- The number of individual birds found at the Yasuní Biosphere Reserve has dropped by half, according to a study published earlier this year.
- Other studies have shown a similar trend in preserved rainforests, pointing to habitat deterioration and pesticides as the usual causes of widespread bird decline in the Northern Hemisphere, but this does not explain the phenomenon in tropical sites.
- Researchers point to a few possible causes for the declines, such as signs of reduction in insect abundance, but climate change is the common suspect in all cases.
When John Blake and Bette Loiselle arrived at Tiputini for the first time, they found exactly what they’d been looking for. For years, the two University of Florida professors had been working in Costa Rica, studying how resources — fruits in particular — influence the way birds use their habitat. But as the forests around La Selva Biological Station, their old study site, started to get cut down, they were forced to change course.
“We could no longer really separate the anthropogenic effects of land use change from the effects of changing fruit resources,” Loiselle told Mongabay. “That really motivated us to begin to look at other places to do our work.”
Tiputini Biodiversity Station is part of the Yasuní Biosphere Reserve, a 2.7-million-hectare (6.6-million-acre) area of Amazon Rainforest in eastern Ecuador and one of the most biodiverse hotspots on the planet. A tropical forest as pristine as a researcher can hope to access on Earth today.
“We just decided that this would be a perfect place to start a long-term study on birds,” Blake told Mongabay. “There was no thought at the time that we’re going to document declines in the bird population.”
This March, 23 years after their arrival in Tiputini, the two scientists published a study in Global Ecology and Conservation showing an alarming trend. Relying on observations and mist net captures on two terra firme forest plots about 1.5 kilometers (0.9 miles) apart, the researchers documented a 50% decline in the number of individual birds found at the station throughout the years.
Signs of wide-ranging declines in bird abundance are not new. Reports of continentwide reduction in avian populations in the Northern Hemisphere have been piling up over the last 10 years.
A 2019 study published in Science analyzing 529 species in the United States and Canada showed a net decline of 29% in their population over 48 years. That’s 2.9 billion birds fewer than there were in the 1970s. Europe has also witnessed widespread population reductions, with recent studies showing declines in insect and seed-eaters and common bird species as a whole.
The culprits seem clearer in these cases, with habitat deterioration (mostly due to agricultural intensification and urbanization) and pesticide use leading the rankings. But the same can’t be said about Tiputini.
During the 23 years of Blake and Loiselle’s study, the forest didn’t go through any human-induced change. In fact, aside from the occasional tree fall, it didn’t change much at all. There were also no farmlands close to their study site, and therefore no impacts from pesticides. There was no hunting — at least in any substantial fashion — going on at the station, and no sign of a pathogen or exotic species that could account for the decline.
According to Blake, after about 2009, populations simply started going down without any visible cause. “[It] was probably around 2012 when we really started to think that something was actually happening in a negative way,” he said. “There’s something called the dawn chorus, which is typical in tropical forests where lots of birds sing just before dawn. And over the last 10 years, that has just been going quieter and quieter with very few birds singing in the morning.”
Bird declines in Central and South America
Blake and Loiselle weren’t the only researchers to notice a somewhat mysterious decline in birds in mostly undisturbed tropical forest tracts. Philip Stouffer, a professor at Louisiana State University, led another long-term study on bird abundance in the Brazilian Amazon, published in 2021 in Ecology Letters.
His study was conducted at a Biological Dynamics of Forest Fragments Project (BDFFP) site, with more than 90% of its area preserved. The BDFFP is a long-term, large-scale ecological study region near the city of Manaus, capital of the Brazilian state of Amazonas. Other scientists had already sampled birds there in the early 1980s, so when Stouffer gathered funding for fieldwork in 2007, he was excited. “[We] had a lot of things going in the field,” he told Mongabay.
The mostly untouched forest was supposed to serve as a control region — a baseline for comparison with the BDFFP’s fragmented and impacted areas. But something seemed off at the site, particularly with the birds specialized in eating insects.
“I had a couple of really good birders who were really keen to see … some of these specialized birds,” Stouffer said. But it was really hard to find them, according to him. “And that prompted us to think, ‘geez, are they really less common than they used to be?’”
Among 79 bird species analyzed by Stouffer and colleagues, 38 showed decreases in abundance from 1980-84 and 2008-16, while 24 showed increases.
