Birds in tropical forests plummet 90% in 40 years

The number of birds living in tropical forests has plummeted by up to 90 per cent in just 40 years, according to new research.

Conservationists described the decline as "concerning" after finding that the vast majority of species studied in a Panamanian rainforest had seen declines in numbers of at least 50 per cent between 1977 and 2020.

Study lead author Dr Henry Pollock, of the University of Illinois in the United States, said: "Many of these are species you would expect to be doing fine in a 22,000-hectare national park that has experienced no major land use change for at least 50 years.

"It was very surprising."

Co-author Professor Jeff Brawn, also of 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 only one park. We can’t necessarily generalise to the whole region and say the sky is falling, but it’s quite concerning."

Loss of birds from any habitat can threaten the integrity of the entire ecosystem, warned the researchers.

In the Neotropics - South America, Central America and the Carribean- birds are key seed dispersers, pollinators, and insect eaters. Fewer birds could threaten tree reproduction and regeneration, impacting the entire structure of the forest, a pattern shown elsewhere after major bird declines.

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But the research team haven’t looked at the impacts or the underlying causes yet, focusing instead on documenting the numbers.

University of Illinois scientists initiated a twice-yearly bird sampling effort in 1977. Each year, members of the team set up mist nets in the rainy and dry seasons to capture birds moving through the study site.

Mist nets gently entangle birds, allowing researchers to carefully pluck them out. They then identify, measure, and band the birds before releasing them, unharmed, back into the forest.

Over 43 years and more than 84,000 sampling hours, the researchers captured more than 15,000 unique birds representing nearly 150 species and gathered sufficient data to track 57 of those.

The researchers noted declines in 40 species (70 per cent), and 35 species lost at least half of their initial numbers. Only two species - a hummingbird and a puffbird - increased.

Dr Pollock said: "At the beginning of the study in 1977, we’d catch 10 or 15 of many species.

"And then by 2020, for a lot of species, that would be down to five or six individuals."

Although the birds represented a wide variety of guilds - groups that specialise on the same food resources - the researchers noted declines across three broader categories: common forest birds; species that migrate seasonally across elevations; and "edge" species that specialise in transition zones between open and closed-canopy forest.

Prof Brawn says the decline among common species is most alarming.

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He said: "The bottom line is these are birds that should be doing well in that forest. And for whatever reason, they aren’t. We were very surprised."

Edge species were hardest hit, most declining by 90 per cent or more.

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

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

But over time, the road stopped being maintained and has since turned into a small gravel road and the forest canopy filled in overhead.

Dr Pollock said: "The fact that edge species went away when the road did is not concerning.

"It shows what we would expect with forest maturation and the loss of those successional habitats."

The researchers are reluctant to generalise their results beyond their study site, pointing out the scarcity of similar sampling efforts throughout the tropics.

Dr Pollock said: "Right now, this is really the only window we have into what’s going on in tropical bird populations.

"Our results beg the question of whether this is happening across the region, but unfortunately we can’t answer that. Instead, our study highlights the lack of data in the tropics and how important these long-term studies are.."

The study wasn’t designed to explain why birds are declining in the forest, but the researchers have some ideas they want to follow up on including changing rainfall levels, food resources, and reproductive rates, many of which may be tied to climate change.

Prof Brawn added: "Almost half the world’s birds are in the Neotropics, but we really don’t have a good handle on the trajectories of their populations."

He added: "I think it’s very important more ecological studies be done where we can establish trends and mechanisms of decline in these populations.

"And we need to do it damn quick."

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

SWNS