Science

Thirty Percent of North American Bird Species Face Decline Across Seasons

Two new studies strike similar conclusions on continental bird populations during winter and spring.

Thirteen may be unlucky for humans, but for the birds of North America, one-third is a much more ominous number.

Earlier this week, Audubon scientists published a study in Ecosphere revealing that one-third of wintering North American bird populations have declined since 1966. That's on top of last week's news from the tri-national North American Bird Conservation Initiative (NABCI) that more than one-third of North American bird species are at risk of extinction, unless significant conservation actions are taken.

Both studies tapped valuable citizen science data to reach these conclusions: Audubon’s statistics were derived from 46 Christmas Bird Counts (CBC), while NABCI’s focused on the North American Breeding Bird Survey (BBS) and eBird submissions. But the question is: How did two separate groups of researchers using different data sets come up with such similar numbers?

The overlap may be pure coincidence, Kenneth Rosenberg, a scientist with the Cornell Lab of Ornithology and part of the steering committee that produced the NABCI report, says. “Each group had different goals,” says Rosenberg. “Audubon’s was to answer a very specific question, ‘What was the population change of wintering birds over a set of years?’ while [NABCI's State of North American Birds Report] sought to provide a full assessment of different bird groups across the continent.”

Mild Winters Don't Bring Mild Changes

For the wintering-bird study, Audubon researchers set out to determine population trends of 551 North American species between 1966 to 2013. To prevent skewing the results, they whittled down the original pool to 288 species to eliminate birds that frequent feeders. Then, to figure out how birds are faring in specific areas around the continent, they combined data from December and January of each year and developed population-trend estimates for states, provinces, and other large geographic zones. 

A CBC in New York City's Central Park. Photo: Camilla Cerea/Audubon

What they found is that roughly two-thirds of the species were doing well over those 46 years, but that one-third were declining rapidly. Grassland and woodland birds, like the Northern Bobwhite and the Wood Thrush, and birds that winter along the Gulf Coast and Hawaii showed devastating slumps.

"Identifying wintering-bird-population trends is particularly important because climate warming is happening more rapidly in winter than during the breeding season,” Audubon’s quantitative ecologist Nicole Michel says. She also points out that Audubon found similar patterns in the spring after analyzing BBS, similar to the NABCI scientists.

Where Species Are Most Vulnerable

Instead of focusing strictly on numbers, the group at NABCI created a “conservation vulnerability assessment”—the first of its kind—for all 1,154 native North American bird species. Researchers scored the extinction risk of each species in various regions by taking a holistic look at available citizen science data. Using eBird sightings, the BBS, and a smattering of wintering data from the CBC, they came up with a conservation status based on population size, size of breeding and nonbreeding ranges, and the severity of threats to species’ survival. The researchers then created a “Watch List” of avians most at risk of extinction—a list that includes the Allen’s Hummingbird, Least Storm Petrel, and Wood Thrush (again). 

Wood Thrush. Photo: Kathy Johnston/Audubon Photography Awards

Like the Audubon study, The State of North America’s Birds report found that species in arid, grassland, and coastal habitats are facing the steepest declines. “Birds with the highest conservation scores that then ended up on the Watch List are specialists and have a small range or small populations,” says John Alexander, executive director of the Klamath Bird Observatory and member of NABCI’s science committee. “Climate change, pollution, habitat loss, and predation are risks to these vulnerable populations.”

How does one put two and two together in regard to these new studies? “While they seek and accomplish different goals, there is some agreement,” says Alexander. “The collaborative nature of each project and general similarities in results represent a consensus between migratory bird experts studying wintering and breeding birds.” Moreover, they both embrace the spirit of citizen science, and hope to alert people that birds and their habitats need to be conserved before changes become set in stone.

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Want to help birds like the Wood Thrush and the Allen's Hummingbird fight back against global warming? Here are five ideas.

Correction: An editing error in the headline has been fixed to reflect that only certain species are facing a 30 percent decline. The acronym for North American Bird Conservation Initiative is NABCI, not NACBI. The Audubon paper didn't eliminate birds with a solid wintering range across the study area. The Audubon paper also didn't include the Bobolink, which spends its winters in South America.

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