Researchers have spent decades catching and measuring birds in a wide stretch of forest unaffected by roads or deforestation in a remote section of Brazil’s Amazon rainforest. The experimental plot aimed to provide as a baseline that would illustrate how habitat fragmentation, such as from logging or highways, might hollow down rainforests’ wild menagerie. However, a more subtle change is taking place in this beautiful enclave of wilderness: the birds are disappearing.
Hundreds of Amazonian bird species have plummeted in number during the last 40 years. Researchers reveal in Science Advances that many species have lost over 2% of their average body weight every decade. Furthermore, some species’ wings have gotten longer. The changes coincide with a hotter, more variable climate, which could put a premium on leaner, more efficient bodies that help birds stay cool, the researchers say.
“Climate change isn’t something that will happen in the future. It’s happening now and has been happening for a while, with consequences we haven’t considered,” says Ben Winger, an ornithologist at the University of Michigan in Ann Arbor who wasn’t part in the study but has observed similar shrinkage in migratory birds. Observing the same trends in so many bird species in such diverse settings “speaks to a more universal issue,” he says.
Body size and temperature have long been connected by biologists. It is advantageous to be large in colder locations since having a lower surface area compared to one’s volume reduces heat loss via the skin and keeps the body warmer. As the climate warms, “you’d expect shrinking body sizes to help organisms off-load heat better,” says Vitek Jirinec, an ecologist at the Integral Ecology Research Center in Blue Lake, Calif.
Climate change isn’t something that will happen in the future. It’s happening now and has been happening for a while, with consequences we haven’t considered.
Ben Winger
Winger and colleagues published in Ecology Letters in 2020 that many species of North American migratory birds are shrinking. Climate change is the most likely culprit, according to Winger, but because migrators encounter a wide range of conditions when traveling the world, other causes such as deteriorated habitats that birds may encounter cannot be ruled out.
Jirinec and colleagues examined data on nonmigratory birds gathered from 1979 to 2019 in a 43-kilometer-long intact section of the Amazon to see if they have shrunk as well. The dataset comprises mass and wing length measurements for approximately 11,000 individual birds from 77 species. The researchers also looked at climate data for the area.
The researchers discovered that all species fell in number throughout this time period, including birds as diverse as the Rufous-capped antthrush (Formicarius colma), which snatches insects from the forest floor, and the Amazonian motmot (Momotus momota), which eats fruit high in the trees. Each decade, species lost between 0.1 and nearly 2 percent of their average body weight. Over the course of the trial, the motmot, for example, shrank from 133 grams to around 127 grams.
These changes occurred in tandem with an increase in average temperature of 1 degree Celsius during the wet season and 1.65 degrees Celsius during the dry season. Temperature and precipitation also became more variable over the time period, and these short-term fluctuations, such as an especially hot or dry season, better explained the size trends than the steady increase in temperature.
“The dry season is really difficult for birds,” explains Jirinec. Birds’ mass reduced the most in the year or two after particularly hot and dry spells, supporting the theory that birds are shrinking to cope with heat stress. Other factors, such as decreasing food availability, may also contribute to smaller sizes. However, because birds with wildly disparate diets all fell in mass, Jirinec believes a more general force, such as climate change, is the most likely reason.
Wing length increased in 61 species, with a maximum increase of roughly 1% each decade. Longer wings, according to Jirinec, make for more efficient, and thus cooler, fliers. A fighter jet, for example, requires immense power to move due to its massive bulk and small wings. A light and long-winged glider, by contrast, can cruise along much more efficiently.
“Longer wings may be helping [birds] fly more efficiently and produce less metabolic heat,” which can be beneficial in hotter conditions, he says. “But that’s just a hypothesis.” This body change was most pronounced in birds that spend their time higher up in the canopy, where conditions are hotter and drier than the forest floor.
It is uncertain if these changes in shape and size are an evolutionary adaptation to climate change or just a physiological response to warmer temperatures. Whatever the cause, Jirinec believes the alteration demonstrates the harmful influence of human activities.
He describes the Amazon jungle as “mysterious, remote, and brimming with wildlife.” “This study implies that you can observe signs of climate change even in settings like this, far far from civilization.”
