Bird populations across North America are declining fastest in areas where they were once most abundant, raising concerns about the health of ecosystems previously considered safe havens, according to major new research.

Published in the journal Science, the research analysed nearly 500 bird species and found that three-quarters are in decline across their ranges, with two-thirds experiencing significant population losses. Alarmingly, some of the steepest declines are occurring in grasslands, drylands, and Arctic regions—areas once regarded as strongholds for avian biodiversity.

Using one of the most comprehensive citizen science datasets to date, scientists from the Cornell Lab of Ornithology analysed birdwatcher-submitted observations through the eBird app. These records allowed researchers to track bird population trends between 2007 and 2021 in fine detail—within 27-square-kilometre (10-square-mile) grid cells—revealing steep declines even in areas where birds had thrived just 15 years ago.

“We’ve known for several years that a lot of bird species in North America have been declining. With this study, we were aiming to understand in much finer spatial resolution where birds were declining and where they might be increasing. Rather than having a range-wide trend to see if a species is going up or down, we want to know where it is going up and down,” said Alison Johnston, director of the Centre for Research into Ecological and Environmental Modelling at the University of St Andrews, UK, who led the study.

“The main ecological finding is that the locations where these species were thriving in the past, where the environments were really well suited to birds, are now the places where they are suffering the most,” she said.

The researchers said further studies were needed to explain the reasons behind the changes, many of which were dramatic, with populations falling by over 10% a year in some areas.

While the exact causes of the declines remain uncertain, global heating and habitat loss are considered likely contributors. Some regions have seen population drops of more than 10% per year. Global heating and habitat change were suggested as the main theories, but Johnston said they ultimately did not know.

“The way I interpret this result is that it’s indicative of major changes in our world,” she said. “The fact that where birds used to have strongholds, where there used to be a lot of resources, where the environments were really suitable, are now the places where they are declining most, that suggests to me that we are just seeing fundamental changes to the environments around us. The birds are like the canary in the coalmine,” she said.

The findings add to a growing body of research showing alarming declines in bird numbers even in nature reserves and protected habitats. However, the study also identified small pockets of stability, particularly in the Appalachians and western mountain ranges. Encouragingly, 97% of the bird species studied showed population increases in at least some parts of their range.

Cornell’s team has pioneered techniques to convert crowd-sourced observations into reliable data, ensuring the study included only statistically sound results.

Co-author Prof Amanda Rodewald emphasised that the detailed maps generated from this method will help conservationists focus efforts where they are most urgently needed.

“It is this kind of small-scale information across broad geographies that has been lacking and it’s exactly what we need to make smart conservation decisions,” she said. “These data products give us a new lens to detect and diagnose population declines and to respond to them in a way that’s strategic, precise and flexible. That’s a gamechanger for conservation.”

Ian Burfield, global science coordinator at BirdLife International, praised the study—though not involved himself—and noted that it highlights critical areas for further investigation into the causes of the widespread declines.

“North American birds are one of very few taxonomic groups and regions where such data exist to facilitate this approach. This emphasises the vital need for more field data collection, both through formal monitoring schemes and citizen science efforts, in many other parts of the world, especially in the biodiversity-rich tropics,” he said.

 

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