass, a key intrinsic biological trait, influenced timings of the annual cycle using Bayesian structural equation models. We find that body mass has a strong direct effect on departure date from non-breeding and breeding sites, and indirect effects on arrival date at breeding and non-breeding sites, mainly through its effects on migration distance and a carry-over effect. Our results suggest that environmental factors strongly affect the timing of spring migration, while body mass affects the timing of both spring and autumn migration. Our study provides a new foundation for future research on the causes of species distribution and movement.

Artificial light at night is a top predictor of bird migration stopover density.

As billions of nocturnal avian migrants traverse North America, twice a year they must contend with landscape changes driven by natural and anthropogenic forces, including the rapid growth of the artificial glow of the night sky. While airspaces facilitate migrant passage, terrestrial landscapes serve as essential areas to restore energy reserves and often act as refugia-making it critical to holistically identify stopover locations and understand drivers of use. Here, we leverage over 10 million remote sensing observations to develop seasonal contiguous United States layers of bird migrant stopover density. In over 70% of our models, we identify skyglow as a highly influential and consistently positive predictor of bird migration stopover density across the United States. This finding points to the potential of an expanding threat to avian migrants: peri-urban illuminated areas may act as ecological traps at macroscales that increase the mortality of birds during migration.

Seasonal species richness of birds on the world's islands and its geographical correlates.

The presence of migratory birds on islands results in seasonal variation in species richness. These patterns and their geographical correlates within the context of island biogeography theory have not been examined. We used 21 years of bird observations on 690 islands from eBird to determine how seasonal species richness estimates vary as a function of island area, isolation and latitude. Species richness was highest on islands within the northern mid-latitudes during migration and on islands within tropical latitudes during the non-breeding season. Area defined positive, nonlinear relationships with species richness across seasons, with the steepest slopes occurring with islands greater than 1100 km2. Distance to mainland defined negative, nonlinear relationships with species richness across seasons, with the strongest slopes occurring with islands located greater than 150 km from the mainland. Species-area relationships were weakest for the most remote islands and strongest for islands at intermediate distances to the mainland. Intermediate proximity to other islands was a poor predictor of species richness. Our findings emphasize the presence of seasonally dynamic geographical relationships, the enhanced role of evolutionary processes on larger islands, the unique ecology of the world's most remote islands, and the importance of islands as stopover sites and wintering grounds for migratory bird species.

GPS tracking data of Eurasian oystercatchers (Haematopusostralegus) from the Netherlands and Belgium.

We describe six datasets that contain GPS and accelerometer data of 202 Eurasian oystercatchers (Haematopusostralegus) spanning the period 2008-2021. Birds were equipped with GPS trackers in breeding and wintering areas in the Netherlands and Belgium. We used GPS trackers from the University of Amsterdam Bird Tracking System (UvA-BiTS) for several study purposes, including the study of space use during the breeding season, habitat use and foraging behaviour in the winter season, and impacts of human disturbance. To enable broader usage, all data have now been made open access. Combined, the datasets contain 6.0 million GPS positions, 164 million acceleration measurements and 7.0 million classified behaviour events (i.e., flying, walking, foraging, preening, and inactive). The datasets are deposited on the research repository Zenodo, but are also accessible on Movebank and as down-sampled occurrence datasets on the Global Biodiversity Information Facility (GBIF) and Ocean Biodiversity Information System (OBIS).

Nocturnal foraging lifts time constraints in winter for migratory geese but hardly speeds up fueling.

Climate warming advances the optimal timing of breeding for many animals. For migrants to start breeding earlier, a concurrent advancement of migration is required, including premigratory fueling of energy reserves. We investigate whether barnacle geese are time constrained during premigratory fueling and whether there is potential to advance or shorten the fueling period to allow an earlier migratory departure. We equipped barnacle geese with GPS trackers and accelerometers to remotely record birds' behavior, from which we calculated time budgets. We examined how time spent foraging was affected by the available time (during daylight and moonlit nights) and thermoregulation costs. We used an energetic model to assess onset and rates of fueling and whether geese can further advance fueling by extending foraging time. We show that, during winter, when facing higher thermoregulation costs, geese consistently foraged at night, especially during moonlit nights, in order to balance their energy budgets. In spring, birds made use of the increasing day length and gained body stores by foraging longer during the day, but birds stopped foraging extensively during the night. Our model indicates that, by continuing nighttime foraging throughout spring, geese may have some leeway to advance and increase fueling rate, potentially reaching departure body mass 4 days earlier. In light of rapid climatic changes on the breeding grounds, whether this advancement can be realized and whether it will be sufficient to prevent phenological mismatches remains to be determined.

