This story was originally published by The Guardian and appears here as part of the Climate Desk collaboration.

The world’s birds, described as the planet’s “canaries in the coal mine,” are disappearing in large numbers as the colossal impact of humanity on the Earth grows, a global review has found.

There are about 11,000 species of bird spanning the globe, but the populations of half of them are falling, while just six per cent are increasing. Their flight and song make them easier to study than many animals, meaning they are the best-studied large group.

Bird populations are also affected by all the damage caused by human activity, from the destruction of wild habitat, the climate crisis, and pesticides and other pollution, to overhunting and impacts of alien species and disease. This makes them the best living indicators of global change, the scientists said.

Billions of birds have been lost in recent decades in North America and Europe alone, and while there are more species in the tropics, a higher proportion are at risk of extinction in temperate and largely richer nations, the review found.

Conservation efforts have been successful at rescuing individual species in specific locations from the brink, but political will and funding are needed to reverse the global decline, the researchers said.

“Birds are a much more powerful taxa [than others] to tell us a story about the health of the planet,” said Alexander Lees, at Manchester Metropolitan University, U.K., and who led the review. “We know so much about them — we don’t even know how many insect species there are.”

“Currently, we’re triaging the species at risk, but we’re not stopping the flow of species towards extinction,” he said. “There’s only so much we can do with site-based conservation.” While conservation work has improved populations of 70 species enough to lower their risk of extinction since 1988, 391 species have deteriorated, the review found.

Global data collated by Birdlife International underpinned the review and the conservation group’s CEO, Patricia Zurita, said: “Birds truly are the canary in the coal mine as indicators for the health of our planet, given their sensitivity to ecosystem changes, their ubiquity around the planet, and how well studied they are. [We] need to listen and act upon what birds are telling us, as they disappear ever faster.”

Billions of birds are disappearing because of humanity’s impact on the Earth, a global review finds. #ClimateCrisis #GlobalHeating

One exception to the broader decline is among water birds, where populations living in wetlands in North America and Europe have grown by 13 per cent since 1970. The restoration of relatively small wetland areas can have a huge impact, while birds living in grasslands and forests need much bigger areas.

The review, published in the journal Annual Review of Environment and Resources, found that 48 per cent of bird species are known or suspected to be undergoing population declines, compared with 39 per cent with flat trends, six per cent showing increases and seven per cent with unknown trends.

Most long-term data is from Europe, North America, India and some sites in Africa but more recent monitoring in Latin America and Asia shows similar results. The population of birds in the U.S. and Canada has fallen by three billion since 1970, while 600 million have disappeared from Europe since 1980.

The review notes the extraordinary range of birds, from Antarctic petrels nesting 200 kilometres inland in Antarctica to the Hornby’s storm-petrel found nesting in the Atacama desert. A Rüppell’s vulture has been reported flying at an altitude of 11,300 metres, while emperor penguins can dive more than 500 metres below the sea surface. Birds have huge cultural value but are also vital to ecosystems, including dispersing seeds and eating pests.

Birds are affected by all the impacts of human activity. For example, 2.7 million are estimated to die every year in Canada alone from eating pesticides, while domestic cats may kill 2.4 billion a year in the U.S. The most threatened families of birds are those which are larger and take longer to reproduce, including parrots, albatrosses, cranes and stocky birds like the Australian brushturkey. All countries host at least one globally threatened bird species and 10 nations have more than 75, the review found.

Farmland species are declining precipitously, the review said, down 57 per cent in Europe since 1980. This is due to the intensive agriculture that supplies cheap food, Lees said, adding: “If we want farmers to farm wildlife, we have to pay for that as a society.”

Individual species have been rescued, such as the Mauritius kestrel, which was down to one breeding female but has now been revived to a population of hundreds, and the Alagoas curassow in Brazil, which was extinct in the wild but has been restored from birds held by private collectors.

But the review concluded: “The growing footprint of the human population represents the ultimate driver of most threats to avian biodiversity. A lack of progress in conserving [birds] usually reflects a lack of resources or political will, rather than a lack of knowledge of what needs to be done.”

Prof. Stuart Pimm at Duke University said the review was excellent and authoritative, even with the sparse availability of data in some regions. “What is certain is that about two-thirds of all bird species live in tropical forests, and human actions are shrinking those habitats,” said Pimm. “Even without detailed population estimates, their numbers are surely in decline.”

Lees said people need not feel powerless in helping to reverse the decline but added: “We all have connections [to birds]. If a company is associated with deforestation in Brazil, don’t buy stuff from them. And if everyone spares as much land as possible within their gardens for nature, then that adds up to quite a large area. Another lever is voting — we get the politicians we vote for.”

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