In the Tropics, a Troubling Echo of North America’s Bird Declines

From the Autumn 2024 issue of Living Bird magazine. Subscribe now.

In 2001, two ornithologists—John Blake and Bette Loiselle—headed deep into the Ecuadorian Amazon. Getting there required a day’s long trek down the Napo and the Tiputini rivers into the heart of Yasuní National Park and the Tiputini Biodiversity Station.  

“The boat ride was the most exhil­arating experience,” recalls Loiselle. “The area was so full of birds and monkeys hanging from the trees.” Sounds from antbirds, antthrushes, and leaftossers filled the air at Tiputini. Nearly everywhere they went they could hear the slowly accelerating series of hollow notes of the Striated Antthrush and the clear whistle, then stuttering trill, of the Black-faced Antthrush. More than 500 different bird species have been historically documented in this forest.  

The duo, now both professors in the Wildlife and Ecology Conservation department at the University of Florida, have returned to the region every year for more than two decades to check in on the bird populations. But starting around 2011 or 2012, things started to change.  

“We started hearing fewer and fewer birds,” says Blake. “We used to hear the diagnostic song of the Striated Antthrush all the time, but now they’ve basically disappeared.”  

“It started to get a bit depressing to watch the birds decline,” says Loiselle.

A lot of media attention has been paid to the staggering bird losses in North America—set off by research published in the journal Science in 2019 that documented a 29% overall decline in breeding bird populations in the U.S. and Canada. But in South America, there are also troubling trends quietly brewing for birds in the rainforest. A collection of long-term bird population surveys in and around the Amazon region—conducted at research sites in remote and intact rainforest, the kinds of places that should be immune to population declines—are exhibiting even steeper losses than the declines reported for forest birds in North America.  

“The thing that is troubling,” says Kelly Swing, founder of the Tiputini Biodiversity Station in Ecuador, “is that Tiputini is in a remote spot. It’s not like we have a tremendous amount of defor­estation or a lot of pesticides. We are in the middle of nowhere.”

One of the Most Biodiverse Places on Earth

In northeastern Ecuador, nestled deep in the Amazon, lies one of the most biodiverse places on Earth. Yasuní National Park is home to the Waorani people and at least two Indigenous tribes living in voluntary isolation, as well as thousands of wildlife species. Plant surveys here have found 655 species of trees, shrubs, and vines in just a single hectare—almost as many as there are tree species in the entire continental U.S. A study published in the journal PlosOne in 2010 found that the local diversity of amphibians, birds, mammals, and plants is higher in Yasuní than anywhere else on Earth.  

Within the park, along the Tiputini River, sits Tiputini Biodiversity Station, founded in 1995 by the Universidad San Francisco de Quito in Ecuador, in col­laboration with Boston University. Blake and Loiselle first arrived at Tiputini in 2001 as professors from the University of Missouri, St. Louis, looking for a place to set up a long-term research site and monitor populations of tropical birds. As soon as they arrived at the station, they knew Tiputini was the place.  

“To be dropped into this Amazon site with tens of kilometers of forest all around you, it was really a wonderful opportunity,” says Loiselle.

The pair returned to their study sites at the Tiputini Biodiversity Station year after year. But in this magical hyper-biodiverse place that feels worlds apart from the plight of deforestation and habitat loss afflicting other parts of the tropics, Blake and Loiselle found populations of tropical forest birds in steep decline—down nearly 50% since 2001. Using observational surveys and data logs from capturing and tagging birds using mist nets, Blake and Loiselle noted several different groups of birds were in trouble.  

“At least 90% of the species that we had enough data to look at are declining, and very few species have shown increases,” says Loiselle. Some of the hardest hit were the insectivores, species that forage for insects on the ground and in the canopy. Terrestrial insectivores such as White-breasted Wood-Wren, Black-faced Antbird, and Spot-backed Antbird declined sharply, as did species such as White-plumed Antbird and White-cheeked Antbird that gobble up insects fleeing army ants on the forest floor.  

“As an Amazonian, it’s really concerning what is happening to the birds and all of Mother Nature,” wrote Geovanny Rivadeneyra, a naturalist and guide from the Indigenous Kichwa com­munity of Añangu along the Napo River, in an email. “We haven’t heard the songs of several terrestrial insectivores such as Scaled Antpitta, Striated Antthrush, and Rufous-capped Antthrush for the last 15 years. In all of my travels throughout the Amazon, species such as White-lored Antpitta and Thrush-like Antpitta have been decreasing to the point that they are no longer being heard.”  

“It’s not just the insectivores that have declined,” adds Blake. “Other groups have declined as well.” Fruit-eaters, such as toucans and aracaris, and omnivores (birds that eat insects and fruits) also showed steady declines.  

In June 2024, a summary of Blake’s and Loiselle’s 23 years of research at Tiputini was published in the journal Global Ecol­ogy and Conservation, under the troubling title “Sharp declines in observation and capture rates of Amazon birds in absence of human disturbance.” Perhaps even more alarming, Tiputini is not unique—similar bird declines are being reported in other protected forests of the tropics.  