Ontogenetic niche shifts as a driver of seasonal migration.

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.

Cascading effects of climate variability on the breeding success of an edge population of an apex predator.

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. Here, we unravelled the full mechanistic path by which a climatic driver, the Atlantic trade winds, determines the viability of a bird population. 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. 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. 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.

Los factores ambientales a gran escala afectan a la biodiversidad a distintos niveles de organización. El clima en particular, a menudo se asocia a la distribución de especies y gradientes de diversidad. Sin embargo, los mecanismos que lo vinculan con la dinámica de poblaciones siguen siendo poco conocidos. En este estudio revelamos el mecanismo a través del cual un factor climático, los vientos Alisios atlánticos, determinan la viabilidad de una población de una especie de ave rapaz. Monitorizamos la población canaria de Halcón de Eleonor durante una década (2007­2017) e integramos distintos datos y métodos para reconstruir cómo la disponibilidad de alimento (pequeñas aves migratorias) es regulada por los vientos Alisios. Además, monitorizamos los movimientos de caza de adultos reproductores mediante GPS y el inicio de la migración de sus presas mediante un radar meteorológico, y simulamos la trayectoria de migración de estas presas utilizando un modelo espacialmente explícito basado en el individuo. Demostramos que los patrones de vientos del este regulan el flujo de aves migratorias que determina la disponibilidad de alimento para los halcones y sus pollos y, por tanto, su éxito reproductor. Al reconstruir cómo las aves migratorias son desviadas hasta las Islas Canarias por los vientos Alisios conseguimos explicar la mayor parte de la variación (hasta el 86%) en la productividad anual de los halcones durante una década. Este estudio ilustra cómo un fenómeno climático a gran escala puede afectar a los procesos demográficos a escala local y aporta nueva evidencia de que el viento puede ser un importante factor determinante de la eficacia biológica de un predador.

Decline of the North American avifauna.

Species extinctions have defined the global biodiversity crisis, but extinction begins with loss in abundance of individuals that can result in compositional and functional changes of ecosystems. Using multiple and independent monitoring networks, we report population losses across much of the North American avifauna over 48 years, including once-common species and from most biomes. Integration of range-wide population trajectories and size estimates indicates a net loss approaching 3 billion birds, or 29% of 1970 abundance. A continent-wide weather radar network also reveals a similarly steep decline in biomass passage of migrating birds over a recent 10-year period. This loss of bird abundance signals an urgent need to address threats to avert future avifaunal collapse and associated loss of ecosystem integrity, function, and services.

Projected changes in wind assistance under climate change for nocturnally migrating bird populations.

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.

Aeroecology of a solar eclipse.

Light cues elicit strong responses from nearly all forms of life, perhaps most notably as circadian rhythms entrained by periods of daylight and darkness. Atypical periods of darkness, like solar eclipses, provide rare opportunities to study biological responses to light cues. By using a continental scale radar network, we investigated responses of flying animals to the total solar eclipse of 21 August 2017. We quantified the number of biological targets in the atmosphere at 143 weather radar stations across the continental United States to investigate whether the decrease in light and temperature at an atypical time would initiate a response like that observed at sunset, when activity in the atmosphere usually increases. Overall, biological activity decreased in the period leading to totality, followed by a short low-altitude spike of biological activity during totality in some radars. This pattern suggests that cues associated with the eclipse were insufficient to initiate nocturnal activity comparable to that occurring at sunset but sufficient to suppress diurnal activity.

Seasonal abundance and survival of North America's migratory avifauna determined by weather radar.

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.

Arctic Geese Tune Migration to a Warming Climate but Still Suffer from a Phenological Mismatch.

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 [4-6], 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 [12-14]. 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.

High-intensity urban light installation dramatically alters nocturnal bird migration.

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.

Analyzing time-ordered event data with missed observations.

