3 billion birds

The western tanager is among the birds whose populations have decreased by 29% since 1970. Loss of habitat, foods, and other factors, from cats to glass windows, were implicated in a newly published study led by Cornell University. Total losses are estimated as three billion birds.

A study published Sept. 19 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 a widespread ecological crisis. 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 including sparrows.

“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. “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 findings showed 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 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, 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 animals showing massive declines, including insects and amphibians,” said coauthor Peter Marra, senior scientist emeritus and former head of the Smithsonian Migratory Bird Center “It’s imperative to address immediate and ongoing threats, both because the domino effects can lead to the decay of ecosystems that humans depend on for our own health and livelihoods — and because people all over the world cherish birds in their own right. Can you imagine a world without birdsong?”

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.

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. It noted that the largest factor driving these declines is loss of habitat 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.

The study also documents a few promising rebounds resulting from galvanized human efforts. Waterfowl (ducks, geese, and swans) have made a remarkable recovery over the past 50 years, due to 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.

“It’s a wake-up call that we’ve lost more than a quarter of our birds in the U.S. and Canada,” said coauthor Adam Smith from Environment and Climate Change Canada. “What our birds need now is an historic, hemispheric effort that unites people and organizations with one common goal: bringing our birds back.”

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