Autumn stopover hotspots and multiscale habitat associations of migratory landbirds in the eastern United States.

Halting the global decline of migratory birds requires a better understanding of migration ecology. Stopover sites are a crucial yet understudied aspect of bird conservation, mostly due to challenges associated with understanding broad-scale patterns of transient habitat use. Here, we use a national network of weather radar stations to identify stopover hotspots and assess multiscale habitat associations of migratory landbirds across the eastern United States during autumn migration. We mapped seasonal bird densities over 5 y (2015 to 2019) from 60 radar stations covering 63.2 million hectares. At a coarse scale, we found that landbirds migrate across a broad front with small differences in migrant density between radar domains. However, relatively more birds concentrate along the Mississippi River and Appalachian Mountains. At a finer scale, we identified radar pixels that consistently harbored high densities of migrants for all 5 y, which we classify as stopover hotspots. Hotspot probability increased with percent cover of all forest types and decreased with percent cover of pasture and cultivated crops. Moreover, we found strong concentrating effects of deciduous forest patches within deforested regions. We also found that the prairie biome in the Midwest (now mostly cropland) is likely a migration barrier, with large concentrations of migrants at the prairie-forest boundary after crossing the agricultural Midwest. Overall, the broad-front migration pattern highlights the importance of locally based conservation efforts to protect stopover habitats. Such efforts should target forests, especially deciduous forests in highly altered landscapes. These findings demonstrate the value of multiscale habitat assessments for the conservation of migratory landbirds.

A place to land: spatiotemporal drivers of stopover habitat use by migrating birds.

Migrating birds require en route habitats to rest and refuel. Yet, habitat use has never been integrated with passage to understand the factors that determine where and when birds stopover during spring and autumn migration. Here, we introduce the stopover-to-passage ratio (SPR), the percentage of passage migrants that stop in an area, and use 8 years of data from 12 weather surveillance radars to estimate over 50% SPR during spring and autumn through the Gulf of Mexico and Atlantic coasts of the south-eastern US, the most prominent corridor for North America's migratory birds. During stopovers, birds concentrated close to the coast during spring and inland in forested landscapes during autumn, suggesting seasonal differences in habitat function and highlighting the vital role of stopover habitats in sustaining migratory communities. Beyond advancing understanding of migration ecology, SPR will facilitate conservation through identification of sites that are disproportionally selected for stopover by migrating birds.

Urban areas affect flight altitudes of nocturnally migrating birds.

Urban areas affect terrestrial ecological processes and local weather, but we know little about their effect on aerial ecological processes. Here, we identify urban from non-urban areas based on the intensity of artificial light at night (ALAN) in the landscape, and, along with weather covariates, evaluate the effect of urbanization on flight altitudes of nocturnally migrating birds. Birds are attracted to ALAN; hence, we predicted that altitudes would be lower over urban than over non-urban areas. However, other factors associated with urbanization may also affect flight altitudes. For example, surface temperature and terrain roughness are higher in urban areas, increasing air turbulence and height of the boundary layer, and affecting local winds. We used data from nine weather surveillance radars in the eastern United States to estimate altitudes at five quantiles of the vertical distribution of birds migrating at night over urban and non-urban areas during five consecutive spring and autumn migration seasons. We fit Generalized Linear Mixed Models by season for each of the five quantiles of bird flight altitude and their differences between urban and non-urban areas. After controlling for other environmental variables and contrary to our prediction, we found that birds generally fly higher over urban areas compared to rural areas in spring, and marginally higher at the mid-layers of the vertical distribution in autumn. We also identified a small interaction effect between urbanization and crosswind speed, and between urbanization and surface air temperature, on flight altitudes. We also found that the difference in flight altitudes of nocturnally migrating birds between urban and non-urban areas varied among radars and seasons, but was consistently higher over urban areas throughout the years sampled. Our results suggest that the effects of urbanization on wildlife extend into the aerosphere and are complex, stressing the need of understanding the influence of anthropogenic factors on airspace habitat.

Artificial light at night confounds broad-scale habitat use by migrating birds.

