Our Best Estimate

I found this story by Christopher Joyce from August on NPR's website.

Modern architecture loves glass. Glass makes interiors brighter and adds sparkle to cityscapes. But glass also kills millions of birds every year when they collide with windows. Biologists say as more glass buildings go up, more birds are dying... A lot of them do, and die. "Our best estimate is 100 million to a billion" birds in North America die this way every year, she says

It sounds like ornithologists have a problem but they don't know how big it is. Perhaps they could have said, "We have a problem, but we don't know how big it is."