The spatial extent and intensity of artificial light at night (ALAN) has increased worldwide through the growth of urban environments. There is evidence that nocturnally migrating birds are attracted to ALAN, and there is evidence that nocturnally migrating bird populations are more likely to occur in urban areas during migration, especially in the autumn. Here, we test if urban sources of ALAN are responsible, at least in part, for these observed urban associations. We use weekly estimates of diurnal occurrence and relative abundance for 40 nocturnally migrating bird species that breed in forested environments in North America to assess how associations with distance to urban areas and ALAN are defined across the annual cycle. Migratory bird populations presented stronger than expected associations with shorter distances to urban areas during migration, and stronger than expected association with higher levels of ALAN outside and especially within urban areas during migration. These patterns were more pronounced during autumn migration, especially within urban areas. Outside of the two migration periods, migratory bird populations presented stronger than expected associations with longer distances to urban areas, especially during the nonbreeding season, and weaker than expected associations with the highest levels of ALAN outside and especially within urban areas. These findings suggest that ALAN is associated with higher levels of diurnal abundance along the boundaries and within the interior of urban areas during migration, especially in the autumn when juveniles are undertaking their first migration journey. These findings support the conclusion that urban sources of ALAN can broadly effect migratory behavior, emphasizing the need to better understand the implications of ALAN for migratory bird populations.

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.

Migratory animals are affected by various factors during their journeys, and the study of animal movement by radars has been instrumental in revealing key influences of the environment on flying migrants. Radars enable the simultaneous tracking of many individuals of almost all sizes within the radar range during day and night, and under low visibility conditions. We review how atmospheric conditions, geographic features and human development affect the behavior of migrating insects and birds as recorded by radars. We focus on flight initiation and termination, as well as in‐flight behavior that includes changes in animal flight direction, speed and altitude. We have identified several similarities and differences in the behavioral responses of aerial migrants including an overlooked similarity in the use of thermal updrafts by very small (e.g. aphids) and very large (e.g. vultures) migrants. We propose that many aerial migrants modulate their migratory flights in relation to the interaction between atmospheric conditions and geographic features. For example, aerial migrants that encounter crosswind may terminate their flight or continue their migration and may also drift or compensate for lateral displacement depending on their position (over land, near the coast or over sea). We propose several promising directions for future research, including the development and application of algorithms for tracking insects, bats and large aggregations of animals using weather radars. Additionally, an important contribution will be the spatial expansion of aeroecological radar studies to Africa, most of Asia and South America where no such studies have been undertaken. Quantifying the role of migrants in ecosystems and specifically estimating the number of departing birds from stopover sites using low‐elevation radar scans is important for quantifying migrant–habitat relationships. This information, together with estimates of population demographics and migrant abundance, can help resolve the long‐term dynamics of migrant populations facing large‐scale environmental changes.

The number of wind farms operating in the Isthmus of Tehuantepec, southern Mexico, has rapidly increased in recent years; yet, this region serves as a major migration route for various soaring birds, including Turkey Vultures (Cathartes aura) and Swainson's Hawks (Buteo swainsoni). We analyzed the flight trajectories of soaring migrant birds passing the La Venta II wind farm during the two migratory seasons of 2011, to determine whether an avoidance pattern existed or not. We recorded three polar coordinates for the flight path of migrating soaring birds that were detected using marine radar, plotted the flight trajectories and estimated the number of trajectories that intersected the polygon defined by the wind turbines of La Venta II. Finally, we estimated the actual number of intersections per kilometer and compared this value with the null distributions obtained by running 10,000 simulations of our datasets. The observed number of intersections per kilometer fell within or beyond the lower end of the null distributions in the five models proposed for the fall season and in three of the four models proposed for the spring season. Flight trajectories had a non-random distribution around La Venta II, suggesting a strong avoidance pattern during fall and a possible avoidance pattern during spring. We suggest that a nearby ridgeline plays an important role in this pattern, an issue that may be incorporated into strategies to minimize the potential negative impacts of future wind farms on soaring birds. Studies evaluating these issues in the Isthmus of Tehuantepec have not been previously published; hence this work contributes important baseline information about the movement patterns of soaring birds and its relationship to wind farms in the region.

1. Flying birds have been documented to respond in different ways to the presence of wind farms. Such responses are species-and site-specific, with wind farm design playing an important role. Between 2009 and 2014, the length of rows of wind turbines within our study area increased from 3Á4 km to $ 7Á5 km, and the total area occupied by wind farms increased from 6Á47 km 2 to 14Á21 km 2 . This area is located on an important migratory corridor in southern Mexico. 2. We used marine radar and hawk-watch monitoring stations to collect data during six consecutive autumn seasons from a single wind farm. We analysed the response of migrating raptors to the presence of two new wind farms by comparing the mean bearing of flight trajectories and the number of intersections km À1 of trajectories with wind farm areas between 2009-2011 and 2012-2014, representing the pre-and post-construction stages of the new wind farms. 3. Mean raptor count was >600 000 individuals for the six seasons. The most abundant species were Turkey Vulture Cathartes aura, Swainson's Hawk Buteo swainsoni and Broadwinged hawk Buteo platypterus. Between 79% and 97% of migration occurred in October. Radar monitoring overlapped with the peak migratory activity each season. 4. We observed significant differences between periods, involving more scattering in flight bearings and less intersections km À1 of trajectory in the post-than in the pre-construction period, implying an avoidance of the new wind farms. 5. Synthesis and applications. We show that migrating raptors adjusted their flight trajectories to avoid new wind farms, but also discuss the extent and limitations of our findings. Our results from our hawk-watch monitoring station, which represent the first published account about the seasonality and intensity of raptor migration in the area, could be used by decision-makers for careful planning of future wind energy developments in the area. Our results might aid in the conservation of those species of raptors that migrate through the Isthmus of Tehuantepec.

