Common Birds in Crisis: New Study Reveals Hidden Drivers of North America’s Avian Decline
Long-term surveys show that losses are not led by rare species, but by steep declines in once-abundant birds whose numbers once defined the landscape
The shifting balance of bird populations
North America has lost nearly a third of its birds over the past half century, but a new study published in Science Advances reframes the crisis. The research, led by Gates Dupont and Andy Dobson, analysed almost 39,000 Breeding Bird Surveys spanning 57 years and 244 species. The results reveal a striking pattern: it is the common species - once abundant and seemingly secure - that have driven the steepest declines, while many less common birds have modestly increased.
This finding overturns the conventional narrative that rare species are the most vulnerable to disappearance. Instead, species that were formerly near their carrying capacity, dominating communities, are now shrinking fastest. In contrast, birds that were scarce have often grown in number, though their gains are too small to offset the losses of dominant species. Across the six regions studied, total abundance fell by an average of 31 per cent, with biomass down by 19 per cent.
A handful of species driving losses
Just a tiny fraction of species account for most of the decline. In each region, roughly 5 per cent of species - those that were among the most common - contributed more than 80 per cent of total losses. In the Appalachian Mountains, for instance, only six species were responsible for the majority of declines. This concentration underscores how vulnerable ecosystems become when heavily reliant on a few numerically dominant birds.
The data also show that community evenness has increased - with the most common species reduced and the least common species gaining - but this levelling has come at the expense of overall abundance and ecosystem services.
Land-use change and human pressures
The study identifies human land-use change as the principal driver of the pattern. Birds that thrive in open or agricultural habitats have declined most steeply, while some forest-preferring species have fared better thanks to reforestation in parts of North America. Larger-bodied species and those with extensive ranges were more likely to have been abundant in the past - and are now among the steepest decliners, an effect the authors describe as “numerical exposure.”
In essence, being common has made these birds more exposed to human disturbance across wide landscapes, turning what was once an advantage into a liability.
Conservation successes and failures
Not all is bleak. Species such as the Bald Eagle (Haliaeetus leucocephalus) have rebounded spectacularly thanks to targeted conservation and the banning of DDT. Yet these bright spots are exceptions. For most abundant species, decline has been steady and severe. Unlike rare birds, which attract focused conservation attention, common species often slip beneath the radar until their absence is undeniable.
The researchers stress that the decline of common birds carries especially grave consequences for ecosystem services such as pollination, seed dispersal, pest control and nutrient cycling. Because common species provide the bulk of these services, their losses jeopardise both natural ecosystems and human well-being.
Rethinking conservation priorities
The authors argue that conservation must shift emphasis from an exclusive focus on rare and endangered species to include the stewardship of common birds. Monitoring and protecting abundance at scale, rather than just preventing extinction, may be the only way to preserve ecological function.
As Dupont and Dobson conclude, “We need to adopt a more inclusive approach to biodiversity loss - one that tracks the fate of common species whose declines reshape entire communities and threaten the ecological services on which we all depend.”
September 2025
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