In Panama, a team of scientists from four universities in the United States and Canada analyzed data from a 44-year study of mist-net bird surveys in an area that has been protected as part of Soberanía National Park since 1980. Their study, published in the journal Proceed­ings of the National Academy of Sciences in 2022, reported widespread declines of birds with different foraging strategies across 54,000 acres of relatively intact tropical forest along the Panama Canal. Species that forage in the midstory, such as the Olive-streaked Flycatcher, or on the ground, such as the Wing-banded Antbird, were among the biggest declin­ers. Altogether, the authors noted 35 bird species showing declines of more than 50% from 1977 to 2020.  

Scientists in Brazil are also reporting worrisome declines among forest birds. Working in a 37,000-acre protected area (about the size of Bryce Canyon National Park), a team of scientists from Brazil, Colombia, and the United States used mist nets to resurvey bird populations at study sites that were previously surveyed in the 1980s. Their resurveys from 2008 to 2016 found declines among 21 species, particularly birds that forage for insects near the ground. According to their research, published in the journal Ecol­ogy Letters in 2020, the bird communities in relatively undisturbed rainforests of the Brazilian Amazon now have species compositions trending toward those in disturbed forests.

“Species that started declining in abundance, some of them upwards of more than 50%, were the same species that would disappear from forest frag­ments due to edge effects,” says Jared Wolfe, an ornithologist at Michigan Tech who worked on the study. When forests are fragmented, explains Wolfe, sunlight and wind infiltrate the forest, changing the forest microclimates and ultimately affecting insect populations and the ability of birds to find suitable climates. Wolfe thinks that these edge effects impacted forest bird species such as Rufous-bellied Antwren, Black-tailed Leaftosser, and Musician Wren—all spe­cies now scarcely detected or much rarer at these Amazon rainforest sites.

“Whenever you talk about declines, we always look for the guilty party, and usually the guilty party is pretty easy to identify,” says Kelly Swing of the Tiputini Biodiversity Station. But in the case of these puzzling bird declines in areas pro­tected from deforestation and habitat loss, “it’s not so easy to point fingers,” he says.

Signs of Bird Declines at Remote Science Stations Across Central and South America

Soberanía National Park, Panama
Mist-net surveys from 1977 to 2020 revealed that populations of 35 out of 57 understory forest bird species had declined by 50% or more.

Tiputini Biodiversity Station, Ecuador
Observational and mist-net surveys showed forest bird populations decreased nearly 50% since 2001, particularly among insectivores.

Biological Dynamics of Forest Fragments Project, Brazil
Fourteen insect-eating, ground-dwelling bird species experienced significant population declines when resurveyed in the 2010s, compared to the 1980s.

Map of Central and South America showing areas of forest cover. Bright green indicates intact forest landscapes*; lighter green indicates other types of forest cover. Map source: based on Hansen/UMD/Google/USGS/NASA, 2013.

*Defined as an unbroken expanse of natural ecosystems within the zone of current forest extent, showing no signs of significant human activity and large enough that all native biodiversity could be maintained

Stark Declines in Insect Numbers

For clues as to what might be going on with tropical forest birds, some scientists are looking at insects. But data on tropical insects are hard to come by, so much of what is known about insect populations comes from observation.

In the early 2000s, Swing started taking note of the insects attracted to lights near the Tiputini research station in Ecuador.

“Early along it was easy to get repre­sentatives of at least 10 genera of insects and usually twice that number of species. But now … if you see two genera, that’s pretty good,” Swing says. “Insects that used to be rare are now nonexistent, and the insects that used to be common are now pretty rare.”  

Observations of insect numbers in a tropical forest in Costa Rica mirror Swing’s experience. Daniel Janzen and Winnie Hallwachs, tropical ecologists at the University of Pennsylvania, documented large declines in insects in a 2020 study published in the journal Proceedings of the National Academy of Sciences. Janzen and Hallwachs monitored the insect pop­ulation and photographed insects coming to light traps in Costa Rica starting in 1984. The study includes a photo from 1984 that shows several thousand insects, completely filling the frame. But by 2019 on the same date and in the same location, only a few hundred insects dotted the image.

Insect declines could be a factor in the drop-offs among insectivorous tropical forest birds, but as Blake points out, “the fact that frugivores and omnivores have also declined, it suggests that there’s something else going on.”

“Whether it’s different impacts of cli­mate change, or whether the birds got hit particularly hard with the La Niña event and simply don’t have the reproductive capacity to increase rapidly, we don’t really know,” he says.

One thing is clear, especially to the people living in the region: “The climate is definitely changing,” says Rivadeneyra. “In the summer there is a lot of rain, causing the rivers to flood, and in the rainy season there is too much sun—many plants dry out and the soil cracks.”

Back in Brazil, Michigan Tech orni­thologist Jared Wolfe and colleagues have documented temperature and pre­cipitation changes at their study sites in the Amazon. Their research, published in the journal Science Advances in 2021, showed that temperature had increased by 1° Celsius and precipitation increased by 13% in wet seasons since 1966, while during the dry season tem­peratures had increased 1.65° Cel­sius with a 15% decrease in precipitation.  