The distributions of birds during migratory stopovers are influenced by a hierarchy of factors. For example, in temperate regions, migrants are concentrated near areas of bright artificial light at night (ALAN) and also the coastlines of large water bodies at broad spatial scales. However, less is known about what drives broad-scale stopover distributions in the tropics. We quantified seasonal densities of nocturnally migrating landbirds during spring and fall of 2011–2015, using two weather radars on the Yucatan peninsula, Mexico (Sabancuy and Cancun). We tested the influence of environmental predictors in explaining broad-scale bird stopover densities. We predicted higher densities in areas (1) closer to the coast in the fall and farther away in spring and (2) closer to bright ALAN and with lower ALAN intensity in both seasons. We found that birds were more concentrated near the coastline in the fall and away from it in spring around Cancun but not Sabancuy. Counter to our expectations, we detected increased bird densities with increased distance from lights in spring around Sabancuy, and in both seasons around Cancun, suggesting avoidance of bright areas during those seasons. This is the first evidence of broad-scale bird avoidance of bright areas during stopover.

Although many tropical countries have functional wind farms, most of the information on their impact on bat populations has come from temperate zones. Our study is based on a 5-year study (2009–2013) of bat captures using mist nets, acoustic recordings, and carcass searches at a wind farm in tropical southern Mexico. We investigated the composition of bat species, estimated the number of bat fatalities per turbine, and assessed the effect of the spatial attributes of vegetation cover near turbines on numbers of bat carcasses recovered by trophic guild. We recorded 29 bat species at the wind farm. The family Phyllostomidae was best represented in terms of number of species and individuals captured. Glossophaga soricina and G. morenoi exhibited the highest relative abundance, whereas Balantiopteryx plicata had the highest acoustic activity. We found 203 carcasses, including 73 Pteronotus davyi individuals (35.9%); other frequent species were Mormoops megalophylla, Molossus sinaloae, and Lasiurus intermedius. The total number of carcasses found within a year ranged from 17 to 83 (2012 and 2009, respectively), with the corrected estimates ranging from 410 to 1,980, or 4.18–20.20 fatalities/turbine. The number of carcasses recorded was positively correlated with secondary vegetation surrounding turbines but negatively correlated with agricultural fields. The spatial attributes of vegetation surrounding turbines influenced numbers of bat carcasses differentially depending on the bats’ trophic guild and habitat use. Contrary to findings from United States and Canadian wind farms, most of the carcasses observed in our study were resident species. Notably, the most commonly captured and acoustically active species were not the most commonly found in carcass searches. To obtain more accurate information about the most vulnerable species and how to reduce the impact on bat mortality, we advise the use of alternative monitoring methods in pre-construction studies. Aunque muchos países tropicales han promovido el establecimiento de parques eólicos en sus territorios, mucha de la información referente al impacto de esta actividad sobre las poblaciones de murciélagos proviene de zonas templadas. Usando datos de 5 años (2009–2013) de muestreos con redes de niebla, grabaciones acústicas, y búsquedas de cadáveres en un parque eólico al sur del trópico mexicano, investigamos la composición de especies de murciélagos, estimamos el número de cadáveres/turbina y evaluamos el efecto de los atributos espaciales de la vegetación que rodea a las turbinas sobre el número de cadáveres de murciélagos por gremio trófico. Registramos 29 especies de murciélagos en el parque, la familia Phyllostomidae fue la mejor representada en cuanto al número de especies e individuos capturados. Glossophaga soricina y G. morenoi fueron las especies con mayor abundancia relativa, mientras que Balantiopteryx plicata fue la especie con mayor actividad acústica. Se registraron 203 cadáveres de murciélagos en el parque eólico, el 35.9% pertenecían a Pteronotus davyi, otras especies frecuentes fueron Mormoops megalophylla, Molossus sinaloae y Lasiurus intermedius. El número total de cadáveres encontrado va de 17 a 83 (2012 y 2009 respectivamente), mientras que la estimación corregida va de 492–1,980, o 4.18–20.20 cadáveres/turbina. La cantidad de cadáveres estimada para los 5 años combinados es de 4,782. El área ocupada por vegetación secundaria alrededor de los aerogeneradores se relacionó significativa y positivamente con el número de cadáveres registrados. Nuestros resultados también indican que la ubicación de los aerogeneradores afecta de forma diferencial a las especies de murciélagos según sus gremios tróficos y uso de hábitat. Contrario a lo encontrado en parques eólicos de Estados Unidos y Canadá, los cadáveres de murciélagos registrados en el parque eólico tropical son de especies residentes, incluyendo aquellas que forman grandes colonias. Ya que algunas de las especies frecuentemente registradas en las redes y acústicamente, no fueron las que se encontraron frecuentemente en los cadáveres, sugerimos diversificar los métodos de monitoreo en estudios de pre-construcción para identificar las especies más vulnerables y tomar acciones que reduzcan el impacto sobre sus poblaciones.

1. Urban areas affect terrestrial ecological processes and local weather, but we know little about their effect on aerial ecological processes.2. 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.3. 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. 4. 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.5. 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. 6. 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.