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

Abstract

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.

Figure 1

Worldwide distribution of artificial light at night (ALAN - yellow areas over black background). Lines connect centroids of wintering and breeding ranges of nocturnally-migrating bird species considered for analysis (n = 298). Line color represents distances shorter (white) or longer (grey) than the overall median distance. Shorter distances overlap areas with high concentration of ALAN, mainly in North America and eastern Asia. ALAN corresponds to the “vcm-orm-ntl” product (cloud and ephemeral lights free, background set to zero) from the 2015 VIIRS nighttime lights annual composite produced by the Earth Observation Group, NOAA National Geophysical Data Center^,. Map was created with ArcMap version 10.4 (http://desktop.arcgis.com/en/arcmap/).

Figure 2

Partial dependence plots of variables associated with intensity of light pollution in the geographic ranges of 298 nocturnally-migrating bird species. Y-axes show the marginal intensity of ALAN in units of 1E9*nanoWatts*cm⁻²*sr⁻¹; notice different scales among plots. Rug plots shown inside the top two main plots on the x-axis show deciles of the distribution of predictor values. Values in parenthesis of X-axis label indicate the relative influence of each predictor variable.

Figure 3

Partial dependence plot for the interactions of seasonal range area and migration distance on the mean artificial lights at night (ALAN) within the geographic ranges of 298 nocturnally-migrating bird species around the world.

Figure 4

Species richness in (a) breeding, (b) migration, and (c) non-breeding ranges for 298 nocturnally-migrating bird species, estimated by intersecting geographic range map polygons for each phase of annual cycle. This map was created in R version 3.3.2 (https://cran.r-project.org/) with geospatial data from the Birds of the World Geodatabase, and edited with ArcMap version 10.4 (http://desktop.arcgis.com/en/arcmap/).