Three billion fewer birds in North America since 1970, suggesting a widespread ecological crisis
Bird populations in the US and Canada have declined by 29% since 1970 -- a loss of almost 3 billion birds, indicating a widespread ecological crisis. Given the current pace of global environmental change, quantifying change in species abundances is essential to assess ecosystem impacts, says a research team from multiple institutions which did the assessment. The analysis shows a loss of more than one in four birds in the past 50 years - a net loss of 2.9 billion birds.
Some of the hardest hit are familiar birds. The results show tremendous losses across diverse groups of birds and habitats - from iconic songsters such as meadowlarks to long-distance migrants such as swallows and backyard birds. According to the research team, more than 90% of this loss can be attributed to 12 bird families, including sparrows, warblers, swallows, and finches. They are common and widespread species that play influential roles in food webs and ecosystem functioning, from seed dispersal to pest control, says the study published in the journal Science.
"Multiple, independent lines of evidence show a massive reduction in the abundance of birds. We expected to see continuing declines of threatened species. But for the first time, the results also showed pervasive losses among common birds across all habitats, including backyard birds," says the study's lead author, Ken Rosenberg, who is a senior scientist at the Cornell Lab of Ornithology and American Bird Conservancy.
Birds are excellent indicators of environmental health and ecosystem integrity, and the ability of scientists "to monitor many species over vast spatial scales far exceeds that of any other animal group," says the research team. They evaluated population change for 529 species of birds in the US and Canada, covering 76% of breeding species.
Citizen-science participants contributed critical scientific data to show the international scale of losses of birds, says coauthor John Sauer from the US Geological Survey (USGS). The study included citizen-science data from the North American Breeding Bird Survey coordinated by the USGS and the Canadian Wildlife Service (the main sources of long-term, large-scale population data for North American birds), the Audubon Christmas Bird Count, and Manomet's International Shorebird Survey.
The analysis shows that grassland birds are especially hard hit, with a 53% reduction in population, which is over 720 million birds since 1970.
"Across breeding biomes, grassland birds showed the largest magnitude of total population loss since 1970 - more than 700 million breeding individuals across 31 species - and the largest proportional loss (53%); 74% of grassland species are declining. All forest biomes experienced large avian loss, with a cumulative reduction of more than 1 billion birds. Wetland birds represent the only biome to show an overall net gain in numbers (13%), led by a 56% increase in waterfowl populations. Surprisingly, we also found a large net loss (63%) across 10 introduced species," say the researchers.
A total of 419 native migratory species experienced a net loss of 2.5 billion individuals. Shorebirds, most of which frequent sensitive coastal habitats, have lost more than one-third of their population.
"Species overwintering in temperate regions experienced the largest net reduction in abundance (1.4 billion), but the proportional loss was greatest among species overwintering in coastal regions (42%), southwestern arid lands (42%), and South America (40%). Shorebirds, most of which migrate long distances to winter along coasts throughout the hemisphere, are experiencing consistent, steep population loss (37%)," the findings state.
Why this decline?
The researchers say that birds are indicators of environmental health, signaling that natural systems across the US and Canada are now being so severely impacted by human activities that they no longer support the same robust wildlife populations.
The steep drop in North American birds parallels the losses of birds elsewhere in the world. According to the researchers, the largest factor driving these declines is likely the widespread loss and degradation of habitat, primarily due to agricultural intensification and urbanization. They face many other threats as well: from collisions with glass, buildings and other structures to pervasive use of toxic pesticides and widespread declines in insects, an essential food source for birds.
Climate change will compound all these problems by accelerating the loss of habitats birds need, and threatening plant communities they need to survive, says the research team, adding that this loss signals a broader crisis in the natural world, one that ultimately affects us all.
"Population loss is not restricted to rare and threatened species, but includes many widespread and common species that may be disproportionately influential components of food webs and ecosystem function. Furthermore, losses among habitat generalists and even introduced species indicate that declining species are not replaced by species that fare well in human-altered landscapes," says the study.
Monitoring data suggest that bird declines will continue without targeted conservation action, "triggering additional endangered species listings at a tremendous financial and social cost."
According to coauthor Peter Marra, former head of the Smithsonian Migratory Bird Center and now director of the Georgetown Environment Initiative at Georgetown University, it is imperative to address immediate and ongoing threats, as the domino effects can lead to the decay of ecosystems that humans depend on for their own health and livelihoods. Birds provide numerous benefits to ecosystems (for example, seed dispersal, pollination, pest control) and economies (47 million people spend $9.3 billion annually through bird-related activities in the US). Hence, their population reductions and possible extinctions will have severe direct and indirect consequences, warn scientists.
What can be done?
The findings, say the researchers, can provide insights into actions that can be taken to reverse the declines. The analysis shows that not all species are on the decline. Some bird species, including raptors and waterfowl, have shown population gains - likely due to focused conservation efforts and endangered species legislation.
Waterfowl (ducks, geese, and swans) have made a remarkable recovery over the past 50 years, made possible by investments in conservation by hunters and billions of dollars of government funding for wetland protection and restoration. Raptors such as the Bald Eagle have also made spectacular comebacks since the 1970s, after the harmful pesticide DDT was banned and recovery efforts through endangered species legislation in the US and Canada provided critical protection.
The team says it is time to expand the conservation and habitat management efforts to help the rest of the birds recover too. "The story is not over. There are so many ways to help save birds. Some require policy decisions, such as strengthening the Migratory Bird Treaty Act. We can also work to ban harmful pesticides and properly fund effective bird conservation programs. Each of us can make a difference with everyday actions that together can save the lives of millions of birds - actions like making windows safer for birds, and protecting habitat," says coauthor Michael Parr, president of American Bird Conservancy.
The researchers say reversing the bird population decline will require concerted action by individuals, governments, and the private sector. They recommend strengthening bird conservation objectives in public land and water management decisions and increasing federal bird conservation incentives for private lands. The scientists suggest implementing a sustainable net-gain policy for all high-quality bird habitat (for example, designated critical habitat and important bird areas), and restoring natural disturbance and water flows to key landscapes.
"Preserve and maintain the quality of bird habitat in protected areas and across large landscapes, and develop/enhance restoration projects across birds' life-cycles and along flyways. Restore and expand full regulatory protection and funding for migratory and endangered birds," say the researchers.