Oology is the science of eggs. Birds’ eggs come in a huge array of colors even though most of us see only the brown and white eggs in the supermarket. It has been illegal to collect wild birds’ eggs in the United States since 1918 with the passage of the Migratory Bird Treaty Act. If you want to see wild bird eggs, you have to go to a museum like the Chicago Field Museum, the source of the 600 eggs shown in my colleague Mark Hauber’s extraordinary The Book of Eggs. Or you can see 90 New York City bird eggs in the Field Guide. New technologies enable biologists, chemists, and ornithologists to learn more about bird eggs. But as an artist, it is color, shape, and variety that most fascinate me. Eggs are basically made up of calcium carbonate, which looks white to the human eye. Birds’ eyes are tetrachromatic—they have four photoreceptor cells so they see color in the UV range not perceptible to us. The egg colors come from two basic pigments: biliverdins, which make the blue-green colors and protoporphyrins, which make the yellow, red, brown and other rusty colors. The two pigments together can form colors like violet and exotic greens. So it is that within one species a remarkable variety of egg colors can be found. The photo shown here was shared with me by Mark Hauber; it shows 12 Common Murre, Uria aalge, eggs. These birds live in huge colonies and it is thought that the color variety allows parents to pick out their own egg from thousands on a rock cliff. It’s clear that there is a lot to “ooh” about oology.