With many of the world's migratory bird populations in alarming decline, broad-scale assessments of responses to migratory hazards may prove crucial to successful conservation efforts. Most birds migrate at night through increasingly light-polluted skies. Bright light sources can attract airborne migrants and lead to collisions with structures, but might also influence selection of migratory stopover habitat and thereby acquisition of food resources. We demonstrate, using multi-year weather radar measurements of nocturnal migrants across the northeastern U.S., that autumnal migrant stopover density increased at regional scales with proximity to the brightest areas, but decreased within a few kilometers of brightly-lit sources. This finding implies broad-scale attraction to artificial light while airborne, impeding selection for extensive forest habitat. Given that high-quality stopover habitat is critical to successful migration, and hindrances during migration can decrease fitness, artificial lights present a potentially heightened conservation concern for migratory bird populations.

Fat, weather, and date affect migratory songbirds' departure decisions, routes, and time it takes to cross the Gulf of Mexico.

Approximately two thirds of migratory songbirds in eastern North America negotiate the Gulf of Mexico (GOM), where inclement weather coupled with no refueling or resting opportunities can be lethal. However, decisions made when navigating such features and their consequences remain largely unknown due to technological limitations of tracking small animals over large areas. We used automated radio telemetry to track three songbird species (Red-eyed Vireo, Swainson's Thrush, Wood Thrush) from coastal Alabama to the northern Yucatan Peninsula (YP) during fall migration. Detecting songbirds after crossing ∼1,000 km of open water allowed us to examine intrinsic (age, wing length, fat) and extrinsic (weather, date) variables shaping departure decisions, arrival at the YP, and crossing times. Large fat reserves and low humidity, indicative of beneficial synoptic weather patterns, favored southward departure across the Gulf. Individuals detected in the YP departed with large fat reserves and later in the fall with profitable winds, and flight durations (mean = 22.4 h) were positively related to wind profit. Age was not related to departure behavior, arrival, or travel time. However, vireos negotiated the GOM differently than thrushes, including different departure decisions, lower probability of detection in the YP, and longer crossing times. Defense of winter territories by thrushes but not vireos and species-specific foraging habits may explain the divergent migratory behaviors. Fat reserves appear extremely important to departure decisions and arrival in the YP. As habitat along the GOM is degraded, birds may be limited in their ability to acquire fat to cross the Gulf.

Body mass and wing shape explain variability in broad-scale bird species distributions of migratory passerines along an ecological barrier during stopover.

Migrating birds are under selective pressure to complete long-distance flights quickly and efficiently. Wing morphology and body mass influence energy expenditure of flight, such that certain characteristics may confer a greater relative advantage when making long crossings over ecological barriers by modifying the flight range or speed. We explored the possibility, among light (mass <50 g) migrating passerines, that species with relatively poorer flight performance related to wing shape and/or body mass have a lower margin for error in dealing with the exigencies of a long water crossing across the Gulf of Mexico and consequently minimize their travel time or distance. We found that species-mean fat-free body mass and wing tip pointedness independently explained variability among species distributions within ~50 km from the northern coast. In both spring and autumn, lighter (i.e., slower flying) species and species with more rounded wings were concentrated nearest the coastline. Our results support the idea that morphology helps to shape broad-scale bird distributions along an ecological barrier and that migration exerts some selective force on passerine morphology. Furthermore, smaller species with less-efficient flight appear constrained to stopping over in close proximity to ecological barriers, illustrating the importance of coastal habitats for small passerine migrants.

Seasonal patterns and protection status of stopover hotspots for migratory landbirds in the eastern United States.

Migratory landbirds in North America are experiencing dramatic population declines. Although considerable research and conservation attention have been directed toward these birds' breeding and wintering grounds, far less is known about the areas used as stopover sites during migration. To address this knowledge gap, we used 5 years of weather surveillance radar data to map seasonal stopover densities of landbirds across the eastern United States during spring and autumn migration. We identified stopover hotspots covering 2.47 million ha that consistently support high densities of migratory landbirds in spring or autumn. However, only 16.7% of these sites are hotspots in both seasons. The distribution of hotspots is shifted eastward in autumn compared with spring. Deciduous forest is the most important habitat type in both seasons, with deciduous forest fragments embedded in broadly deforested regions having the highest probability of being hotspots. The concentration of birds in these forest fragments is stronger in spring, especially in the agricultural Midwest. We found generally higher stopover densities in protected areas than in unprotected areas in both seasons. Nonetheless, only one-third of identified stopover hotspots have some sort of protected status, and more than half of these protected hotspots are subject to extractive uses. A well-distributed network of well-protected stopover areas, complementing conservation efforts on the breeding and wintering grounds, is essential to sustaining healthy populations of migratory landbirds in North America.

