Protected tropical forest sees major bird declines over 40 years

Birds have been declining in the Panamanian rain forest for 44 years. 70% of understory bird species declined in the forest between 1977 and 2020 according to a new University of Illinois-led study. The majority of them are down by half or more.

Henry Pollock, a researcher in the Department of Natural Resources and Environmental Sciences, says that many of these are species you would expect to be doing well in a national park.

Jeff Brawn has a word for it. Brawn is a co-author of the study. For more than 30 years, he has investigated birds at the study site.

It is one of the longest studies of its kind in the Neotropics. We cannot generalize to the whole region and say the sky is falling.

Pollock says that the integrity of the entire system can be threatened by the loss of birds. These birds are key in the Neotropics. The entire structure of the forest could be impacted by fewer birds, a pattern shown elsewhere after major bird declines.

The researchers haven't looked at the underlying causes yet. Pollock, Brawn, and their colleagues focused on documenting the numbers.

Jim Karr, who is now a professor at the University of Washington, started a twice-yearly bird sampling effort back in 1977. The mist nets are set up in the rainy and dry seasons to capture birds. Researchers can carefully remove birds from mist nets. They identify, measure, and band the birds before releasing them back into the forest.

Over the course of 43 years, the researchers captured more than 15,000 unique birds and gathered enough data to track 57 of them. The researchers noted declines in 40 species, or 70%, and 35 species lost at least half of their initial numbers. There are only two bird species that have increased.

At the beginning of the study, we caught 10 or 15 species. Pollock says that by 2020 there would be five or six individuals for a lot of species.

Although the birds represented a wide variety of guilds that specialize in the same food resources, the researchers noted declines across three broader categories.

The decline of common species is alarming.

The birds should be doing well in that forest. They are not for whatever reason. We were surprised.

Declines in the other two groups were not as dramatic. The forest in Panama became increasingly fragmented in the last several decades, but birds that migrate to high altitudes require some degree of forest connection to be successful.

The decline of the edge species was the most severe. Pollock and Brawn were not surprised. The disappearance of edge species boosted their confidence. 40 years ago, a paved access road cut through the site. The ideal edge habitat for birds is in the forest canopy. Over time, the road stopped being maintained and has since turned into a gravel road with a forest canopy overhead.

Pollock says that the fact that edge species went away when the road did is not concerning.

The scarcity of similar sampling efforts throughout the tropics makes researchers reluctant to generalize their results beyond their study site.

Pollock says that this is the only window they have into what is happening in tropical bird populations. Our study highlights the lack of data in the tropics and how important long-term studies are.

The study was not designed to explain why birds are declining in the forest. Climate change may be tied to things like changing amounts of rainfall, food resources, and reproductive rates.

Whatever the cause, the researchers were determined to figure it out.

Almost half the world's birds are in the Neotropics, but we don't have a good handle on their populations. We need to do it quickly, because it is important to establish trends and mechanisms of decline in these populations.

The article "Widespread and Severe Declines of Understory Birds in a protected Neotropical forest" is published in the Proceedings of the National Academy of Sciences. Authors include Henry Pollock and Judith Toms.

More information: Long-term monitoring reveals widespread and severe declines of understory birds in a protected Neotropical forest, Proceedings of the National Academy of Sciences (2022). DOI: 10.1073/pnas.2108731119. Journal information: Proceedings of the National Academy of Sciences Citation: Protected tropical forest sees major bird declines over 40 years (2022, April 4) retrieved 4 April 2022 from https://phys.org/news/2022-04-tropical-forest-major-bird-declines.html This document is subject to copyright. Apart from any fair dealing for the purpose of private study or research, no part may be reproduced without the written permission. The content is provided for information purposes only.