Amazonian birds are changing shape in response to global warming

In the deepest, most pristine areas of the Amazon Forest, birds are isolated from almost any direct contact with humans. Yet, data collected over 40 years in this,  the largest rainforest in the world, shows that even here, birds are being impacted by the effects of global warming.

The study, conducted by scientists from the Louisiana State University, found that the number of sensitive resident birds has declined throughout the Amazon rainforest during the past four decades. In addition, the body sizes and wing lengths of birds have changed. According to the researchers, these physical changes are associated with increasingly hot and dry conditions in the dry season, between June and November each year.

“Even in the middle of this pristine Amazon rainforest, we are seeing the global effects of climate change caused by people, including us,” said study lead author Vitek Jirinec, associate ecologist at the Integral Ecology Research Center and lead author of this study. 

More than 15,000 individual birds were captured, measured, weighed, marked with a leg band and released, over the 40-year study period. Analysis of the data set reveals that, in nearly all of the 77 different species recorded, body size has decreased since the 1980s. 

On average, birds have lost about two percent of their body weight every decade. For an average bird species that weighed about 30 grams in the 1980s, the population now averages about 27.6 grams. 

“These birds don’t vary that much in size. They are fairly fine-tuned, so when everyone in the population is a couple of grams smaller, it’s significant,” said study co-author Philip Stouffer, who is the Lee F. Mason Professor in the LSU School of Renewable Natural Resources.

The birds were collected over a large range of the rainforest, and therefore the changes recorded are not related to any specific locality. This indicates that the phenomena of lighter bodies and longer wings are most likely happening in all regions of the rainforest.

“This is undoubtedly happening all over and probably not just with birds,” said Professor Stouffer. “If you look out your window, and consider what you’re seeing out there, the conditions are not what they were 40 years ago and it’s very likely plants and animals are responding to those changes as well. We have this idea that the things we see are fixed in time, but if these birds aren’t fixed in time, that may not be true.”

The 77 different species recorded represent inhabitants at all forest levels, from the cool, dark forest floor to the warmer, sunlit midstory. The results showed that the species living in the higher sections of the forest midstory had the most dramatic changes in body weight and wing length. These are the birds most exposed to the hotter and drier environmental conditions. They also tend to do more flying as they travel from tree to tree.

The researchers hypothesize that these Amazonian bird species have adapted to hotter, drier conditions by reducing the energy required to fly around and find food. A lighter body with longer wings will result in reduced wing loading and this will make flight more efficient. If a bird has a higher wing loading, it needs to flap its wings faster to stay aloft, which requires more energy and produces more metabolic heat. Reducing body weight and increasing wing length leads to more efficient resource use while also keeping cooler in a warming climate.

LSU alumnus Ryan Burner (Ph.D. ’19) conducted much of the analysis that revealed the variation among the groups of birds over the years. Burner, who is now a research wildlife biologist at the U.S. Geological Survey Upper Midwest Environmental Sciences Center, is the second author on this study.

As the rainforest environment becomes increasingly hotter and drier, the capacity of these Amazonian birds to deal with the conditions will be tested further. The limit of changes in body weight and wing length is not currently known, but at some stage this strategy may fail to deliver the necessary adaptation to changing climate. The same situation no doubt also exists in a lot of other places and for other species that are experiencing the pressures of environmental extremes.

“There may be other researchers in other places who have relevant data from the 1970s and 1980s that could be compared to modern data, because the bird banding protocol we used is pretty standard. So, if you measure mass and wing, maybe there will be more datasets that will emerge and we’ll be able to get more of an idea of the variation across space and how it might be changing in different systems,” said Professor Stouffer.

The results of the study are published today in the journal Science Advances.

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By Alison Bosman, Earth.com Staff Writer