US skyscrapers kill 600 million -- yes, million -- birds every year

Chicago was the most dangerous city in the USA for migrating birds.

(CNN)Around 600 million birds die every year in the United States after striking tall buildings -- with Chicago, Houston and Dallas being especially deadly, according to research from the Cornell Lab of Ornithology.

Attracted to the artificial light in high-rise blocks, birds can become disorientated and crash into walls -- many of which are made of glass -- suffering injury and often death.
The issue is particularly stark during migratory seasons in spring and fall, when billions of birds pass through the central United States on their way between Canada and Latin America, according to the ornithology lab, which has just issued a list of the most dangerous cities for migrating birds. It estimates that well over half a billion birds are dying in this way every year in America.
    Chicago is the most dangerous place for birds during both seasons, closely followed by Houston and Dallas. There, accidents kill birds that have covered thousands of miles on their journeys only to perish in seconds as they collide with glass buildings.
      Kyle Horton is the lead author of the study, published in the journal Frontiers in Ecology and the Environment last week.
      "Chicago, Houston, and Dallas are uniquely positioned in the heart of North America's most trafficked aerial corridors. This, in combination with being some of the largest cities in the US, make them a serious threat to the passage of migrants, regardless of season," he said in a statement posted online by The Cornell Lab of Ornithology.
      An American redstart killed in a building collision
      It is hard to quantify the exact number of casualties, but research published by The Condor: Ornithological Applications in 2014 suggested that as many as 1 billion birds die as a result each year in the US alone.
        "This magnitude of mortality would place buildings behind only free-ranging domestic cats among sources of direct human-caused mortality of birds," that report concluded.
        Skyscrapers are at the heart of the problem, according to New York City Audubon, which aims to protect wild birds and their natural habitat in the city -- which ranked seventh-worst for the spring migration and was the fifth-worst during the fall.
        The problem is exacerbated by the migration's timing, as many birds fly at night. Attracted by the glow of skyscrapers in the dark, they are vulnerable to collision -- either with each other or the buildings. For some, the light can prove so disorientating that they flutter around for hours, eventually becoming exhausted and landing in inhospitable environments.
        Birds can also become confused by windows with plants behind them, mistaking them for a safe landing spot.