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Let's imagine a 28-year-old, 140-pound woman suddenly acquiring the attributes of a house wren.

It's spring -- baby time.

At some point in late May, Wren Woman will give birth to an 18-pound baby. She'll do it again the next day, and once each following day, until she has eight or so babies.

Her mate will help her with diapers (removing the fecal sac) and feeding. He'll also sing a lot. She won't sing.

Wrens lay eggs equal to 13 percent of their body weight, hence the proportionally large baby.

It takes the wren about 20 hours to make an egg. She'll lay it in the morning. That way she doesn't carry the weight or endanger the egg as she forages through the day.

Wren eggs and chicken eggs have much in common, except size, of course. (You could fit 75 wren eggs in one chicken egg.) So, consider the chicken egg.

The shell is mineral; the bird finds the necessary calcium as it feeds. (I put oyster shell near my feeders in the spring; it's a good source of calcium.)

The shell is held together by a thin membrane that adheres to its inner surface. A second membrane surrounds the egg white. This is the one that gives you trouble when you peel a hard-boiled egg.

The yolk consists of protein and fat to feed the embryo. On the yolk, you can see a small white spot. This is the blastoderm. It would develop into the chick if the egg was fertilized.

The shell breathes. Oxygen comes in, to fuel development. Carbon dioxide and waste water produced during growth go out. Rub a bit of oil on an egg, and you'll seal it like a coffin.

At the blunt end of the egg is an air space. When you peel a hard-boiled egg, you can see the depression where this air is stored. This is what the developed chick breathes as it's breaking out of the shell.

The chick has what is called an egg tooth on the end of its upper mandible. This disposable tool is used as the bird pokes its way into the world

Eggs are colored as needed. Wren eggs are white to pink to brown, usually with dark speckling. Eggs are marked with speckles or dots or dashes to help hide them from the eyes of predators. If nest predation is a lesser threat, eggs will be pale and unmarked.

Birds can be either altricial (species in which the young are born or hatched helpless) or precocial (species in which the young are relatively mature and mobile from the moment of birth or hatching).

Altricial birds, including wrens, hatch as blind, featherless creatures that must be intensely tended in the nest for two weeks or longer. Eggs for altricial birds tend to be proportionately smaller, because much development occurs post-hatch.

Precocial birds, including waterfowl, game birds and chickens, are up and around within an hour or two of hatching. These birds' eggs are up to a third larger proportionately because more nourishment is needed inside the egg to produce a fully developed chick.

Human babies are altricial. The largest recorded human baby that survived weighed 22 pounds. Even at that size, it was altricial. It was born to an Italian mother in the 1950s. According to Google, she had only one.

Lifelong birder Jim Williams can be reached at woodduck38@gmail.com. Join his conversation about birds at www.startribune.com/wingnut.