The Incredible Egg

Photo Group
Yellow Warbler Nest
Photo by: 
Linda Williams

There are many things in life we take for granted. Before I started studying birds, eggs certainly fell into that category. Eggs were for either eating, dying and hunting or occasionally throwing. These wonders of nature are incredibly strong, diverse in shape and color and enable birds to nest successfully from the driest deserts to the coldest conditions on the planet.

An egg's shape is a function of strength with the functionality of a specific species needs. There are four basic shapes of egg. The majority of eggs are similar in shape to the classic chicken egg while others are quite unique. The classic or elliptical shape enables eggs to fit together nicely so the parent can incubate all of them evenly.

Larger birds like eagles lay eggs that are pretty much round and birds like gulls and seabirds that nest on cliffs lay eggs that are strongly pear shaped. Think about that if you will. If you lay your eggs on a flat rock inches from a 300 ft drop, don’t you think an egg that rolls in a circle if bumped is a good idea? Perhaps the egg shape that caught my attention the most were the football shaped eggs of grebes and cormorants. One theory is this shape is good for unstable nest like the floating nests of grebes.

We all know there are white, tan and blue eggs but the variation in spots, squiggly lines and blotches on many eggs is truly fascinating. The spots and lines are produced by glands in the oviducts. As the eggs pass through, pigments from the glands are “squeezed” onto the eggs. The movement of eggs within the ducts dictates the location and shape of their spots and lines.

If you have ever looked for a Killdeer’s eggs in a gravel driveway, you know that concealment is a primary function of color for most birds but there are other functions. For birds like seabirds that nest in huge colonies, it is believed that females learn their unique color pattern to locate them.

It may not surprise you to know that small birds lay small eggs, but did you know that small birds lay proportionately much larger eggs than larger birds? If you really want to be impressed, look up the famed Australian bird the Kiwi. The eggs they lay are about half the size of their body. Ouch!

The next time you go to crack open an egg for your favorite recipe, think about your favorite birds and try to guess how their eggs are shaped and colored.

By Mark McKellar

Spring 2008