Deep in the Panamanian rainforest, bird populations have been quietly declining for 44 years.

A new study led by the University of Illinois shows that between 1977 and 2020, up to 70 percent of lower bird species in forests declined. The vast majority of them fell by half or more.

Henry Pollock, a postdoctoral researcher in the University's Department of Natural Resources and Environmental Sciences (NRES) and lead author of the study, said many of these are species you would expect to do well in the 22,000-hectare national park. The national park has not experienced major land-use changes in at least 50 years. This is very surprising.

Birds that lose any habitat can threaten the integrity of the entire ecosystem. In the Neotropics, these birds are key seed dispersers, pollinators, and insect consumers. Fewer birds could threaten tree reproduction and regeneration, affecting the entire forest structure, a pattern seen elsewhere after major bird declines.

But researchers have yet to study the effects or root causes. Pollock, Brawn, and their colleagues first focused on recording numbers.

Over 43 years and more than 84,000 sampling hours, the researchers captured more than 15,000 unique bird species, representing nearly 150 species, and collected enough data to track 57 of them.

The researchers noted that 40 species declined, or 70 percent. Thirty-five species lost at least half of their initial numbers. Only two species -- the hummingbird and the puff bird -- were added.

Marginal species were hit the hardest, with most declining by 90 percent or more. But Pollock and Brown were not surprised. The disappearance of marginal species has boosted their confidence in the results.

This is because, 40 years ago, a paved passageway passed through the site. It creates the ideal fringe habitat for birds that prefer openings in the forest canopy. But over time the road ceased to be maintained and it became a small gravel road with the forest canopy filling up overhead.

The article, "Long-Term Monitoring Shows Widespread and Severe Decreases in Lower-middle Birds in Protected Neotropical Forests," was published in the Proceedings of the National Academy of Sciences. Authors include Henry Pollock, Judith Toms, Corey Tarwater, Thomas Benson, James Karr, and Jeffrey Brawn.