A common problem with observational datasets is that not all events of interest may be detected. For example, observing animals in the wild can difficult when animals move, hide, or cannot be closely approached. We consider time series of events recorded in conditions where events are occasionally missed by observers or observational devices. These time series are not restricted to behavioral protocols, but can be any cyclic or recurring process where discrete outcomes are observed. Undetected events cause biased inferences on the process of interest, and statistical analyses are needed that can identify and correct the compromised detection processes. Missed observations in time series lead to observed time intervals between events at multiples of the true inter-event time, which conveys information on their detection probability. We derive the theoretical probability density function for observed intervals between events that includes a probability of missed detection. Methodology and software tools are provided for analysis of event data with potential observation bias and its removal. The methodology was applied to simulation data and a case study of defecation rate estimation in geese, which is commonly used to estimate their digestive throughput and energetic uptake, or to calculate goose usage of a feeding site from dropping density. Simulations indicate that at a moderate chance to miss arrival events (p = 0.3), uncorrected arrival intervals were biased upward by up to a factor 3, while parameter values corrected for missed observations were within 1% of their true simulated value. A field case study shows that not accounting for missed observations leads to substantial underestimates of the true defecation rate in geese, and spurious rate differences between sites, which are introduced by differences in observational conditions. These results show that the derived methodology can be used to effectively remove observational biases in time-ordered event data.

From Agricultural Benefits to Aviation Safety: Realizing the Potential of Continent-Wide Radar Networks.

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.

Unexpected diversity in socially synchronized rhythms of shorebirds.

The behavioural rhythms of organisms are thought to be under strong selection, influenced by the rhythmicity of the environment. Such behavioural rhythms are well studied in isolated individuals under laboratory conditions, but free-living individuals have to temporally synchronize their activities with those of others, including potential mates, competitors, prey and predators. 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). The behavioural rhythms that emerge from such social synchronization and the underlying evolutionary and ecological drivers that shape them remain poorly understood. Here we investigate these rhythms in the context of biparental care, a particularly sensitive phase of social synchronization 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 captivity. The risk of predation, not the risk of starvation, may be a key factor underlying the diversity in these rhythms.

Innovative Visualizations Shed Light on Avian Nocturnal Migration.

Globally, billions of flying animals undergo seasonal migrations, many of which occur at night. The temporal and spatial scales at which migrations occur and our inability to directly observe these nocturnal movements makes monitoring and characterizing this critical period in migratory animals' life cycles difficult. Remote sensing, therefore, has played an important role in our understanding of large-scale nocturnal bird migrations. Weather surveillance radar networks in Europe and North America have great potential for long-term low-cost monitoring of bird migration at scales that have previously been impossible to achieve. Such long-term monitoring, however, poses a number of challenges for the ornithological and ecological communities: how does one take advantage of this vast data resource, integrate information across multiple sensors and large spatial and temporal scales, and visually represent the data for interpretation and dissemination, considering the dynamic nature of migration? We assembled an interdisciplinary team of ecologists, meteorologists, computer scientists, and graphic designers to develop two different flow visualizations, which are interactive and open source, in order to create novel representations of broad-front nocturnal bird migration to address a primary impediment to long-term, large-scale nocturnal migration monitoring. We have applied these visualization techniques to mass bird migration events recorded by two different weather surveillance radar networks covering regions in Europe and North America. These applications show the flexibility and portability of such an approach. The visualizations provide an intuitive representation of the scale and dynamics of these complex systems, are easily accessible for a broad interest group, and are biologically insightful. Additionally, they facilitate fundamental ecological research, conservation, mitigation of human-wildlife conflicts, improvement of meteorological products, and public outreach, education, and engagement.

Optimal orientation in flows: providing a benchmark for animal movement strategies.

Animal movements in air and water can be strongly affected by experienced flow. While various flow-orientation strategies have been proposed and observed, their performance in variable flow conditions remains unclear. We apply control theory to establish a benchmark for time-minimizing (optimal) orientation. We then define optimal orientation for movement in steady flow patterns and, using dynamic wind data, for short-distance mass movements of thrushes (Turdus sp.) and 6000 km non-stop migratory flights by great snipes, Gallinago media. Relative to the optimal benchmark, we assess the efficiency (travel speed) and reliability (success rate) of three generic orientation strategies: full compensation for lateral drift, vector orientation (single-heading movement) and goal orientation (continually heading towards the goal). Optimal orientation is characterized by detours to regions of high flow support, especially when flow speeds approach and exceed the animal's self-propelled speed. In strong predictable flow (short distance thrush flights), vector orientation adjusted to flow on departure is nearly optimal, whereas for unpredictable flow (inter-continental snipe flights), only goal orientation was near-optimally reliable and efficient. Optimal orientation provides a benchmark for assessing efficiency of responses to complex flow conditions, thereby offering insight into adaptive flow-orientation across taxa in the light of flow strength, predictability and navigation capacity.