Unlike the general declines found in Ecuador, a particular guild — a group of species defined by the way they use resources — suffered most losses at the BDFFP. Eleven out of the 14 species (78.5%) that declined the most were near-ground and terrestrial insectivorous birds.
Henry Pollock, adjunct instructor at the University of Illinois Urbana-Champaign, led another study published in 2022 in PNAS revealing widespread bird decline in a protected tropical forest, this time at Soberanía National Park in Panama. He and colleagues report that, between 1977 and 2020, 70% of bird species captured in mist nets (40 of 57) showed reductions in abundance — 35 of them equal or above 50% — while only two increased in numbers.
Pollock’s study region, however, is a bit different from Tiputini and the BDFFP site Stouffer worked in. The 22,000 hectares (54,363 acres) of Soberanía are free from fragmentation or recent land use change, but they were crossed by a paved road flanked by 25 meters (82 feet) of second-growth forest — a recently regenerated woodland — on each side until the late 1970s. The road was abandoned after the park was established, and it’s now a 5-meter- (16.4-foot-) wide dirt line surrounded by closed-canopy forest. The park has also lost connectivity with neighboring wetter and high-elevation forests in recent decades.
The authors pointed out that these factors probably explained the decline of some species — specifically those that fare better in secondary forests or move across altitudinal landscapes in specific seasons — but given the quantity and diversity of affected birds, these impacts were not alone in the authors’ suspect list.
A common suspect
“It was a lot of rain, even more rain than normal,” remembered John Blake, thinking back at the severe La Niña events which coincided with the early stages of Tiputini’s bird declines. “It was impacting what we could do. We were thinking maybe this is going to have some impact on the birds as well.”
The excessive rain heralded the downfall of Tiputini’s birds. Species abundance continued to decline even after La Niña relented, but the unusual weather brought climate change into the conversation.
Though species from several guilds declined in Tiputini, insectivores — like in the BDFFP — were the most severely affected. “Quite a few of the terrestrial insectivores that … had been relatively common, over the last 10 years, basically disappeared from the plots,” Blake said. Together with Loiselle, he suggested that a reduction in insect abundance caused by climate change is likely part of the explanation behind the mysterious bird declines.
Pollock and collaborators also pointed to direct and indirect effects of climate change as possible causes of the declines in Soberanía — such as unusual rainfall, rising temperatures, changes in resource abundance and perhaps even increases in predation and parasitism.
Stouffer agreed. “In the central Amazon, we know that dry seasons are getting hotter and drier. My hunch is that these conditions are physiologically stressful and also limit some prey, all of which makes life a little harder.” According to him, “a little harder” is all it takes. “The kind of population trends we’re seeing probably don’t require much reduction in reproduction or survival if the trends are consistent.”
Stouffer and his colleagues did more than simply look at the abundance of birds in the BDFFP. In a study published in 2021 in Science Advances, led by Vitek Jirinec, an associate ecologist at the Integral Ecology Research Center, the team was able to link changes in climate with slight changes in bird body size. All 77 species analyzed showed decreases in the mean mass of individuals since the 1980s, and a third of them also showed increases in wing length. As a general rule, smaller body sizes are more efficient in dispersing heat — and thus favorable in a warmer world — but the scientists suspect there’s more to this reduction.
“The morphological changes we’re seeing are consistent with [the birds] needing to fly more efficiently,” Stouffer said. “One thing I wonder about is the extent to which some of this — both the bird body size change and the abundance change — could be driven by it being a little harder for birds to find food.”
A few other possibilities have also been raised to explain the declines. The idea that these are long-term natural fluctuations in bird populations, for example, cannot be ruled out entirely.
“Perhaps this is simply a normal fluctuation in bird numbers, and then in another 20 years things will go back up again,” Blake said. Given the diversity of species and places affected, though, this doesn’t seem to be the best hypothesis.
“[The declines are] certainly not limited just to the local area around Tiputini because bird guides working along the Napo [River] have noticed declines in birds over the same period,” Blake added. “It’s not just something related to our two study plots.”
In Tiputini, gas flares have also been noted as a possible culprit. The practice was supposedly banned in September 2021, but according to Kelly Swing, professor at Universidad San Francisco de Quito, it hasn’t completely died out.
“A little bit of travel across the region readily reveals that these structures are somewhat frequently encountered along roads,” Swing said, “even in not-so-remote areas. Because gas flares attract some taxa of insects from great distances, they are undoubtedly responsible for the elimination of huge numbers of prey items for all the larger organisms that depend upon them as food.”
