Protected Tropical Forest Witnesses Significant Bird Decline Over 40 Years: Study

Over the last 44 years, bird populations in a Panamanian rainforest have been steadily declining. According to a new study led by the University of Illinois, 70 percent of understory bird species declined in the forest between 1977 and 2020. And the vast majority have been reduced by half or more.

Findings of Study:

"Many of these are species you'd expect to be doing fine in a 22,000-hectare national park with no major land-use change in at least 50 years," says Henry Pollock, a postdoctoral researcher in the University of Illinois' Department of Natural Resources and Environmental Sciences (NRES) and the study's lead author. "It was quite unexpected."

Jeff Brawn describes it as "concerning." Brawn is a co-author of the study and holds the Levenick Chair in Sustainability at NRES. He's also spent more than 30 years studying birds at the study site, Parque Nacional Soberana.

"This is one of the longest studies of its kind in the Neotropics, if not the longest," Brawn says. "It is, after all, only one park. We can't necessarily say the sky is falling across the entire region, but it's cause for concern."

Pollock claims that the extinction of birds in any habitat can jeopardize the ecosystem's integrity. These birds are important seed dispersers, pollinators, and insect eaters in the Neotropics. Fewer birds could jeopardize tree reproduction and regeneration, affecting the forest's entire structure, as has happened in the past after major bird declines.

However, the researchers have yet to investigate the consequences or the underlying causes. First and foremost, Pollock, Brawn, and their colleagues concentrated on compiling the data.

Jim Karr, a former University of Illinois faculty member who is now an emeritus faculty member at the University of Washington, started a twice-yearly bird sampling effort in 1977. During the rainy and dry seasons, members of the team set up mist nets to capture birds moving through the study site. Mist nets entangle birds gently, allowing researchers to pluck them out with care. The birds are then identified, measured, and banded before being released back into the forest, unharmed.

The researchers captured over 15,000 unique birds representing nearly 150 species over the course of 43 years and 84,000 sampling hours and gathered enough data to track 57 of them. The researchers found that 40 species, or 70% of them, have declined, and 35 species have lost at least half of their original numbers. Only two species grew in number: a hummingbird and puffbird.

"We'd catch 10 or 15 of a variety of species when we first started the study in 1977. Then, by 2020, many species would be down to just five or six individuals" Pollock explains.

The researchers noticed declines in three broad categories of birds: common forest birds, species that migrate seasonally across elevations, an "edge" species that specialize in transition zones between the open and closed-canopy forests.

According to Brawn, the decline of common species is the most concerning. "In the end, these are birds that should be thriving in that forest. They aren't, for whatever reason. We were taken aback."

The decreases in the other two groups were less pronounced. Birds that migrate to high elevations require some degree of forest connectivity, but Panama's forest, like that of most other countries, has become increasingly fragmented in recent decades.

Edge species took the brunt of the damage, with the majority of them declining by 90% or more. Pollock and Brawn, on the other hand, were not surprised. In fact, the extinction of edge species increased their confidence in their findings. That's because a paved access road ran through the property 40 years ago. It created the ideal edge habitat for birds that prefer forest canopy openings. However, the road ceased to be maintained over time and has since degenerated into a small gravel road, with the forest canopy filling in overhead.

"It's not alarming that edge species vanished along with the road," Pollock says. "It demonstrates what we would expect with forest maturation and the loss of those successional habitats," says the researcher.

The researchers are hesitant to extrapolate their findings beyond their research site, citing the scarcity of similar sampling efforts in the tropics.

Pollock explains, "Right now, this is really the only window we have into what's going on in tropical bird populations." "Our findings raise the question of whether this is occurring across the region, but we are unable to provide an answer." Instead, our research emphasizes the scarcity of data in the tropics and the importance of long-term studies."

The study wasn't intended to explain why forest birds are declining, but the researchers have some ideas they'd like to pursue. Climate change may be linked to changes in rainfall, food resources, and reproductive rates, among other things. Regardless of the cause, the researchers expressed a strong desire to discover it.

"The Neotropics are home to nearly half of the world's birds, but we don't have a good handle on their population trends. As a result, I believe it is critical that more ecological studies be conducted in order to identify trends and mechanisms of decline in these populations" Brawn declares. "And we have to do it right now."