Artificial light alters natural patterns of light and dark within ecosystems and contributes to the deaths of millions of birds each year.

Light pollution can cause birds to change their migration patterns, foraging behaviours and vocal communication, resulting in disorientation and collisions.

Migrating birds are attracted to artificial light at night – particularly when there are low cloud conditions, fog, rain, or when flying at lower altitudes – luring them to dangers in cities.

Birds become disorientated and, as a result, may end up circling in illuminated areas. With their energy reserves depleted, they risk exhaustion, or worse.

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Light pollution is increasing, with artificially lit outdoor areas rising by 2.2 percent per year from 2012 to 2016, according to one study cited by the Secretariat of the Convention on Migratory Species of Wild Animals (CMS), a UN treaty.

"Many nocturnally migrating birds such as ducks, geese, plovers, sandpipers and songbirds are affected by light pollution causing disorientation and collisions with fatal consequences," said Jacques Trouvilliez, executive secretary of the African-Eurasian Waterbird Agreement (AEWA), another UN treaty.

"Seabirds such as petrels and shearwaters are attracted by artificial lights on land and become prey for rats and cats."

Governments, cities, companies, and communities around the world are taking action to address a significant and growing threat to wildlife – including many species of migratory birds – light pollution.

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The issue was the focus of World Migratory Bird Day Saturday observed under the theme "Dim the Lights for Birds at Night."

Natural darkness has a conservation value in the same way as clean water, air, and soil, said Amy Fraenkel, executive secretary of CMS.

However, more than 80 percent of the world's population is now estimated to live under a "lit sky," and the figure is closer to 99 percent in Europe and North America.

Two years ago, countries that are party to the CMS endorsed guidelines on light pollution covering marine turtles, seabirds, and migratory shorebirds.

The recommendations call for environmental impact assessments to be conducted for projects that could result in light pollution.

Projects should consider the main sources of light pollution at a certain site, the likely wild species to be affected, and facts about proximity to important habitats and migratory pathways.

New guidelines focused on migratory landbirds and bats are now being developed and will be presented for adoption at a CMS conference next year.

Solutions to light pollution are readily available, said Frankel.

More and more cities worldwide are taking measures to dim building lights during migration phases in spring and autumn, according to the UN Environment Programme (UNEP).