Swing also worried about the possibility that heavy metals are being aerosolized by burning natural gas.
A grim picture?
Not every research article investigating trends in tropical bird abundance reached the same conclusion. One dissident stands among the few long-term avian studies looking around and beneath the Equator.
In 2023, a study led by Ari Martínez from California State University, Long Beach, revisited a somewhat famous forest plot in Manu National Park, in Peru. It was the same site where John Terborgh, in 1982, had analyzed the entire structure and organization of the local bird community — a revolutionary effort at the time. The authors found little change compared with what Terborgh had reported four decades prior.
The conclusion, though, was based on a 97-hectare (239.6-acre) plot in a hyperproductive flood plain forest, an environment that is quite different from the terra firme and lowland forests where the other studies took place.
“One of the things that they suggested in their paper was that, because it’s a flood plain … there’d be more moisture,” Blake said. “That might mitigate any impacts on insects, which could then reduce the possibilities for bird declines.”
As far as more ordinary kinds of rainforests go, the pattern seems worrying. For Stouffer, the heart of the issue is whether the observed decline is correlated with changes in the birds’ immediate landscape.
“[In] our system, we’re pretty sure that it’s not. And I think in John and Bette’s case, they’re pretty sure it’s not. In Panama, that’s a little different,” Stouffer said. “But still it is long-term, and it is forest, and they did have the changes.” The absence of local explanations seems to point to a broader cause.
“I think the bottom line is that pretty much everybody who has enough data to really be able to look at this has found it,” Stouffer concluded.
Whatever the causes and scope of the problem, the future of bird populations at Tiputini, Soberanía and the BDFFP seems a cause for concern.
“They don’t have a high reproductive rate,” Loiselle said. “Recovery may take a long time for a tropical bird population — in the order of perhaps decades — if they experience declines like they’re experiencing now.”
Ecuador referendum halts oil extraction in Yasuní National Park
Citations:
Blake, J. G., & Loiselle, B. A. (2024). Sharp declines in observation and capture rates of Amazon birds in absence of human disturbance. Global Ecology and Conservation, 51, e02902. doi:10.1016/j.gecco.2024.e02902
Rosenberg, K. V., Dokter, A. M., Blancher, P. J., Sauer, J. R., Smith, A. C., Smith, P. A., … Marra, P. P. (2019). Decline of the North American avifauna. Science. Retrieved from https://www.science.org/doi/full/10.1126/science.aaw1313
Bowler, D. E., Heldbjerg, H., Fox, A. D., De Jong, M., & Böhning‐Gaese, K. (2019). Long‐term declines of European insectivorous bird populations and potential causes. Conservation Biology, 33(5), 1120-1130. doi:10.1111/cobi.13307
Inger, R., Gregory, R., Duffy, J. P., Stott, I., Voříšek, P., & Gaston, K. J. (2014). Common European birds are declining rapidly while less abundant species’ numbers are rising. Ecology Letters, 18(1), 28-36. doi:10.1111/ele.12387
Stouffer, P. C., Jirinec, V., Rutt, C. L., Bierregaard, R. O., Hernández‐Palma, A., Johnson, E. I., … Lovejoy, T. E. (2020). Long‐term change in the avifauna of undisturbed Amazonian rainforest: Ground‐foraging birds disappear and the baseline shifts. Ecology Letters, 24(2), 186-195. doi:10.1111/ele.13628
Pollock, H. S., Toms, J. D., Tarwater, C. E., Benson, T. J., Karr, J. R., & Brawn, J. D. (2022). Long-term monitoring reveals widespread and severe declines of understory birds in a protected neotropical forest. Proceedings of the National Academy of Sciences, 119(16). doi:10.1073/pnas.2108731119
Jirinec, V., Burner, R. C., Amaral, B. R., Bierregaard, R. O., Fernández-Arellano, G., Hernández-Palma, A., … Stouffer, P. C. (2021). Morphological consequences of climate change for resident birds in intact Amazonian rainforest. Science Advances, 7(46). doi:10.1126/sciadv.abk1743
Martínez, A. E., Ponciano, J. M., Gomez, J. P., Valqui, T., Carnes, B. H., Munarriz, R. H., … Terborgh, J. (2023). The structure and organisation of an Amazonian bird community remains little changed after nearly four decades in Manu National Park. Ecology Letters. Retrieved from doi.org/10.1111/ele.14159
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