A wiry black bird called a cormorant dives under waves and comes up with a fish in its beak. This is a familiar sight to many people who spend time near the ocean.
Cormorants are such good hunters, the 10-pound birds need little time to snag their prey.
“They don’t spend that long underwater — maybe half a minute,” says Jakob Christensen-Dalsgaard. He’s a biologist at the University of Southern Denmark.
That is why he was surprised to find out that at least one of 40 species of the bird — great cormorants — have a superpower to help them: excellent underwater hearing. In fact, cormorants’ hearing may be almost as good as the hearing of frogs and turtles that spend most of their lives with their heads submerged.
Christensen-Dalsgaard and his colleagues discovered that great cormorants have “pulsations of air bubbles behind their eardrums, and that gives them underwater sensitivity.” Their eardrums are also stiffer and thicker than those of birds that do not dive. This might protect them from being harmed by water pressure.
It also means that the cormorants’ hearing is not so great when they’re flying through the air. This probably makes them more vulnerable to the sea eagles that hunt them, because they can’t hear them coming. Christensen-Dalsgaard says this is the trade-off for being able to listen for the sounds of tasty fish as they swim.
The scientists’ research is the first to study and document the physiology — the way living things work — of any birds’ underwater hearing. Next they’re planning to research birds that spend lots of time underwater: penguins. They were just about to test a tiny helmet on a penguin, designed to measure its hearing, when the coronavirus pandemic shut down their lab.
Why do researchers care about how birds hear? One reason is that it’s cool to know how they have adapted over millions of years.
Another reason, Christensen-Dalsgaard says, is that the study of how human noise affects animals has become a hot topic in the past 10 years.
“It’s very much an area of concern,” he says. On land, “it’s clear that noise in cities makes birds increase their sound levels, like we raise our voices to be heard.”
And if cormorants and other diving birds use their hearing sensitivity for hunting fish, “they would be vulnerable to intense sounds” and perhaps lose their ability to hunt so well. Those loud sounds include explosions from dynamiting the sea floor, for example, which is a common fishing practice in some places. It’s bad for the fish, bad for coral reefs and probably bad for birds, too.
With so many people staying at home now, the world has become a much quieter place — on land and in the oceans. Christensen-Dalsgaard says that he has colleagues in Europe who have been recording the human silence. Suddenly “birds’ songs have changed, because it is more quiet than usual,” he says.
Birds, it seems, are taking this time of less human noise to make new and beautiful noise of their own.