One of the most basic field research techniques in bird conservation is counting birds and their nests. Tallying songbirds in a suburban garden is one thing; spotting seabirds is another. Seabirds, which reflect the health of their marine ecosystems, often build their nests in inaccessible areas—wedged into vertical cliffs or on remote islands battered by intense waves. Many lay their eggs in burrows more than a meter deep to protect them from weather. And on top of that, most are nocturnal, making them hard to see even in plain sight.

“There is a vast number of shearwater and petrel colonies for which we have absolutely no idea whether there are hundreds of pairs, or thousands, or tens of thousands,” says Steffen Oppel of the Royal Society for the Protection of Birds. So Oppel and his colleagues have come up with a new method for monitoring the populations without being able to see them: they listen.

Oppel and his team found that they could use audio recordings to estimate a bird's population size after testing the method on a raucous colony of Cory's shearwaters that breeds on a small island in the Azores called Corvo. They describe their findings in the journal Nature Conservation. By combining acoustic data from nine microphones placed around Corvo with information about nest density, the researchers developed an algorithm that took terabytes of audio recordings and automatically counted the number of individual calls. They found, as expected, that more nests meant louder recordings and more calls. That correlation enabled them to extrapolate an estimate for the seabird population for the entire island—6,000 breeding pairs—something that in the past has been little more than a simple guess.

Even if the estimate is off somewhat, Oppel's method will be useful for detecting increases or decreases in populations over time. From that, scientists can infer the health of the seabirds' food chains as well as observe how rookeries are faring in the face of invasive predators and climate change.