Bird populations are declining in the Neotropics

A new longitudinal study led by the University of Illinois Urbana-Champaign has found that deep in the Panamanian rainforest – a region belonging to the so-called “Neotropics” – bird populations have been declining over the past 44 years. According to the researchers, 70 percent of understory bird species have declined between 1977 and 2020, with the vast majority of them being down by half or even more.

“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,” said study lead author Henry Pollock, a postdoctoral researcher in Ecology at the University of Illinois. “It was very surprising.”

According to Dr. Pollock, loss of birds from any habitat can threaten the integrity of the entire ecosystem, since the birds play fundamental ecological roles such as seed dispersion, pollination, or insect consumption. Declines in bird populations could endanger tree reproduction and regeneration, thus impacting the entire structure of the forest.

Over the past 43 years, the scientists captured more than 15,000 unique birds representing nearly 150 species and gathered sufficient data to closely follow the dynamics of 57 of these species. They noted marked declines in 40 of these species (70 percent), and 35 species lost at least half of their initial populations. During this period, only a species of hummingbird and one species of puffbird appeared to increase.

“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,” said Pollock.   

Dr. Pollock and his colleagues noted declines across three broad categories: common forest birds, species which migrate seasonally across different elevations, and “edge” species living in the transition zones between open and closed-canopy forests.

While the declines among the last two groups were easily explained by increased forest fragmentation in the past years, the decline among common species is the less understood and most alarming phenomenon. 

“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,” said study co-author Jeff Brawn, a professor of Sustainability at the University of Illinois.

The scientists hypothesize that some of the causes leading to these declines include changing amounts of rainfall, food resources, and reproductive rates, many of which can be tied to climate change.

“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. So, I think it’s very important that 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,” Professor Brawn concluded.

The study is published in the journal Proceedings of the National Academy of Sciences. 

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By Andrei Ionescu, Earth.com Staff Writer