The decline of nature’s song

North American bird populations have declined by 29% since 1970, according to a report published in Science.

The loss equates to almost three billion birds, with results showing profound reductions across diverse groups and habitats – including iconic songbirds such as meadowlarks, 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,” says lead author Ken Rosenberg, from the Cornell Lab of Ornithology, New York, 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 study notes that birds are indicators of environmental health, so the results signal that natural systems across the US and Canada are now so severely affected by human activities that they no longer support the same robust wildlife populations.

The findings show that of the lost species, 90% belong to 12 bird families, including sparrows, warblers, finches and swallows – common and widespread species that play influential roles in food webs and ecosystem function, from seed dispersal to pest control.

Among the steep declines recorded, grassland birds were especially hard hit, suffering a 53% reduction in population – a loss of more than 720 million birds – since 1970.

Shorebirds, mostly denizens of sensitive coastal habitats, were already at dangerously low numbers in the 1970s and have lost more than one-third of their population since.

The study also noted that the volume of spring migration, measured by radar in the night skies, has dropped by 14% in the past decade.

Evidence was gathered over more than a decade from the 143 NEXRAD weather radar stations across North America, which detect migratory birds in the air, in addition to the nearly 50 years of data collected through multiple monitoring efforts on the ground.

“These data are consistent with what we’re seeing elsewhere with other taxa showing massive declines, including insects and amphibians,” says co-author Peter Marra, former head of the Smithsonian Migratory Bird Centre and now director of the Georgetown Environment Initiative at Georgetown University, in Washington, DC.

“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?”

The study didn’t extend to the causes of declines, but it noted that the steep reduction of North American bird populations parallels avian losses elsewhere in the world, suggesting overlapping causes that reduce breeding success and increase mortality. 

The study notes that the largest factor driving declines is likely the widespread loss and degradation of habitat, especially due to intensive agricultural and the expansion of urban areas.

Causes of mortality detailed in other studies include predation by domestic cats; collisions with glass, buildings, and other structures; and the pervasive use of pesticides – a leading factor in the widespread decline in insects, an essential food source for birds.

Climate change is predicted to amplify these problems by altering habitats and threatening the plant communities that birds need to survive.

As a welcome counterpoint, the study also points to a few promising North American species rebounds that have resulted from concerted human effort.

Waterfowl (ducks, geese, and swans) have benefitted from conservation investment by hunters and billions in public funding for wetland protection and restoration. Raptors such as the iconic bald eagle have also made spectacular comebacks since the 1970s, assisted by bans on the harmful pesticide DDT and critical protection provided by US and Canadian endangered species legislation.

“It’s a wake-up call that we’ve lost more than a quarter of our birds in the US and Canada,” says co-author Adam Smith, from Environment and Climate Change Canada.

“But the crisis reaches far beyond our individual borders. Many of the birds that breed in Canadian backyards migrate through or spend the winter in the US and places farther south – from Mexico and the Caribbean to Central and South America.

“What our birds need now is an historic, hemispheric effort that unites people and organisations with one common goal: bringing our birds back.”