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Why do some gulls have a red spot on their beaks?

I was at Granville Island watching gulls steal food from unsuspecting tourists. Have you ever noticed how some of them look like they have a spot of ketchup on their beaks — the gulls, I mean? I decided to find out.

The Glaucous-Winged Gull (not to be confused with the Glaucous Gull) seems to be the most common gull at Granville Island. Glaucous is a fancy way to say “gray”. I wonder if it sounds better if I say, “I’m going glaucous”?

Many gulls also sport a red spot on the beak. They all seem to have evolved from a common ancestor, so they probably inherited the red spot from that ancestor and perhaps maintain it for similar reasons. 
Male and female gulls get the red spot when they are all grown up, in about four years. The spot can be a sign of fitness in mates, reflecting how healthy they are and how good their eggs will be. An oil spill off Spain was linked to poorer health in the gulls there and smaller spots on their beaks. I wonder if the beauty marks on Madonna or Marilyn Monroe had anything to do with their desirability.

Gull chicks also react to the red spot on the beak of an adult gull. Nobel-prize winning animal behaviour scientist Niko Tinbergen found that newborn herring gull chicks instinctively beg when they see a red spot (even when the spot is not on a gull). My daughter does this when she sees White Spot. When the chicks peck at the red spot (both males and females feed the young), the grown-up barfs up breakfast. I do not respond like that, although I do often cough up. Here are some Glaucous-Winged Gull chicks (covered in brown spots) getting fed and growing up in Vancouver.

More recent studies suggest Tinbergen kind of fudged his data, maybe like Mendel and his peas. Tinbergen's conclusions still sort of apply, but the initial instinct gets modified by experience. The red spot occurs in larger species with relatively smaller chicks. It seems to help the chicks focus their behaviour. Perhaps it started with some  kind of heritable mark on the beak that resulted in better fed chicks.

But wait, there's more. Researchers manipulated the spots of yellow-legged gulls with chicks and found that this affected the feeding behaviour by the mates. If a gull had a bigger spot, the partner contributed more food, regardless of how much the chicks begged. When the spot was smaller, the partner fed chicks less, but fed more if the chicks begged more.

I did not come across any research specifically on Glaucous-Winged Gulls and their red spot, so this may be something to explore further, the next time I am down at Granville Island watching  all the gulls go by.