Scientists have known for some time now that tropical birds are falling in number. In general, the cause has been attributed to forest degradation and fragmentation. But a 2020 study has shown that the populations of certain bird species living in regions of the Amazon still undisturbed by human activities have also dropped.

Scientists from Brazilian and North American institutions have been taking a closer look at possible causes by analyzing the behavior of insect-eating understory birds — those who live closer to the ground, beneath the forest canopy — in a preserved area of the Amazon rainforest some 80 kilometers (50 miles) from the city of Manaus.

The study, which was only recently published, confirmed something that the researchers had suspected: Climate changes are transforming the lives of birds even in areas previously considered to be refuges, with more stable microclimates, intact forest canopy and plenty of food supply.

Analyses of data collected over the past 27 years showed a drop in population of 24 of the 29 bird species studied. The main causes were longer dry seasons and less rain in recent years.

“This article unequivocally links climate changes with the birds’ survival. It was just a hypothesis until now, but this analysis confirms that they are playing a significant role in bird deaths in central Amazonia,” says biologist Jared Wolfe from Michigan Technological University, U.S.

Wolfe and his colleagues arrived at an estimate that an average increase of 1° Celsius (1.8° Fahrenheit) during the dry season in the Amazon will reduce the average survival rate in birds by 63%. While this tiny jump on the thermometer may seem insignificant to some people, it is not for animals Wolfe says. And the change has been consistently recorded throughout central Amazon since the start of this century.

But the study warns not only of shrinking bird populations in the planet’s largest tropical forest, but also of their possible extinction in the Amazon and, over time, their full disappearance in other locations as well.

“What makes Amazonia so special is that it wasn’t impacted by Earth’s glacial events. The forests there have existed for millions and millions of years, and this allowed birds to evolve without extinctions. And the results are incredible. It’s like an evolutionary playground, where birds display fantastic colors and behaviors. The tanagers’ dances are one example,” Wolfe explains.

He says he believes that the level of diversity and evolution found here was only possible because the Amazon was able to provide climatic stability, which made it globally unique. “These birds obviously experienced temperature changes of 1 ^oC (1.8 ^oF) or 2 ^oC (3.6 ^oF) over these millions of years, but they occurred slowly over time, allowing the species time to adapt. Right now, we are talking about rapid changes over 10 or 20 years, and these birds don’t have the [ability] to keep up with them. And it’s very sad,” he says.

Climatic stress and less to eat

According to the researchers, there are two hypotheses for why more birds are dying. “The first is physiological: the drier climate makes it harder for them to survive. If it’s very hot or if it rains less, these are direct climate effects,” says David Luther, co-author of the study and a professor at George Mason University, Virginia, U.S. “But an indirect impact would be that there is less food available, especially the insects upon which they feed.”

Therefore, Luther says, the second hypothesis is the most plausible explanation for what is happening in the Amazon. He also participated in a study in Panama, where birds captured in the forest were exposed to higher temperatures and, to the surprise of the researchers, they appeared to tolerate the changes well.

This implies that the increased mortality of Amazonian birds would then be part of a cascade effect resulting from climate changes. Insect populations would fall in both numbers and diversity because they can’t adapt to extreme temperatures and drier topsoil, which in turn makes them less available as food to the birds.

Greater impact on species with longer lifespans

Wolfe commented on another point raised by the study: Climate changes in the Amazon appear to be mostly affecting birds with longer lifespans. In general, tropical understory birds live longer and invest more in their survival than in reproduction.

“If there is less food, these birds will most likely reduce their reproductive efforts even more or completely stop reproducing simply because they don’t have the energetic capacity to do it,” Wolfe says.

Next, the biologists plan to compare their results with a new study that will evaluate the decline of bird populations in fragmented parts of the forest and with a second project already underway together with Amazonas Federal University, Brazil, in which a section of the forest is being irrigated throughout the dry season to be able to analyze the birds’ response to the experiment.

“We’re measuring the insects and analyzing bird metabolisms to understand just how well-fed they are and what their reproductive conditions are, so we can know exactly how temperature affects these microclimate regions,” Wolfe says.

What is already certain and needs no further study is the fact that, definitively, global warming is already affecting the survival of birds in the Amazon.

“If high temperatures persist and dry periods intensify, most of the species will continue to diminish to the point at which they will no longer exist,” Luther warns. “And as more and more species are lost, this whole enormous and complex ecosystem will be impacted.”

 
Banner image: A white-plumed antbird (Pithys albifrons). Image by Philip Stouffer.

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Citations:

Wolfe, J.D., Luther, D.A., Jirinec, V., Collings, J., Johnson, E.I., Bierregaard, R.O., Stouffer, P.C. (2025). Climate change aggravates bird mortality in pristine tropical forests. Science Advances, 11(5). Doi:10.1126/sciadv.adq8086

Pollock, H.S., Rutt, C.L., Cooper, W.J., Brawn, J.D., Cheviron, Z.A., Luther, D.A. (2023). Equivocal support for the climate variability hypothesis within a Neotropical bird assemblage. Ecological Society of America, 105(2). doi:0.1002/ecy.4206