A gull’s egg in my garden – how eggshell pigment is formed

I caught sight of this broken egg shell when I was checking my back lawn for stones and large twigs prior to mowing.  Quite what it was doing in the middle of my lawn I have no idea, but it is beautiful, with a remarkable set of muted colours.  I had never seen one before.  It took me a while of following hyperlinks (most of which were about eating gulls’ eggs) before I found an explanation of how the colours are formed, on the All About Birds website in an article by Pat Leonard entitled The Beauty and Biology of Egg Colour:

An egg’s story begins in a female bird’s single ovary. When an ovum is released into the oviduct and fertilized, it is just a protein-packed yolk. The albumen—the gelatinous egg white—is added next. The blobby mass then gets plumped up with water and encased in soft, stretchy membrane layers. The first globs of the calcium carbonate shell are then deposited on the exterior, with the mineral squirting from special cells lining the shell gland (uterus). Pigmentation, if any, comes next, with an overall protein coating added before the egg is laid. It takes about 24 hours to build a single egg.

In his book, The Most Perfect Thing: Inside (and Outside) a Bird’s Egg, University of Sheffield zoologist Tim Birkhead compares the pigmentation process to an array of “paint guns.” Each gun is genetically programmed to fire at a certain time so that the signature background color and spotting of a species’ eggs is produced.

“Examination of birds’ oviducts at the time the color is placed on the egg suggests that the color is produced and released over a very short time frame,” Birkhead says, “usually in the last few hours before the egg is laid, and that makes it very hard to study.”

Despite the variety of egg colors and patterns, the palette is surprisingly small. Egg pigments are versatile substances made of complex molecules synthesized in a bird’s shell gland. Only two pigments are at work. Protoporphyrin produces reddish-brown colors. Biliverdin produces shades of blue and green. More of one pigment, less of the other, and the egg gets a different background color, spots of a different color, or a combination of both.

The speckling is thought to be camouflage, to disguise the egg and hide it from potential predators, and is common to nearly all foreshore birds.

There is loads more truly fascinating information in the article.  Did you know, for example, that an egg loses 18 percent of its mass, on average, between laying and hatching, mostly from water loss through shell pores. Or that up to 10 percent of the calcium used for shell formation can come from the female’s bones.  The article is well written and is well worth reading, so if you have a moment do go and have a look.

 

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