Climate change may be shrinking tropical birds

a collared puffbird perched on a human hand
Image Source: Many tropical birds, such as this collared puffbird (Bucco capensis), are shrinking in size. The change may help the birds stay cool in a warming climate. (Science News)

Researchers have spent decades in a remote area of Brazil’s Amazon jungle collecting and measuring birds in a huge swath of forest that has not been deforested or paved over by roads. The experimental plot was intended to serve as a baseline to show how habitat fragmentation, such as logging or road construction, can decimate rainforests’ native fauna. The birds, on the other hand, are dwindling in this pristine area of wilderness. Several Amazonian bird species have fallen in numbers over the past 40 years. The average body weight of several species has decreased by roughly 2% per decade, according to a study published November 12 in Science Advances. In addition, some species have developed wings that are longer than others. This may be because of a warming climate that favors slimmer, more efficient bodies that help birds keep cool, according to the researchers. Temperature has been connected to body size for a long time. Having a lower surface area compared to one’s volume reduces heat loss via the skin and keeps the body warm in colder climes, hence it pays to be large. A ecologist at a research center in Blue Lake, California, says that as the environment heats, “you’d expect reduced body sizes to help species off-load heat better.”

Migrating birds in North America are getting smaller, according to a study published in Ecology Letters in 2020. Winger believes that climate change is the most likely culprit, but other variables such as deteriorated habitats that birds may encounter can’t be ruled out because of the vast range of conditions that migratory birds encounter. As part of their research, Jirinec and colleagues examined data on nonmigratory birds collected between 1979 and 2019 in an intact Amazon zone that covers 43 kilometers. More than 11,000 birds of 77 species are included in the collection, which includes measures such as weight and wing length. Climate data for the area was also studied by the researchers. As for climate change, it isn’t something that is going to happen in the future. Ben Winger, an ornithologist at the University of Michigan in Ann Arbor who wasn’t involved in the research but has reported similar shrinking in migratory birds, says, “It’s happening now and has been happening and has ramifications we haven’t thought of.” “It’s a global occurrence,” he adds when he sees the same patterns in so many bird species in so many diverse environments. Researchers observed that the mass of all species decreased throughout this period, including birds as diverse as the Rufous-capped antthrush (Formicarius colma) and the Amazonian motmot (Momotus momota). Over the course of the last century, species have lost anything from 0.1 percent to nearly 2 percent of their average body weight. For example, the motmot dropped from 133 grams to 127 grams over the research.

In the wet season, the average temperature rose by 1 degree Celsius, while in the dry season, it rose by 1.65 degrees Celsius. It was found that short-term changes in climate (such as a very hot or dry season) explained the observed size trends better than long-term trends (such as an increase in temperature). According to Jirinec, “the dry season is particularly stressful for birds.” According to the theory that birds are getting smaller to cope with heat stress, the mass of birds decreased the greatest in the year or two after extremely hot and dry spells. This is not the only element that might contribute to lower sizes. Climate change is the most likely culprit because birds from a variety of diets all decreased in mass, Jirinec said. Wing length also increased for 61 species, with a maximum rise of roughly 1% per decade for the majority of them. Longer wings, according to Jirinec, provide for cooler flying. It requires a lot of power to fly a fighter jet because of its large weight and small wingspan. A glider, on the other hand, is a considerably more efficient mode of transportation. Birds may be able to fly more efficiently and emit less metabolic heat because of longer wings, according to the author. “It’s only a theory,” he said. At higher elevations when temperatures are hotter and drier than those on the ground, birds’ bodies changed dramatically.

We don’t know if these changes in shape and size are an evolutionary adaptation to climate change, or if they are just a physiological response to warmer temperatures. As long as Jirinec is correct, it reveals the destructive force of human activities (SN: October 26, 2011). As he puts it: “The Amazon rainforest is strange, distant, and full of life.” Climate change may be seen in regions like these, which are distant from civilization, according to a new study.

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