, this university is one of the most famous universities in the world.

A new study has found that North American migratory (迁徙的) birds have been getting smaller over the past 40 years. Researchers say the finding suggests a warming climate could be affecting bird growth in North America—and across the world.

The study was recently published in Ecology Letters. Researchers measured the size of 80,000 birds killed from 1978 to 2016 during the spring and fall migrations in the city of Chicago, Illinois. Over the 40-year period, body size decreased in all 52 species. The average body weight fell by 2. 6 percent. Leg bone length dropped by 2. 4 percent. The one area of growth was the wingspan, which increased by 1. 3 percent.

The researchers said the wing growth likely happened to allow the birds to continue making long migrations with smaller bodies. The study considered a principle known as Bergmann's rule, in which individuals within a species grow smaller in warmer areas and grow larger in colder ones.

Brian Weeks, a biologist at the University of Michigan's School for Environment and Sustainability, helped lead the research. He said that he believes the results show a clear connection between a warming climate and the growth of the birds. "In other words, climate change seems to be changing both the size and shape of these species," he said.

The study found a direct connection between the average summer temperature and the body size of the birds. David Willard works with Chicago's Field Museum, which is in charge of measuring all the birds. He said nearly everyone agrees that the climate is warming, but examples of just how that is affecting the natural world are only now coming to light.

"We had good reason to expect that increasing temperatures would lead to reductions in body size, based on earlier studies," Weeks said. "I was incredibly surprised that all of these species are responding in such similar ways. "

The researchers plan to continue studying the Field Museum data in an effort to find additional evidence to support their findings. They will also further examine the idea that an individual's physical development can change to adapt to changing environmental conditions.

Walk through the Amazon rainforest today and you will find it steamy, warm, damp and thick. But if you had been there around 15,000 years ago, during the last ice age, would it have been the same? For more than 30 years, scientists have been arguing about how rainforests might have reacted to the cold, dry climate of the ice ages, but till now, no one has reached a satisfying answer.

Rainforests like the Amazon rainforest are important for mopping up CO₂ from the atmosphere and helping to solve global warming. Currently the trees in the Amazon rainforest take in around 500 million tons of CO₂ each year, equal to the total amount of CO₂ given off in the UK each year. But how will the Amazon rainforest react to the future climate change? If it gets drier, will it survive and continue to draw down CO₂? Scientists hope that they will be able to learn in advance how the rainforest will manage in the future by understanding how rainforests reacted to climate change in the past.

Unfortunately, collecting information is incredibly difficult. To study the past climate, scientists need to look at fossilized pollen(花粉) kept in lake mud. Going back to the last ice age means drilling down into lake sediment(沉积物), which requires specialized equipment and heavy machinery. There are very few roads and paths, or places to land helicopters and aeroplanes. Rivers tend to be the easiest way to enter the rainforest, but this still leaves vast areas between the rivers completely unsampled(未取样). So far, only a handful of cores have been drilled that go back to the last ice age and none of them provide enough information to prove how the Amazon rainforest reacted to climate change.