Urban areas affect flight altitudes of nocturnally migrating birds

Abstract

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.

Extreme altitude changes between night and day during marathon flights of great snipes

“…8 However, a recent radar study found that night migrating birds generally flew at higher altitudes over urban areas than over non-urban areas. 13 In addition, the low amount of artificial light in the Sahara, where both great snipes and great reed warblers 6 still fly much lower at night, speaks against light attraction as a general explanation for the pattern we found. It may be beneficial for migratory birds to ascend to higher altitudes at daylight to locate important distant landmarks for orientation or landfall.…”

Artificial Light at Night is Related to Broad-Scale Stopover Distributions of Nocturnally Migrating Landbirds along the Yucatan Peninsula, Mexico

“…Nocturnally migrating birds travel long distances between their breeding and nonbreeding grounds, facing conditions along the way that could hinder or halt their journeys. For example, ecological barriers [1,2] and human alterations to the environment such as the addition of bright lights at night and wind turbines or other tall structures (e.g., communication towers [3]) are likely to influence the flight trajectories [4], altitudes [5,6], orientation and distributions of migrating birds [7]. Billions of Nearctic-Neotropical birds negotiate large bodies of water during migration including the Great Lakes, the western Atlantic Ocean, the Caribbean Sea, and the Gulf of Mexico (GOM) [8][9][10][11][12].…”

High-Resolution Spatial Distribution of Bird Movements Estimated from a Weather Radar Network

“…Additionally, the light emitted from cities at night can influence the altitudinal distribution of birds moving during the night. A recent study investigating this effect found a relatively small effect meaning that these changes will not have a strong effect on our VID estimates [42].…”