[ti:Study: Climate Change May Be Causing Smaller American Birds][by:shang05.com][00:00.00]更多听力请访问shang05.com[00:00.04]A new study has found that North American migratory birds have been getting smaller over the past 40 years.[00:12.36]Researchers say the finding suggests a warming climate[00:17.32]could be affecting bird growth in North America -- and across the world.[00:24.96]The study was recently published in Ecology Letters.[00:31.00]It involved data on nearly 80,000 birds killed from 1978 to 2016 in the city of Chicago, Illinois.[00:44.52]Researchers measured the size of the birds.[00:48.28]The birds included in the study had died after crashing into buildings during the spring and fall migrations.[00:59.72]Fifty-two species of birds were studied. Most were different kinds of songbirds.[01:08.36]These birds reproduce in cold areas of North America and spend their winters in areas south of Chicago.[01:17.96]Over the 40-year period, body size decreased in all 52 species.[01:26.88]The average body mass fell by 2.6 percent.[01:31.76]Leg bone length dropped by 2.4 percent.[01:37.84]The one area of growth was the wingspan, which increased by 1.3 percent.[01:46.20]The researchers said the wing growth likely happened to permit the birds to continue[01:52.92]making long migrations with smaller bodies.[01:58.92]The study considered a principle known as Bergmann's rule, in which individuals within a species[02:07.40]grow smaller in warmer areas and grow larger in colder ones.[02:14.96]Brian Weeks is a biologist at the University of Michigan's School for Environment and Sustainability.[02:24.72]He helped lead the research.[02:27.32]He told the Reuters news agency that he believes the results show a clear link[02:34.16]between a warming climate and the growth of the birds.[02:40.00]"In other words, climate change seems to be changing both the size and shape of these species," he said.[02:51.32]The study found a direct link between the average summer temperature and the body size of the birds.[03:00.52]Dave Willard works with Chicago's Field Museum, which was in charge of measuring all the birds.[03:08.76]He said nearly "everyone agrees that the climate is warming,[03:13.88]but examples of just how that is affecting the natural world are only now coming to light."[03:23.16]The study provides new evidence of worrisome developments for North American birds.[03:30.96]A study published in September documented a 29-percent drop in the bird population[03:38.48]in the United States and Canada since 1970.[03:44.20]The study said this represented a net loss of about 2.9 billion birds.[03:52.84]Weeks said the new study is the largest examination yet to measure body size reactions[04:00.48]to warming temperatures involving a large, diverse group of species.[04:07.92]"We had good reason to expect that increasing temperatures[04:12.56]would lead to reductions in body size, based on previous studies," Weeks added.[04:20.92]"The thing that was shocking was how consistent it was.[04:25.96]I was incredibly surprised that all of these species are responding in such similar ways."[04:35.72]The researchers plan to continue studying the Field Museum data[04:40.84]in an effort to find additional evidence to support their findings.[04:47.56]They will also further examine the idea that an individual's physical development[04:53.52]can change to fit changing environmental conditions.[04:59.40]I'm Bryan Lynn. 更多听力请访问shang05.com