How Do You Take Your Eggs?

Eggs.  What comes to mind when you read that word?  I think of my favorite summertime breakfast served on a table with fresh cut roses from my garden.  It’s the best-scrambled eggs, homegrown sliced tomatoes sprinkled with salt and Texas Pete and served with grits, like any self-respecting southern girl must have.

But since learning about bird anatomy and physiology, I almost never crack an egg without thinking of everything a female bird’s body must do to produce such a marvel.  The fact that unfertilized hen eggs are produced in such massive quantities as to nourish millions of humans daily is almost beyond my comprehension.

Here is a general summary of the formation, components and function of bird eggs.

1.  Like female humans, female birds have ovaries that produce a mature ovum.  As the ovum passes through the reproductive tract, layers of albumen and shell are added.

2.  The first layer of albumen is added in the part of the reproductive tract known as the magnum.

3.  Next along the reproductive tract is the isthmus, where the inner membrane, the outer shell and more albumen are added.

4.  The next portion of the reproductive tract is the uterus with a shell gland.  Here, more albumen is added,  along with more shell and shell pigments.  The demand for calcium to make the egg shell is very high.  Typically, the level of calcium in the blood of birds is twice as much as mammals.

5.  Next along the reproductive tract are the uterovaginal junction and the location of sperm storage tubules.  This is where fertilization takes place.  Or doesn’t.   The fact that some bird species can store sperm, or eject it when their last mate is not looking, is an adaptation that has theoretically allowed females to select the sperm they have concluded will result in offspring most likely to survive.  Maybe birds even choose the sperm of breeding partners they find most charming or handsome.  Why not?  For several song bird species, different DNA has been found in the same brood, meaning the baby daddy is not the same for all those hatchlings.  It is humorous, but the fact is this approach increases the odds of having at least some of the chicks survive to adulthood.  This is important when you consider, for example, that chickadees only have one brood per season and only four or five eggs per clutch as opposed to bluebirds that have up to five broods and up to twenty eggs per season.

6.  Albumen is ninety percent water and ten percent protein and serves as the embryo’s water supply and as a shock-absorber to help protect the embryo and buffer it from sudden changes in temperature.

7.  Attached to the shell are two membranes, the inner and outer shell membranes that protect the egg from bacterial invasion and help prevent rapid evaporation of moisture from the egg.

8.  The shell contains thousands of pores that permit gas exchange, such as oxygen and carbon dioxide.

9.  Egg colors and markings also serve a purpose to increase odds of survival. For example, eggs that are laid on the ground or in open nests in trees, rather than in cavities, often exhibit cryptic coloration to camouflage the eggs.

10.  Birds’ eggs vary greatly in size, depending on the size of the bird. The largest egg in modern times is the ostrich egg which is more than 2,000 times larger than the smallest egg, which is that of hummingbirds.

Hummingbird nest and two eggs on the left. Ostrich egg on the right. Hummingbird nests are approximately the size of a silver dollar.

Next time you have a chicken egg, whisper a thank you.  There is a lot that goes into one simple egg.  Also, think about composting those eggshells, giving hens more reciprocal thanks for their sacrifice.  Your roses and tomatoes will then thank you with beautiful blooms and bountiful harvests.

All images obtained at http://people.eku.edu/ritchisong/avianreproduction.html