CHICAGO (CBS) — For decades his job was to size up dead birds.

Now, CBS 2’s Vince Gerasole reports, all those numbers from a Chicago scientist add up to a big discovery.

For 40 years in a Field Museum lab, ornithologist David Willard has measured birds.

Dead birds.

Over 70,000 at last count, from 52 species.

“Everything in this drawer has been killed by a building in Chicago,” Willard said.

Killed by concussions, most are collected by volunteers and make their way into Willard’s lab.

“We’ve been measuring the wing length, measuring the bill length and then the leg length and the weight,” Willard noted.

For each bird, he’s painstakingly jotted down the information by hand, in journal after journal. Still, Willard wasn’t sure what to do with all that information.

“I feel I am a way better data generator than I am an analyzer,” Willard said.

But researchers at the University of Michigan flocked to his data, finding that over the years, the birds were actually shrinking in size.

“We are talking in some cases tenths of millimeters, but across a big enough sample that it is actually meaningful,” Willard said.

Overall, their leg length and mass has dropped about 2.5% in that time.

Researchers conclude it’s nature adapting to climate change.

“As the breeding grounds of these birds are getting warmer, it may be advantageous to be smaller,” Willard said.

Willard’s records also found wing length increased by 1%  which could indicate birds are flying farther to find suitable places to breed. Climate change was barely discussed when Willard started taking notes.

Now it’s helping scientists interpret its impact.

“The more examples of what it’s doing to wild populations of things, the more we can put it all together and have a better sense of what the future is going look like,” Willard added.

One of the next steps in this research is to examine the skeletons of these animals to see what they can tell us about changes to bird populations – and the world around us.