Burien, Washington

Andy Waters, Pat Toth, Rhonda Ham

We’re passionate about birds and nature. That’s why we opened a Wild Birds Unlimited Nature Shop in our community.

Burien, Washington

15858 First Avenue South #106
Burien, WA 98148

Phone: (206) 241-3201
Fax: (206) 241-3741
Email: Send Message

Store Hours:
Mon - Sat: 10:00 am - 6:00 pm
Sun: 11:00 am - 4:00 pm

Located near Trader Joe's at the Five Corners shopping mall

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Mewsings from Millie

Hello, and welcome back to my musings!

One of my people occasionally leaves a book here that is all about bird nests and eggs. I was pawing through it the other evening and I couldn't believe how many different colors of eggs there are! White, tan, brown, blue, blue-green, green - some solid colored and some with red or brown speckles or streaks.

This got me wondering about how eggs are formed and how do they get those colors?

Apparently, whether or not they are fertilized, eggs form naturally. First, the yolk, the yellow nutrients of the egg, is released from an ovary and passed as a tiny fluid membrane ball through the female's reproductive tract. As it develops, the egg whites, or albumen, are secreted around it. Next, the shell develops as layers of calcium and other minerals build up around it. The egg is then passed through the body and exits as a fully formed egg.

But what about those colors? As the shell is forming, the egg moves through the oviduct which is a tube from the ovary to the outside. As it moves through, it squeezes against glands that produce colored pigments that combine to form every color in the egg shell spectrum. If the egg is stationary in the oviduct, it will be spotted. If it is moving, the egg will be streaked. Different birds produce specific pigments thus they have their particular shell color like the beautiful blue of a robin's egg.

The whole egg producing process is similar to an assembly line. In the birds body, there is a whole line up of undeveloped eggs, each at a more advanced stage than the one behind it. The eggs continue to develop constantly as long as the bird is alive but the productivity varies depending on the time of year, whether or not the bird has chicks and its age.

Well, this got me wondering how a bird knows it has laid the correct number of eggs? You know, most birds lay a predictable number of eggs like Bald Eagles usually two, Bluebirds four to six, etc. But how do they know when to stop?

To find out scientists experimented by going to nests and repeatedly removing eggs soon after they were laid. Some birds replaced them straight away. One House Sparrow laid 50 in a row!

Birds that do this are called indeterminate layers. And for them it seems the number of eggs is intimately tied to the brood patch. That's the bare skin on a bird's underside that transfers body warmth to the eggs during incubation. It's the tactile sensation involved in this process that seems to let the layer know she's reached the ideal number of eggs.

For some other birds, the scientists' removing eggs had no effect at all on egg numbers. These are known as determinate layers. Just why these two types of layers exist remains a mystery.

Until next time,

Millie, the Muse of Mews