You have probably seen Gibbon pairs duetting in nature documentaries or videos. It is a territory display that shows how the pair is well bonded, and the turn-taking of the duetting is amazingly precise, the pair alternating calls to produce a single 'song' that advertises their presence and ownership of a territory. But you don't need to travel to distant tropical jungles to witness pairs duetting animals. Herring Gulls do it all the time. Because their long call is so familiar we tend not to pay much attention to it, but I was made to stop a couple of days ago when a pair landed on a chimney stack opposite my house (above). One of the pair seemed intent on long calling, and started to call, followed each time by its partner. Long calls become commonplace during the breeding season, when pairs defend territories or food, as it appears to have an aggressive function. Here is a sonogram of it. The gull starts calling with the neck pointing down, the calls well spaced, then the calling becomes faster in a staccato and the gull stretches its neck and points up and then the call dies in intensity. In 'The Herring Gull world, Tinbergen describes the call, saying that 'The voice of the herring gull is wonderfully melodious’‘There are few sounds as evocative, as stirring as the profound, plaintive beauty of their calls.' However, he missed the fact that it often occurs in pairs, there the pair message is not to each other, but to other gulls. The duetting has also been described in other gulls, like in the Western Gull.
Often, one of the pair starts the call, and the other joins in and they end the long call together, necks stretched, bills pointing up.
Why do gulls long call? They seem to do it when they have asserted their ownership of something (food, for example) or finish a squabble, or sometimes the whole Herring Gull colony calls and pairs joins together. I'm surprised I've found so little about this amazing behaviour other than anecdotal references to it, but this is a wonderful photo of a pair duetting.
