Bird eggs are amazing packages

This summer, in one of my routine checks on the nest boxes I tend, I found four wiggling Eastern bluebird chicks and one unhatched egg.

The egg somehow had become wedged between the grassy nest and the box wall. Given the development of the birds in the box and its position out of the nest, this egg was not going to hatch.

Before discarding it, I peeled part of the shell away, and photographed the egg and embryo.

This is a bluebird embryo, my guess about two-thirds into its two-week development as a bluebird chick. (I compared this to a set of photos documenting a chicken egg's three-week journey to hatch.)

Eggs are amazing things. The complete development of the bluebird chick in that egg in 14 days or less is amazing. The day-to-day change, if observed, could almost be considered a slow-motion film.

The yellow blob you see is the yolk. It nourishes the chick during its growth.

I can't see any egg white — albumen — in the photo. The white provides water to the chick along with 40 different proteins.

The shell is bound to the chick by two transparent protein membranes that provide defense against bacteria. They have surprising strength. You can sometimes see them when you crack a chicken egg. They are more evident when you peel a hard-boiled egg.

You can see a portion of one membrane clinging to the top of the embryo's head and on the left eye as you view it. The eye on the right, dark, shows how large eyes are upon hatching.

The shell is porous, as many as 17,000 tiny pores allowing passage of air and water. The shell is almost 100 percent calcium. The female bluebird forages for calcium. Some insects are calcium-rich, millipedes for example. The hen will eat small shells, sand, dirt, or ash, bones from dead animals, even mortar between bricks.

The hen also will draw on the calcium in her bones if necessary. She will sometimes eat the shells of her hatched eggs if there is to be a second brood.

When you peel a hard-boiled egg you might notice that one end is concave. In the shell, this was an air cell. It formed after the egg was laid, when its contents cooled.

Most songbird eggs take about two weeks to hatch. Larger eggs take longer. Incubating birds rarely leave eggs unattended for more than five or 10 minutes.

Eggs sometimes fall from nests. They can be returned to the nest, according to the U.S. Fish and Wildlife Service, if you can locate it. But baby birds found on the ground should be left alone. Parent birds usually are around. They know more about their babies than you do.