For almost 40 years, a dedicated team of researchers and volunteers has been taking the measurements of thousands of birds that have collided with buildings in Chicago, Illinois-and they found a surprising trend.
"Virtually everyone agrees that the climate is warming, but examples of just how that is affecting the natural world are only now coming to light", added Dave Willard, collections manager emeritus at the Field Museum in Chicago who measured all the birds. "The maximum is 300 in a day", he said.
Given the well-established link, many scientists had predicted in recent years that global warming would affect the size of many animals.
The study provides fresh evidence of worrisome trends for North American birds.
Studies of plant and animal response to climate change often focus on shifts in the geographical range of a species or the timing of events such as springtime flowering and migration.
"Despite the diversity of so many species, they are all changing in the same way", said Brian Weeks, an evolutionary ecologist at the University of MI who was the lead author of the study.
"But we see them consistently across so many species, across male and female, across ages and all these things that can introduce variability", said Benjamin Winger, a University of MI evolutionary biologist who was the senior scientist on the project.
"The species were pretty diverse, but responding in a similar way", he said.
The scientists aren't exactly sure why warmer temperatures cause birds to shrink.
Body size had reduced in all 52 species - with wing length increasingly significantly in 40 of them.
The length of the tarsus leg bone, a key indicator of overall body size, declined 2.4% across all 52 species during those decades.
Those with the fastest shortening legs also had the most rapid gains in wing length. The wingspans increased by 1.3%, possibly to enable the species to continue to make long migrations even with smaller bodies. They cited a phenomenon called Bergmann's rule, in which individuals within a species tend to be smaller in warmer regions and larger in colder regions, as reason to believe that species may become smaller over time as temperatures rise. Larger body sizes help animals in cold places stay warm, with smaller bodies holding on to less heat.
Long-distance bird migration is one of the most impressive feats in the animal kingdom. "As humans change the world at an unprecedented rate and scale, there are likely widespread and consistent biotic responses to environmental change".
Dr Weeks said: "Periods of rapid warming are followed really closely by periods of decline in body size - and vice versa". But this study suggests body morphology is a crucial third aspect. "The results of this study highlight how essential long-term data sets are for identifying and analyzing trends caused by changes in our environment", says Willard, a collections manager emeritus at the Field Museum and one of the authors of a new study in Ecology Letters presenting these findings, along with lead author Brian Weeks and senior author Ben Winger from the University of MI.
He said: "It started as a very casual study - someone mentioned that birds sometimes run into McCormick Place, and I was curious, so I went for a walk around the building one morning". "I found a couple dead birds and I brought them back to the museum-I've always wondered if there had been no birds that morning whether I would have ever bothered to go back".
"When we began collecting the data analyzed in this study, we were addressing a few simple questions about year-to-year and season-to-season variation in birds".