About 40 years ago, David Willard was working at the Field Museum in Chicago, this famous natural history museum, when he heard something strange.

A person at another institution in Chicago just mentioned casually that birds often hit the windows

at the big convention center called McCormick Place, which is a building that's right on the lake

and has a lot of glass.

So one morning in 1978, David went to check it out.

I walked down in the morning and just kind of blatantly trespassed around the building out of pure curiosity, not expecting anything.

And sure enough, I found three or four dead birds around the edge of the building, having hit the glass, and brought them back to the museum.

David and his colleagues went back again and again, and they'd usually find more birds on the ground.

We'd walk around the edge of the building with a plastic bag for dead birds,

and we would take four measurements from each bird, putting the information into big ledgers.

Over the last few decades, David and other volunteers have collected over 100,000 dead

birds across downtown Chicago. When I first started, it was just a thought that here are

these dead birds, we might as well at least make them useful for research and bring them back to the museum.

But he started to realize this was a serious opportunity.

It was only as the sample grew that we realized there were a lot of interesting questions that could be addressed using them.

And when scientists examined this huge amount of data,

they started to notice something weird.

Birds over the last 35 years have shrunk.

And this wasn't just one type of bird.

The most amazing thing that they observed is that on all of the species, whether it was a thrush, a warbler, a sparrow, they were showing this declining size.

And the question of just exactly why is an open one.

I'm Noam Hassenfeld, and this week on Unexplainable, why are birds and a whole bunch of other animals shrinking?

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