Wednesday, Apr 02 2014 06:00 AM

Pen in Hand: Oology: discovering the secrets of eggs

Related Photos

House finch eggs in a cup nest. Most birds that use cup nests placed up in trees tend to have bluish or greenish eggs to make less noticeable in the foliage. Photo courtesy of Jon Hammond

These killdeer eggs match nearly perfectly with a neighbor's recycled asphalt driveway. Ground-nesting birds have some of the most cryptically-colored eggs. Photo courtesy of Jon Hammond

An old ornithological chart showing some of the diversity among different types of eggs. Photo courtesy of Jon Hammond

Because they are laid safely down in a hole in the ground, and they are neither at risk for rolling away or being noticed by predators, these burrowing owl eggs are nearly round and white. Photo courtesy of Jon Hammond

A horned lark is another ground-nesting bird with camouflaged eggs, though larks do go to more trouble to build a nest than the simple "scrape" used by killdeer. Photo courtesy of Jon Hammond

Now that spring has arrived, birds throughout the Tehachapi Mountains are going to be nesting and laying eggs. Some species, like ravens and great-horned owls, have already been nest building. While some birds construct elaborate nests and others hardly build any at all, they will all be laying eggs that contain their chicks. There is an amazing diversity of sizes, shapes and coloration of eggs produced by different bird species, and the subject of bird eggs is so interesting and complex that it has its own field of study: Oology.

While "oology" resembles a mere word fragment, like it dropped off the end of a longer scientific term, it actually refers to the study of eggs, bird nests and nesting behavior. It is apparently derived from "oion," the Greek word for "egg." I think it's an amusing coincidence that the "O" actually looks like a physical representation of an egg, followed by "ology," which of course means "the study of," so it's accurate both linguistically and visually.

Bird eggs are fascinating if you stop to consider them for a minute. Why are some nearly round and others elliptical? Why are some white and others elaborately colored? How can a whole large clutch of eggs from a bird like a California quail hatch within an hour or so, when they were laid over a period of two weeks and so some of them are much older than others?

The typical configuration of an egg, which is technically called "ovoid" but referred to by most of us as egg-shaped, serves a useful purpose: when an egg is bumped, it makes a tight circle rather than rolling away, like a round egg would. This is especially important for cliff nesting birds -- many seabirds lay their eggs on rocky ledges without much of a nest, and a round egg would easily roll away like an oversized marble and be dashed on the rocks below. Instead, cliff nesting birds have some of the most conical eggs of all, so these eggs don't roll far when accidently nudged. On the other hand, cavity-nesting birds like many parrots and owls have nearly round eggs, because there isn't much "roll away risk" inside their cozy nests in hollow trees and similar locations.

And though you'd think that the bigger end of an egg would typically be down with the smaller end pointing up, it's actually the reverse, and a conical egg tends to sit on its side in the nest with the larger end pointed up. This is useful, because the larger end is where an air sac is located and there are more air pores on that end of the egg, so tilting the big end upward improves air flow to the developing chick's large head, brain and eyes. Logically, the bird's tail develops at the egg's pointy end, where there is less air and room.

The mechanism that allows eggs laid on different days to all hatch at the same time is yet another egg marvel. There is a temperature called "physiological zero," below which embryos inside eggs remain dormant and won't develop. This is about 68 degrees Fahrenheit, and eggs typically have to brought up to about 99 degrees Fahrenheit to develop properly. When the eggs reach 99 degrees for four hours or more, it's like the starting gun has been fired and the race is on. If the parent birds leave the nest and are delayed in returning so the temperature drops back below physiological zero, development will stop. It can resume when the egg warms back up, but these fluctuations significantly weaken the chick and reduce its chances of hatching successfully. To help keep her eggs warm, the mother (usually) will lose some feathers on her underside, leaving a small area of featherless bare skin called a "brood patch," which she holds right against the eggs to keep them nice and warm.

That's all interesting, but what about the different colors? As you'd expect, those too tend to serve a practical purpose. Ground nesting birds tend to have very cryptically colored eggs to make them harder for predators to find. Just check out the killdeer nest photo on this page -- it took me a good 20 minutes of careful searching to spot these incredibly well-camouflaged eggs blending in with my neighbor's recycled asphalt driveway, and I'm thankful I didn't step on them. Birds that nest higher up in trees tend to have bluish or greenish eggs to blend in with vegetation.

There is so much more to learn about eggs -- like the fact that there are materials called protoporphyrins that are deposited in egg shells as red or brown pigments that not only serve as camouflage, but also strengthen the egg and are deposited when the shells are thin because the mother bird is experiencing a shortage of calcium. So eggs are more heavily spotted and speckled when soil is calcium-deficient, or if they are the last eggs laid in a clutch when the mother is low on calcium.

The science of eggs is an intriguing branch of ornithology, and I continue to be impressed by the intricacies of this field. May our local birds have a successful nesting season despite the rigors of California's drought.

Have a good week.

Jon Hammond has written for the Tehachapi News for over 30 years. Send email to:

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