Using the California Waterfowl Tracker to Assess Proximity of Waterfowl to Commercial Poultry in the Central Valley of California.

Migratory waterfowl are the primary reservoir of avian influenza viruses (AIV), which can be spread to commercial poultry. Surveillance efforts that track the location and abundance of wild waterfowl and link those data to inform assessments of risk and sampling for AIV currently do not exist. To assist surveillance and minimize poultry exposure to AIV, here we explored the utility of Remotely Sensed Moderate Resolution Imaging Spectroradiometer (MODIS) satellite imagery in combination with land-based climate measurements (e.g., temperature and precipitation) to predict waterfowl location and abundance in near real-time in the California Central Valley (CCV), where both wild waterfowl and domestic poultry are densely located. Specifically, remotely collected MODIS and climate data were integrated into a previously developed boosted regression tree (BRT) model to predict and visualize waterfowl distributions across the CCV. Daily model-based predictions are publicly available during the winter as part of the dynamic California Waterfowl Tracker (CWT) web app hosted on the University of California's Cooperative Extension webpage. In this study, we analyzed 52 days of model predictions and produced daily spatiotemporal maps of waterfowl concentrations near the 605 commercial poultry farms in the CCV during January and February of 2019. Exposure of each poultry farm to waterfowl during each day was classified as high, medium, low, or none, depending on the density of waterfowl within 4 km of a farm. Results indicated that farms were at substantially greater risk of exposure in January, when CCV waterfowl populations peak, than in February. For example, during January, 33% (199/605) of the farms were exposed for ≥1 day to high waterfowl density vs. 19% (115/605) of the farms in February. In addition to demonstrating the overall variability of waterfowl location and density, these data demonstrate how remote sensing can be used to better triage AIV surveillance and biosecurity efforts via the utilization of a functional web app-based tool. The ability to leverage remote sensing is an integral advancement toward improving AIV surveillance in waterfowl in close proximity to commercial poultry. Expansion of these types of remote sensing methods, linked to a user-friendly web tool, could be further developed across the continental United States. The BRT model incorporated into the CWT reflects a first attempt to give an accurate representation of waterfowl distribution and density relative to commercial poultry.

