Robin egg blue

“An Egg a Day is Work
If you think laying an egg is easy, think again! Robins lay only one egg per day for good reasons. Female birds have one working ovary, unlike mammals, which have two. Ovaries are the organs where eggs are produced. A bird's ovary looks like a tiny bunch of different-sized grapes. These "grapes" are theova, or actually the yolks. The one ovum about to be released looks huge. One or two are about half this size, a few more are a bit smaller, and the rest of the ova are tiny. About once a day, the largest yolk is ovulated. That means it pops off the ovary and starts traveling down a tube to the outside of the robin's body. This tube is called the oviduct.

If a female robin has mated with a male, the yolk will become fertilized. If the robin hasn't mated, the yolk still goes down the oviduct and will be laid like a normal robin egg, but it won't develop into a robin. As the yolk travels through the oviduct, the tube's walls slowly secrete (drip out) watery proteins called albumen to surround the yolk. Near the end of the trip down the tube, the oviduct secretes calcium compounds. The calcium compounds will become the eggshell, but the egg will remain a bit soft until it is laid. You can imagine why the formation of an egg is a tremendous drain on a mother robin's body!”