Louisiana State University: Amazon Rainforest Birds' Bodies Transform Due To Climate Change

Press release from Louisiana State University:

November 12, 2021

“Even in the middle of this pristine Amazon rainforest, we are seeing the global effects
of climate change caused by people, including us,” said Vitek Jirinec, LSU alumnus
(Ph.D. ’21), associate ecologist at the Integral Ecology Research Center and lead
author to this study published in the journal Science Advances.

Birds in the Amazon rainforest have become smaller and their wings have become longer
over several generations, indicating a response to the shifting environmental conditions
that may include new physiological or nutritional challenges.

This is the first study to discover these changes in non-migratory birds’ body size
and shape, which eliminates other factors that may have influenced these physiological
changes. Jirinec and colleagues studied data collected on more than 15,000 individual
birds that were captured, measured, weighed, marked with a leg band and released,
over 40 years of field work in the world’s largest rainforest. The data reveal that
nearly all of the birds’ bodies have reduced in mass, or become lighter, since the
1980s. Most of the bird species lost on average about 2 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. How significant is this?

“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 co-author
Philip Stouffer, who is the Lee F. Mason Professor in the LSU School of Renewable
Natural Resources. 

The data set covers a large range of the rainforest so the changes in the birds’ bodies
and wings across communities are not tied to one specific site, which means that the
phenomenon is pervasive. 

The scientists investigated 77 species of rainforest birds that live from the cool,
dark forest floor to the warmer, sunlit midstory. They discovered that the birds that
reside in the highest section of the midstory and are the most exposed to heat and
drier conditions, had the most dramatic change in body weight and wing size. These
birds also tend to fly more than the birds that live on the forest floor. The idea
is that these birds have adapted to a hotter, drier climate by reducing their wing
loading therefore becoming more energy efficient in flight. Think of a fighter jet
with a heavy body and short wings that requires a lot of energy to fly fast compared
to a glider plane with a slim body and long wings that can soar with less energy.
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 of this study.

The question of the future capacity of Amazonian birds to deal with increasingly hotter
and drier surroundings, especially in the dry season, remains unanswered. The same
question can be asked for a lot of places and species that live at the edges of even
more environmental extremes. 

This press release was produced by Louisiana State University. The views expressed here are the author’s own.