Las aves acuáticas migratorias son el principal reservorio de los virus de la influenza aviar (con las siglas en inglés AIV), que pueden transmitirse a la avicultura comercial. Actualmente no existen esfuerzos de vigilancia que rastrean la ubicación y densidad de poblaciones de aves acuáticas silvestres y que vinculen esos datos para informar evaluaciones de riesgo y muestreo para influenza aviar. Para ayudar a la vigilancia y minimizar la exposición de la avicultura comercial a influenza aviar se exploró la utilidad de las imágenes satelitales por espectrorradiómetro de imágenes con resolución moderada (con las siglas en inglés MODIS) y de detección remota en combinación con mediciones climáticas terrestres (por ejemplo, temperatura y precipitación) para predecir la ubicación y densidad de aves acuáticas prácticamente en tiempo real en el Valle Central de California (CCV), donde tanto las aves acuáticas silvestres como las aves domésticas están densamente ubicadas. Específicamente, los datos MODIS y climáticos recopilados de forma remota se integraron en un modelo de árbol de regresión reforzado (BRT) desarrollado previamente para predecir y visualizar la distribución de las aves acuáticas en el Valle Central de California. Las predicciones diarias basadas en modelos están disponibles públicamente durante el invierno como parte de la aplicación dinámica en el del rastreador de aves acuáticas de California (California Waterfowl Tracker, CWT) ubicada en la página de internet de Extensión Cooperativa de la Universidad de California. En este estudio, se analizaron 52 días de predicciones del modelo y se produjeron mapas espaciotemporales diarios con densidades de aves acuáticas cerca de las 605 granjas avícolas comerciales en el Valle Central de California durante enero y febrero de 2019. La exposición de cada granja avícola a las aves acuáticas durante cada día se clasificó como alta, media, baja o nula, dependiendo de la densidad de aves acuáticas dentro de los cuatro kilómetros de una granja. Los resultados indicaron que las granjas tenían un riesgo sustancialmente mayor de exposición en enero, cuando las poblaciones de aves acuáticas en el Valle Central de California alcanzan su punto máximo, en comparación con febrero. Por ejemplo, durante enero, el 33% (199/605) de las granjas estuvieron expuestas durante más de un día a una alta densidad de aves acuáticas frente a un 19% (115/605) de las granjas en febrero. Además de demostrar la variabilidad general de la ubicación y densidad de las aves acuáticas, estos datos demuestran cómo se puede utilizar la teledetección para clasificar mejor los esfuerzos de bioseguridad y vigilancia para la influenza aviar mediante la utilización de una herramienta funcional basada en una aplicación en el internet. La capacidad de aprovechar la teledetección es un avance integral hacia la mejora de la vigilancia para influenza aviar en aves acuáticas en las proximidades de la avicultura comercial. La expansión de estos tipos de métodos de teledetección, vinculados a una herramienta en el internet que es fácil de usar, podría desarrollarse aún más en los Estados Unidos continentales. El modelo de árbol de regresión reforzado incorporado en el sistema de rastreo de aves acuáticas de California refleja un primer intento de brindar una representación precisa de la distribución y densidad de las aves acuáticas en relación con las aves comerciales.

Author Correction: Light pollution is greatest within migration passage areas for nocturnally-migrating birds around the world.

A correction to this article has been published and is linked from the HTML and PDF versions of this paper. The error has been fixed in the paper.

Light pollution is greatest within migration passage areas for nocturnally-migrating birds around the world.

Excessive or misdirected artificial light at night (ALAN) produces light pollution that influences several aspects of the biology and ecology of birds, including disruption of circadian rhythms and disorientation during flight. Many migrating birds traverse large expanses of land twice every year at night when ALAN illuminates the sky. Considering the extensive and increasing encroachment of light pollution around the world, we evaluated the association of the annual mean ALAN intensity over land within the geographic ranges of 298 nocturnally migrating bird species with five factors: phase of annual cycle, mean distance between breeding and non-breeding ranges, range size, global hemisphere of range, and IUCN category of conservation concern. Light pollution within geographic ranges was relatively greater during the migration season, for shorter-distance migrants, for species with smaller ranges, and for species in the western hemisphere. Our results suggest that migratory birds may be subject to the effects of light pollution particularly during migration, the most critical stage in their annual cycle. We hope these results will spur further research on how light pollution affects not only migrating birds, but also other highly mobile animals throughout their annual cycle.

Swainson's Thrushes do not show strong wind selectivity prior to crossing the Gulf of Mexico.

During long-distance fall migrations, nocturnally migrating Swainson's Thrushes often stop on the northern Gulf of Mexico coast before flying across the Gulf. To minimize energetic costs, trans-Gulf migrants should stop over when they encounter crosswinds or headwinds, and depart with supportive tailwinds. However, time constrained migrants should be less selective, balancing costs of headwinds with benefits of continuing their migrations. To test the hypotheses that birds select supportive winds and that selectivity is mediated by seasonal time constraints, we examined whether local winds affected Swainson's Thrushes' arrival and departure at Ft. Morgan, Alabama, USA at annual, seasonal, and nightly time scales. Additionally, migrants could benefit from forecasting future wind conditions, crossing on nights when winds are consistently supportive across the Gulf, thereby avoiding the potentially lethal consequences of depleting their energetic reserves over water. To test whether birds forecast, we developed a movement model, calculated to what extent departure winds were predictive of future Gulf winds, and tested whether birds responded to predictability. Swainson's Thrushes were only slightly selective and did not appear to forecast. By following the simple rule of avoiding only the strongest headwinds at departure, Swainson's Thrushes could survive the 1500 km flight between Alabama and Veracruz, Mexico.