A team of researchers set out to understand which areas are most important as stopover locations for the billions of birds migrating across the United States each spring and autumn, revealing a strong link between urban illumination and stopover density.
Until now, we have had a very limited understanding of which areas might serve as key stopover locations for many of the the nearly 500 species of bird that migrate across North America between their breeding and wintering grounds. A team of researchers led by Kyle Horton from Colorado State University set out to gather a large-scale dataset on stopover locations for the first time.
Songbirds such as Kentucky Warbler migrate at night on a broad front, but their apparent attraction to areas of high light pollution presents a potential conservation issue (Evelyne Coronado-Guillaumet).
Knowledge of important stopover areas for passerines has been particularly poor due to songbirds' tendency to migrate across a broad front at night, relying on scattered habitat at which to pause and feed along their migration routes.
In a paper published in Nature Communications, the researchers used 133,000 radar scans to assess the influence of light pollution, forest cover and vegetative productivity on migrant bird stopover density across the contiguous United States.
They found that skyglow was among the biggest positive predictors of stopover density alongside precipitation, elevation and forest cover, while the amount of cultivated crop in an area had a negative association with the number of migrant birds dropping in to rest.
Skyglow, the illumination of the night sky from human light sources, had a positive association with migrant bird stopover density in more than 70% of the 2,500 models used by the scientists. The effect was most obvious on the Western Flyway, where skyglow was the top predictor of stopover density.
The central portion of the US revealed the highest stopover densities in spring, being 1.5 times greater in the Central Flyway than the Eastern Flyway, and nearly three times higher than the Western Flyway. The picture shifted somewhat in autumn, with stopover density being greatest in the south-eastern states, especially Alabama and Tennessee.
The data revealed a stark difference in stopover density between the seasons, with more than 70% of 1-km areas showing higher density in the autumn, with nearly a third of the contiguous States showing doubled stopoever density in autumn,compared to spring.
As it seems to be drawing in large numbers of birds, skyglow could pose a serious threat to nocturnal migrants. Light pollution may be acting as an ecological trap, attracting birds into poor habitat on their long migrations and putting them at greater risk of collision with buildings or vehicles, as well as predation.
The paper says that there is much still to learn about why migrant birds seem to be drawn to light and to what degree bright areas are ecological traps, important ecological regions, or both. Skyglow is growing at a rate of more than 10% per year in North America, so the authors say that widescale collaboration and effective lighting policies will be necessary to reverse the rise of light pollution and its damaging impacts.
The researchers said: "Our maps can help guide conservation efforts to protect critical habitats, and collectively contribute to the full-annual cycle conservation of migratory birds."
Horton, K G, Buler, J J, Anderson, S J, Burt, C S, Collins, A C, Dokter, A M, Guo, F, Sheldon, D, Tomaszewska, M A, and Henebry, G M. 2023. Artificial light at night is a top predictor of bird migration stopover density. Nature Communications. DOI: https://doi.org/10.1038/s41467-023-43046-z