The number of birds living in tropical forests “has dropped by 90% in just 40 years”

According to new research, the number of birds living in rainforests has dropped by 90% in just 40 years.

Ecologists called the decline “worrying” after finding that the vast majority of species studied in a Panamanian rainforest had experienced declines of at least 50% between 1977 and 2020.

The study’s lead author, Dr Henry Pollock, from the University of Illinois in the US, said: “Many of these are species that you would expect to are doing well in a 22,000 hectare national park that has seen no major land use changes for at least 50 years.

“It was very surprising.”

Co-author Professor Jeff Brawn, also from the University of Illinois, said: “This is one of the longest, if not the longest, study of its kind in the Neotropics.

“Of course, it’s just one park. You can’t necessarily generalize to the whole region and say the sky is falling, but it’s quite concerning.”

The loss of birds from any habitat can threaten the integrity of the entire ecosystem, researchers say.

In the Neotropics, birds are the main seed dispersers, pollinators and insect eaters. Fewer birds could threaten tree reproduction and regeneration, impacting the entire structure of the forest, a pattern seen elsewhere after major bird declines.

But the research team has yet to look at the impacts or underlying causes, instead focusing on documenting the numbers.

University of Illinois scientists began a biannual bird sampling effort in 1977. Each year, team members would set up mist nets during the rainy and dry seasons to capture birds moving at the study site.

The mist nets gently entangle the birds, allowing researchers to carefully snatch them. They then identify, measure and band the birds before releasing them, safe and sound, into the forest.

A Kingfisher sits on a branch above the River Wandle at Wandsworth on August 26, 2020 in London, England.
Dan Kitwood/Getty Images

In 43 years and more than 84,000 hours of sampling, researchers captured more than 15,000 unique birds representing nearly 150 species and collected enough data to track 57 of them.

The researchers noted declines in 40 species (70%) and 35 species lost at least half their original numbers. Only two species – a hummingbird and a puffbird – increased.

Dr Pollock said: “At the start of the study in 1977 we were catching 10 or 15 of many species.

“And then by 2020, for a lot of species, it would be down to five or six individuals.”

Although the birds represented a wide variety of guilds – groups that specialize on the same food resources – the researchers noted declines in three broader categories: common forest birds; species that migrate seasonally through altitudes; and “edge” species that specialize in the transition zones between open and closed canopy forest.

Professor Brawn says the decline of common species is most alarming.

He said: “At the end of the day, these are birds that should do well in this forest. And for some reason they don’t. We were very surprised.”

Edge species were the hardest hit, with most declining by 90% or more.

But the researchers weren’t surprised. In fact, the disappearance of edge species increased their confidence in their results.

Forty years ago, a paved access road crossed the site. It has created the ideal edge habitat for birds that like openings in the forest canopy.

But over time the road ceased to be maintained and has since turned into a small gravel road and the forest canopy has filled in above.

Dr Pollock said: “The fact that the edge species disappeared when the road did is of no concern.

“It shows what we would expect with the maturing of the forest and the loss of these successional habitats.”

The researchers are hesitant to generalize their findings beyond their study site, pointing to the paucity of similar sampling efforts across the tropics.

Dr Pollock said: “At the moment this is really the only window we have on what is happening in tropical bird populations.

“Our results raise the question of whether this is happening across the region, but unfortunately we cannot answer this. Instead, our study highlights the lack of data in the tropics and the importance of these long-term studies.

The study was not designed to explain why the birds are in decline in the forest, but the researchers have insights they want to track, including changes in rainfall levels, food resources and reproductive rates , many of which may be linked to climate change.

Professor Brawn added: “Almost half of the world’s birds are found in the Neotropics, but we really don’t have a good idea of ​​their population trajectories.”

He added: “I think it is very important that more ecological studies are carried out to establish the trends and mechanisms of decline in these populations.

“And we have to do it damn fast.”

The findings were published in the journal Proceedings of the National Academy of Sciences.

This story was provided to Newsweek by Zenger News.


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