To the untrained eye - and even a trained one - penguins look alike. But for Charles Bergman, one king penguin stands out.
On a trip to the island of South Georgia in 2011, the seabird approached the nature photographer and writer, threw its head back and released a sound that Bergman describes as a kazoo. It emitted the sound over and over, like a tipsy reveller at a New Year's Eve party.
After completing its vocal performance, the penguin looked at Bergman and waited for a reply. Bergman has spent more than a decade searching for the correct response to that penguin's query, a pursuit he documents in his book,