North American bird populations are shrinking most rapidly in the very areas where they are still most abundant, according to a new study leveraging citizen science data for nearly 500 bird species. The findings reveal both urgent threats and potential opportunities for targeted conservation and recovery. Bird populations are experiencing steep declines globally, with North America losing more than 25% of all breeding birds since 1970. While long-term monitoring has revealed these troubling trends, effective conservation requires knowing where populations are declining most. However, this goal has been limited by the lack of fine-scale, spatially comprehensive data on bird population trends, making it difficult to prioritize efforts or detect localized patterns of decline and recovery. To address this need, Alison Johnston and colleagues compiled citizen science data from over 36 million eBird checklists, spanning 2007 to 2021, to generate fine-scale population trends for 495 bird species across North America, Central America, and the Caribbean. By analyzing changes in bird sightings at a high spatial resolution, the authors were able to separate actual shifts in bird populations from differences in observer behavior. Their approach involved using a specialized machine learning model, which enabled the detection of nuanced population changes with high statistical reliability.
The analysis revealed a complex patchwork of local population dynamics; although overall trends show that 75% of bird species are declining across their ranges – and 65% significantly so – nearly every species (97%) is experiencing both gains and losses depending on location within their ranges. Notably, Johnston et al. found that bird populations are declining fastest in the very places where they remain most abundant. This pattern – observed in 83% of species – suggests that even the strongholds of bird populations are no longer safe. The declines are especially severe in birds that breed in grasslands and drylands, and declines are more closely tied to local abundance than geographic position within a species' range, the findings suggest; this points to ecological stress – climate change and habitat loss – as the primary driver of decline. Habitats that support abundant populations may be more vulnerable to these pressures, while species in marginal habitats may have greater resilience. Yet, despite widespread declines, the study revealed pockets of stability, such as in the Appalachians and western mountains, which may offer refuge or point to conditions that could facilitate recovery.
For reporters interested in trends, a 2019 Research Article in Science reported that North America had lost nearly three billion birds since 1970.