While that might seem like a small change, “Amazonian forests have had relatively stable climate for hundreds of thousands, if not millions of years,” says Wolfe. “So these birds over time have become very locally adapted to their environment and that might make them less resilient to these rapid changes in climate. And that might be one of the reasons why we see such sensitivity in some areas, like the central Amazon and the Tiputini site.”

Changes in rainfall could also be affecting forests in Ecuador. Blake and Loiselle noted several severe La Niña events that brought more rain than nor­mal to the region from 2008 to 2013.  

“With severe La Niña events, there’s lots and lots of rain and that can greatly impact the foraging ability of birds, par­ticularly insectivores,” says Blake. The declines in bird populations Blake and Loiselle reported coincide with increases in La Niña events in Ecuador, suggesting a possible culprit.  

According to Flavio Lehner, a climate scientist at Cornell University, La Niña and El Niño events cause a change in the sea surface temperatures in the Pacific Ocean near the equator. El Niños result in warmer oceans and La Niñas in colder ocean surface temperatures.  

“As a consequence of that, it kind of disrupts the weather around the world,” says Lehner. Some places receive more rain while others receive less. But recent decades have seen a tendency towards more La Niña events with some persistent changes in rainfall patterns. Whether climate change is responsible for that shift towards more La Niñas and whether it will continue, “we frankly don’t know yet,” Lehner says. “It’s actually one of the biggest current hot topics among climate-change scientists.”

Listening for Answers

While scientists may have different ideas about the factors in Neotropical forest bird declines, there’s uniform agreement on what’s needed to get at the root causes—more data.

“We need to be able to scale data collection and trust it,” says Glenn See­holzer, an ornithologist who has been studying birds in the Amazon for the past two decades. He says audio surveys of bird calls and songs provide a comple­mentary methodology for monitoring populations, along with mist-netting and visual counts. “In the Amazon you can detect around 90% of what’s there just by closing your eyes. There’s no place in the world I think is better suited to bioacous­tic monitoring than the Amazon.”

Today Seeholzer is curator of the Cornell Lab of Ornithology’s Macaulay Library, which houses the world’s biggest collection of bird audio recordings. Recently Seeholzer joined up with a coworker, Macaulay Library archivist and sound recordist Jay McGowan, and Juan Francisco Herrera Cueva, a researcher at Universidad San Francisco de Quito, to explore the possibilities of developing avian monitoring tools for the Amazon focused on sound.  

Monitoring bird populations using sound is a great way to collect a lot of data over large areas, and that type of data is sorely needed, says Seeholzer. The first step, he says, is building “a reference sound archive for Amazonian birds that will allow us to train Merlin Bird ID to identify the birds.”

Merlin Bird ID is the free smartphone app from the Cornell Lab that can identify birds from a brief audio recording. But before Merlin’s AI algorithms can identify bird sounds, the app needs a reference database of at least 100 recordings per bird species—representing the complete vocal repertoire of each species.  

Currently the Macaulay Library holds around 38,000 recordings from Amazo­nian birds, which means around 110,000 more sound recordings are needed before Merlin can be used to identify all 1,500 bird species in the Amazon.  

To build the Amazonian bird sound collection, the Macaulay Library and Universidad San Francisco de Quito conducted a weeklong sound-recording workshop for 12 Ecuadorian birders and ornithologists. At the end of the work­shop, participants banded together to create the Ecuador Bioacoustics Club, the first of its kind in the country. While this project is in its infancy, Seeholzer hopes in coming years that the Cornell Lab can work with partners in the Amazon to set up arrays of automatic audio recording units to document and monitor avian diversity of the Amazon, building off the reference library in the Macaulay Library.  

“We need this data to monitor the bird populations of the Amazon at a scale that will convince stakeholders and policy­makers at all levels to take action where needed and assess the impact of those actions,” says Seeholzer.

While it’s easy to get depressed about all the data on Neotropical forest bird declines, many ornithologists say they have no intention of giving up.  

“I don’t want to write obituaries for these birds,” says Wolfe, the Michigan Tech researcher in the Brazilian Amazon. “I want to understand the mechanisms to try to better understand what components of the forest constitute climate refugia for birds, and prioritize those for protection.”  

To that end, Wolfe says he is working with colleagues throughout the tropics at long-term research sites: “We’re in the process of harmonizing the data sets, bringing in the latest, greatest remotely sensed models and estimates of climate to really start understanding how climate is affecting tropical birds at scale across multiple sites.”  

And Wolfe says he is optimistic that getting the word out about tropical bird declines and increasing collaboration among governments, scientists, and the public will spur broader solutions: “The fact that birds are responding so negatively to slight changes in climate should be really alarming for biodiver­sity protection globally.”  

According to Lehner, the Cornell climate scientist, there’s still time to act. Some of the worst-case climate scenarios don’t appear to be coming to fruition, he says. But “at the same time the super-optimistic scenarios where we solve everything by 2050 have become less likely because we’re clearly not moving fast enough.”

“Our chance of reducing warming ultimately depends very much on what humans do,” says Lehner.