In a remote corner of Brazil's Amazon tropical（热带的）rainforest, researchers have spent decades catching and measuring birds. Over 40 years, dozens of Amazonian bird species have declined in mass（质量）. Many species have lost nearly 2% of their average body weight each decade, researchers report November 12 in Science Advances. Some species have grown longer wings. The changes could help birds stay cool in a hotter, more changeable climate, the researchers say.

"Climate change isn't something of the future. It has been happening and has effects we haven't thought of," says Ben Winger, an ornithologist（鸟类专家）at the University of Michigan, who wasn't involved in the research but has documented similar shrinkage（缩水）in migratory birds.

To see if non-migratory birds have also been shrinking, Jirinec and colleagues analyzed data collected from 1979 to 2019 in a remote region in the Amazon that spans 43km. The data include over 11,000 individual birds of 77 species as well as climate for the region.

All species declined in mass over this period, the researchers found. Species lost from about 0.1% to nearly 2% of their average body weight each decade. The motmot, for example, shrunk from 133g to about 127g over the study period.

These changes coincided with an overall increase in the average temperature of 1℃ in the wet season and 1.65%℃ in the dry season. Birds' mass decreased the most in a year or two after especially hot and dry seasons, which tracks with the idea that birds are getting smaller to deal with heat stress.

Wing length also grew for 61 species, with a maximum increase of 1% per decade. Jirinec thinks longer wings make for more efficient, and thus cooler, fliers.

"The Amazon rainforest is mysterious, remote and full of biodiversity," Jirined says. "This study suggests that even in places like this, far removed from civilization, you can see signatures of climate change."

A rainforest is an area covered by tall trees with the total high rainfall spreading（扩大） quite equally through the year and the temperature rarely falling below 16°C. Rainforests have a great influence on the world environment because they can take in heat from the sun and adjust the climate.

Without the forest cover, these areas would reflect（反射）more heat into the atmosphere, warming the rest of the world. Losing the rainforests may also influence wind and rainfall patterns，potentially（潜 在地） causing certain natural disasters all over the world.

In the past hundred years, humans have begun destroying rainforests in search of three major resources ： land for crops, wood for paper and other products, land for raising farm animals. This action affects the environment as a whole. For example, a lot of carbon dioxide in the air comes from burning the rainforests. People obviously have a need for the resources we gain from cutting trees but we will suffer much more than we will benefit.

There are two main reasons for this. Firstly, when people cut down trees, generally they can only use the land for a year or two. Secondly, cutting large sections of rainforests may provide a good supply of wood right now, but in the long run it actually reduces the world’s wood supply. Rainforests are often called the world's drugstore. More than 25% of the medicines we use today come from plants in rainforests. However, fewer than 1% of rainforest plants have been examined for their medical value. It is extremely likely that our best chance to cure diseases lies somewhere in the world’s shrinking（减 少）rainforests.

Natural selection is the process by which one type of animal within a species thrives (兴旺) because of certain characteristics that make it more likely to live than others in its group. The history of the peppered moth (灰蛾) is an example of the natural selection process.

In nineteenth-century England, certain types of peppered moths were able to better blend (融合) into their surroundings. During that time period, great changes were happening in Great Britain. The Industrial Revolution was part of this change, and with it came air pollution. Natural selection often takes hundreds or even thousands of years to occur. For the peppered moth, this process occurred comparatively quickly.

At the beginning of the Industrial Age, most peppered moths in England were light-colored and covered with black markings, although a few moths had dark-colored wings. Because the light-colored moths blended into the light-colored bark on the trees, they could not be easily seen by birds that would eat them. As the air grew more polluted, however, tree trunks became covered with soot (煤烟) and became darker. The light-colored moths became easy for birds to see against the dark tree trunks. Since the dark-colored moths now had the advantage, their numbers grew. Within 50 years, the peppered moth went from being mostly light-colored to being mostly dark-colored.

In the twentieth century, the air cleared up, and the peppered moth population changed again. As tree trunks lightened due to less soot in the air, light-colored moths once again had an advantage. Their numbers increased as soot levels declined. Depending on their environment, the coloration of the moths helped them to be “naturally selected” to survive.