Hazards of Bird Migration

Environmental & Science Education, STEM, Global Climate Change, Nature, Wildlife, Biodiversity

Ed Hessler

"Is Bird Migration Getting More Dangerous?" This is the question that Marc Devokaitis addresses in the Living Bird magazine (Spring 2021).

Bird migration is about big numbers and Devokaitis reports on research published in the journal Science that documented the loss of 3 billion breeding birds in North America since 1970 ... 80% of losses were among migrants. Devokaitis includes a chart showing how those deaths have been distributed: Cats 2.6b, Windows 624m, Vehicles 214m, Power Lines 57m, Commercial Towers 6.8m, Wind Turbines up to 579 thousand.

It used to be thought by researchers that birds stopped on their journeys north and south somewhat randomly,"but now we know birds heading north are concentrating at a few places for longer, places that really have the resources they count on," said Camila Gomez a Cornell Laboratory of Ornithology postdoctoral researcher. Her work has focused on Gray-cheeked Thrush migration and she found that they "spent much more time on the ground at stopover sites than in the air--about 10 days fueling up at a stopover for every two or three days of actual flight." She said, “Birds have been relying on these kinds of spots since prehistory, relying on peak resources to be there at the right time. 

The article includes an detailed map of priority site stopover sites for fall migration only, spring migration only, both spring and fall, and protected areas. And there is another map that provides the scope of light pollution worldwide. The "strategy of migrating by night--when there are fewer predators, fairer winds, and cool damp air that minimizes water loss--has worked wonderfully for birds for millennia, Additionally, when it is clear at night birds can use the sky as an additional navigational aid. Light pollution of the night sky is increasing about 2%/year.

Now birds face a new "starry" sky--"billions of artificial lights beckoning brightly from below as they dim the view above." Devokaitis reports on research done by the Cornell Lab and the New York City Audubon society. It "revealed more than 1 million birds were drawn into NYC over seven nights by the mile-high beams of light pointed skyward during the Tribute in Light, a memorial to 9/11 victims in Lower Manhattan." The effect was that birds circled "pointlessly for hours." 

Birds are being drawn to city lights all across North America. "Chicago was named the most dangerous city in America for birds in migration. For 40 years the city has "meticulously collected and cataloged in spring and fall," with "one building--McCormick Place, a glass-facaded convention center in Chicago's South Loop" accounting for 40,000 dead birds. I was struck by the detail of the data collected, e.g., by window and whether the lights were on. The number of deaths decreased considerably "when the building began turning off the lights at night in the early 2000s."

Weather plays a role, too. Low clouds, unexpected bad weather, wild temperature swings, with climate change playing a role: earlier migrations (two days per decade). Such events have disrupted food sources sufficiently that the migrating birds have starved to death.

Recommendations include "restoration of ecologically degraded migratory stopover sites, developing incentives to protect sites on private lands,  implementing public outreach campaigns like Lights Our and Keep Cats Indoors programs that reduce threats to birds on migration, and working to reduce the exposure to chemical and pollution that can weaken the body condition of long-distance migratory birds."

This essay is far richer than my rough summary of some of the ideas in it. Read it or at least scan it for the maps and photographs.