Field Notes: The Gonydeal Spot

American Herring Gulls in breeding plumage, Fairbanks, Alaska; to the right is a closeup of the right hand bird’s bill

A reader asked why some of the gulls currently being featured in recent Return of Bird of the Week series have a red spot on their bill. More recently, Mrs. WC reminded WC of a classic avian behavioral study closely related to the reader’s question. So, what’s up with the red spot?

It’s not blood, as WC heard one non-birder assert. Rather, it’s called a gonydeal spot, a red (and sometimes black) area on the ridge formed on the lower mandible of some gulls, where the where the two plates that make up the lower mandible separate. That feature is called the  gonydeal angle or gonydeal expansion. In some gull species, the plates expand slightly at that point, creating a noticeable bulge. In the photo above you can see the bulge of the gonydeal expansion and the dull red gonydeal spot. The red is in the leathery, skin-like cover of the bill, called the rhamphotheca. The red area is brighter in breeding season, and generally doesn’t appear until the bird is sexually mature. It’s present in both males and females.

Okay, a red spot. What’s it for? That’s where the avian behavioral study comes in. A pioneer animal behavioral researcher named Niko Tinbergen tracked down a nest of European Herring Gulls with recently hatched young. While the parents were away, he kidnapped the nestlings and presented them with a mock bird head with a mock bill made by a wooden popsicle stick with a red dot painted on it. The nestlings pecked at the red dot. When the researcher presented a plain wooden popsicle stick, identical to the first but without the red spot, the nestlings were massively disinterested. Clearly, the nestlings associate the presence of the stick-shaped bill with a red dot as a signal that a parent was supplying food.

The adults, in turn, took the tap on the red spot as a signal to regurgitate food into the nestlings’ open mouths. Sure, it’s a little gross but gulls don’t have hands, and their feet have evolved for paddling, not clutching, so what else are they going to do?

So the gonydeal spot is a visual cue to nestlings that food is available. There’s some ambivalent evidence that the gonydeal spot may also have a role in sexual selection, with potential mates evaluating each other based on the brightness of the red spot. But the evidence is muddled.

Why is the gonydeal spot only found is larger, white-headed culls? Good question. Ornithologists are working on that.

Tinbergen, together with Konrad Lorenz and Karl von Frisch, shared the 1973 Nobel Prize in the medicine and physiology. All three ethologists were cited for “discoveries concerning the organization and elicitation of individual and social behavior patterns.”

Desmond Morris, author of The Naked Ape, and one of Tinbergen’s first pupils at Oxford, says that most people interested in biology are either lab men or naturalists. “They wear a white coat or Wellington boots, one or the other,” Morris says, “Tinbergen does both. In my book, that makes him the most important person in his field this century.”