A study published in the journal Science reveals that since 1970, bird populations in the United States and Canada have declined by 29 percent, or almost 3 billion birds, signaling what has been considered a widespread ecological crisis.
The results show tremendous losses across diverse groups of birds and habitats — from iconic songbirds such as meadowlarks to long-distance migrants such as swallows, and backyard birds such as sparrows. More research is needed to pinpoint primary causes for declines in individual species.
“Multiple, independent lines of evidence show a massive reduction in the abundance of birds,” said Ken Rosenberg, the study’s lead author and a senior scientist at the Cornell Lab of Ornithology and American Bird Conservancy in an announcement of the research. “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.”
The study calls birds indicators of environmental health, and says they are signaling that natural systems across the U.S. and Canada are now being so severely impacted by human activities that they no longer support the same robust wildlife populations.
The findings show that of nearly 3 billion birds lost, 90 percent belong to 12 bird families, including sparrows, warblers, finches, and swallows — common, widespread species that play influential roles in food webs and ecosystem functioning, from seed dispersal to pest control.
Among the steep declines noted:
- Grassland birds, such as those found in the Washington County Grasslands, are especially hard hit, with a 53-percent reduction in population — more than 720 million birds — since 1970.
- Shorebirds, most of which frequent sensitive coastal habitats such as are found where the Great and Little Chazy Rivers empty into Lake Champlain, were already at dangerously low numbers and have lost more than one-third of their population.
- The volume of spring migration, measured by radar in the night skies, has dropped by 14 percent in just the past decade.
“These data are consistent with what we’re seeing elsewhere with other taxa showing massive declines, including insects and amphibians,” said coauthor Peter Marra, senior scientist emeritus and former head of the Smithsonian Migratory Bird Center and now director of the Georgetown Environment Initiative at Georgetown University.
The studies authors say evidence for the declines emerged from detection of migratory birds in the air from 143 NEXRAD weather radar stations across the continent in a period spanning over 10 years, as well as from nearly 50 years of data collected through multiple monitoring efforts on the ground.
The analysis 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.
Although the study did not analyze the causes of declines, it noted that the steep drop in North American birds parallels the losses of birds elsewhere in the world, suggesting multiple interacting causes that reduce breeding success and increase mortality. It says that the largest factor driving these declines is likely the widespread loss and degradation of habitat, especially due to agricultural intensification and urbanization.
Other studies have documented mortality from predation by free-roaming domestic cats; collisions with glass, buildings, and other structures; and pervasive use of pesticides associated with widespread declines in insects, an essential food source for birds.
Climate change is expected to compound these challenges by altering habitats and threatening plant communities that birds need to survive. A recent study found loss of wintering habitat in the near future will likely be magnified by the long-term effects of climate change. Another found that by the middle of the next century migratory bird populations will experience novel climates during all phases of their annual life cycles.
The study also documents a few promising rebounds resulting from human efforts. Waterfowl (ducks, geese, and swans) have made a 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 and Peregrine Falcon have also made comebacks since the 1970s, after the harmful pesticide DDT was banned and recovery efforts through endangered species legislation in the U.S. and Canada provided protection.
Images provided by Cornell Lab of Ornithology.