Bird Eggs

by Stan Tekiela
© NatureSmart
April 28, 2006

One of the most remarkable things in nature is the bird egg. So, this week we will continue our series on bird nest and eggs. There are over 9,000 species of birds in the world and all of them lay eggs. By contrast not all mammals, of which there are over 4,000 species, reproduce by live birth. In fact there are two kinds of mammal which reproduce with eggs. These are the spiny anteaters and the platypus, both found in Australia. Talk about your strange animals.

Unlike mammals, in which an embryo grows inside the body and is constantly “hooked up” to the mother to obtain its nourishment and other needs, eggs are complete independent from its parents. More importantly an egg needs to be filled with all the protein, carbohydrates, fats, and water that the developing hatchling will need for the duration of the incubation.

Just on the surface of it (pun intended) an egg shell is an amazing structure. To start with the shell is the developing embryos first line of defense. So the shell has to be thick enough to be structurally strong enough to not break every time the mother sits on them, but is also must be thin enough to all gases such as oxygen and nitrogen to move free back and forth and at the same time not entomb the chick within when it comes time to emerge from the egg.

To prove just how important gas exchange through a bird’s egg is, researchers demonstrated that applying just one-half of a drop of oil per day to a Mallard Duck egg kills the embryo after just a couple days. Even one-tenth that amount of oil killed up to 90 percent of the embryos in another study. Adult nesting birds exposed to even low levels of pesticides or oil in water may bring home enough toxic substance on their belly feathers to kill their eggs. Give that some thought the next time you are applying chemical weed killers to your lawn where your friendly robin likes to run around and feed or when you wash oil down the storm drain into your local pond or lake.

The largest eggs of any living bird are those of the Ostrich. They measure approximately 7 by 5 inches and weight nearly 3 pounds. The eggs of some extinct birds such as the elephantbirds of Madagascar measured 15 by 10 inches and weighed as much as 27 pounds or approximately the same as 150 average chicken eggs.

The smallest of all bird eggs are laid by the hummingbirds. No big surprise there. Two West Indian species of hummingbird lay the smallest eggs measuring .4 to .5 inches and weigh less than .04 ounces. 75 of these tiny eggs would fit inside one chicken egg. Now that is small!

In general the larger the bird the larger the eggs they produce. However in proportion of their body size it doesn’t match up. For example, an Ostrich egg is only 1.7 percent of the bird’s weight, whereas a House Wren’s egg is 13. percent of the weight of the mother that lays it.

And there is more interesting aspects of egg size. The females of many species lay smaller eggs in their first nesting season of life then they do later in life. In addition birds that lay a large number of eggs such as Wood Ducks and Mallards tend to lay smaller eggs. In addition the eggs of birds in which their newly hatched chicks leave the nest within hours of hatching (precocial) tend to be larger than similar sized birds whose chicks stay in the nest for many days (altricial) after hatching. Presumably it’s because the larger eggs allow for more development within the egg before hatching instead of doing the final development after hatching.

Maybe next time I will take a look at egg shapes. Until next time…

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