The barred owl has a very distinctive call.

"It's the one that goes 'who cooks for you.

'" <<barred owl call sample>> Connor Wood is a conservation biologist and applied ecologist at the University of Wisconsin-Madison.

Who does a pretty great barred owl impression himself.

<<impression of barred owl>> Barred owls are native to the eastern U.S. But in the last century,they've crossed the Great Plains,flooded into Canada, and taken up roost in the Pacific Northwest… where they're a major threat to northern spotted owls.

"Barred owls are bigger and more aggressive.

And so barred owls have been able to basically drive spotted owls out of their preferred nesting areas.

And also because they're more flexible with what they eat, there can be more barred owls on the landscape."And now,they're invading the Sierra Nevada mountains, further south—the native range of the California spotted owl.

"And the big question was: what is the size of the barred owl population there, and how fast is it growing?"To answer that,Wood and his team deployed audio recorders in roughly 1000 locations, throughout 2300 square miles of the northern Sierra Nevada.

And then they listened—for two years—collecting the faint calls of spotted owls <<spotted owl>> and barred owls,too.