How Many Birds Disappear Between Migration Seasons? We Now Have a Clue.

New research has found that a third of the avian population that winters in the mainland United States might not survive till spring. But why?

An American Robin takes off from its summer home in Montreal, Canada, putting everything on the line to fly 1,600 miles to spend the holidays in West Palm Beach, Florida. Even more grueling is the journey of a Scarlet Tanager that wings 2,700 miles from Dartmouth, Massachusetts, to Cali, Colombia.

Each fall, billions of birds like the robin and tanager make their way to the Lower 48 or to the tropics. But a big slice of them never flies back—casualties of natural causes like weather and predation, and unnatural causes like oil pits, feral cats, and glass collisions. Now scientists from the Cornell Lab of Ornithology in Ithaca, New York, have put a first-ever estimate on those missing travelers. By tracking migratory night flights from 2013 to 2017 on weather radar maps, measuring the magnitude, and plugging the numbers into a cloud-computing service, they tallied how many land birds were—or weren’t—traversing the country. Their findings show a titanic drop-off of 2.6 billion birds between fall and spring ­migrations. What’s more, though species that winter in the United States have shorter fall flights, they suffered the bulk of the losses, indicating that northern populations face greater threats than those in the tropics.

It’s a narrative that’s bound to change, says lead researcher and migration expert Adriaan Dokter. As development in Latin America continues to climb, there will be fewer pristine habitats, which means some birds might not venture as far south. Drier, hotter conditions under climate change could make the trek even more costly for migrants that try to stick it out near the equator. It’s a future we need to plan for, Dokter says. By both protecting tropical wilderness and reducing domestic dangers, we can boost the survival of billions of birds before they fly off the radar.

The “biomass” of land birds that moves north to south in fall differs continent-wide. Fewer individuals choose to linger in the Lower 48 once the breeding season passes, but not so few that the region becomes a ghost town for feathers. Dokter notes that similar patterns are seen on the flip side of the Atlantic, where migrants travel from the Russian Arctic to Europe and from Europe to Africa.

Going the distance has its perks. The tropics offer milder climates and richer resources. So even if the trip there is sapping, a spell in paradise can restore a bird and prepare it for the vital breeding season.

Short migrations mean lower survivorship. Species that spend less time in transit face fewer flight risks. But in exchange for their quick trip, they endure harsher winter conditions, which drive up mortality rates. Widespread urbanization and other human impacts in the contiguous United States also pose potential hardships that take a big bite out of the population.

Life compensates death. To make up for low survival rates on their stateside wintering grounds, northern breeding species generally crank out more chicks. Most of the young recruits won’t survive fall migration, but their high numbers still give the population a boost before the grind begins anew.

This story originally ran in the Winter 2018 issue as “The Billion-Bird Question​.” To receive our print magazine, become a member by making a donation today.