A study from Cornell University last month found the US and Canada has lost one in four of their birds – or nearly 3 billion breeding adults – since 1970. This first-ever comprehensive assessment of net population changes in the U.S. revealed across-the-board declines that scientists called “staggering” with devastating losses among birds in every biome.

Grassland birds have been hit hard, for example, with a 53% reduction in population. Much of the grassland (or prairie) that used to dominate much of the centre of the continent has been lost to ranching and development.

Migrants that overwinter in the tropics and breed in boreal forests have been hit by huge habitat clearance/conversion and deforestation. The tracking of mass movements of birds by radar has found a 14% drop in the volume (or biomass) of spring migration in just the last decade.

Shorebird populations have shrunk by 70% across North America since 1973.  Rapidly changing climate conditions in the Arctic are taking a toll, but coastal development, hunting in the Caribbean and the same agricultural shifts affecting grassland species are all having an impact.

Some of the species involved in the population crashes may not be familiar to many people, but it appears that it’s the ‘common’ birds – the species that many people see every day – that have suffered the greatest losses, according to the study. More than 90% of the losses (more than 2.5 billion birds) come from just 12 families including the sparrows, blackbirds, warblers, and finches.

These include species regularly seen at bird feeders, such as Dark-eyed Juncos (down by 168 million) and White-throated Sparrows (down by 93 million). Eastern and Western Meadowlarks are down by a combined 139 million individuals. Even the ubiquitous Red-winged Blackbird—a common sight in virtually every marsh and wet roadside across the continent—has declined by 92 million birds.

Cornell Lab director John Fitzpatrick called the declines detailed in the report, “A staggering loss that suggests the very fabric of North America’s ecosystem is unraveling.”

Whilst climate change (two-thirds of bird species in North America are at risk of extinction because of the climate crisis, according to a recent report from researchers at the Audubon Society), and habitat loss are now the most widely-recognised drivers in this population crash, other less obvious factors are also involved.

Domestic cats (thought to kill approximately 2.4 billion birds every year in the US), collisions with glass and buildings (a report in 2014 put the likely annual kill between 365 million and 1 billion birds, with a median estimate of 599 million), and a decline in the insects birds eat – because of widespread pesticide use – also contribute to such enormous losses

Bird population losses of course mirror similar catastrophic declines being seen among insects and amphibians across the planet, and in the depletion of the UK’s nature described in the recently-released State Of Nature Report 2019.

Our War On Wildlife will have consequences that we don’t yet fully understand, but as Brooke Bateman, the senior researcher who wrote the Cornell report, puts it: “Birds are indicators of the health of our environment. If they disappear we have to understand that the environment is changing for us as well.”

Feature image Baltimore Oriole by Gary Mueller/Macaulay Library at Cornell Lab of Ornithology