The Chicken and the Egg

October 25th, 2022 by georgann

The ChickenAnd the Egg  

Mention eggs to most people and they have visions of gray cardboard or pastel styrofoam cartons in the dairy section of their favorite grocery.  Inside those deceiving cartons are boring white eggs; to be scrambled, fried, poached, or boiled.  In the real world, most bird eggs are much more entertaining:  blue, green, brown, or pink with streaks, swirls, spots, and blotches of black, brown, and purple.  Any egg found in the grocery store that fits that description must be rotten.

The egg or ovum develops in the bird’s left ovary (her only remaining ovary).  The ovary may contain 25,000 to 100,000 follicles that mature no more than about 50 ova in a bird’s lifetime.  The yellow yolk is actually the egg or ovum and is composed of about 49 percent water, 33 percent fat, 16 percent protein, 1 percent carbohydrates, and 1 percent minerals.  It erupts from the follicle and is embraced by the first section of the oviduct, the infundibulum.  This section will actually engulf any suitable small object.  Cork balls have been experimentally substituted for yolks in the infundibulum, with the result that the bird laid eggs with cork centers instead of yolk.

After about 18 minutes, the yolk is passed on to the next section, the magnum where it spends four hours getting its albumen (egg white).  The yolk and albumen move on to the isthmus where the shell membranes are added.  That’s the membrane that makes it difficult to peel a hard-boiled egg unless you immediately rinse it under cold water.  The membranes separate at the round end of the egg allowing for an airspace where the baby bird will breathe before hatching.

The next 18-20 hours are spent in the uterus where the hard, but porous, calcium egg shell is secreted around the yolk and albumen.  This is also where the shell becomes pigmented by porphyrins from hemoglobin to produce the brown and olive tones, or by cyanins from bile to produce the blues and greens. 

Background shell colors range from white in most chickens, to sky blue in robins, rich green in gray catbirds, soft brown in loons, and stunning black in tinamous.  The spots, streaks, swirls, and blotches are added to the shell as the egg moves through the pigment glands just before being laid.  For example, the fine brown stippling on a Carolina Wren’s egg is concentrated in a “wreath” pattern at the rounded end because the egg pauses there while being laid. 

Since egg size, shape and color is genetically controlled, it is relatively easy to identify an egg’s mother.  Woodpeckers and owls have round to oval eggs that are glossy white (easier to find in a dark cavity).  Chickadees, titmice, and nuthatches also nest in cavities but have tiny white eggs finely covered by brown stippling which indicates that they originally nested in the open, outside of any cavity.  Mockingbirds have a beautiful pale greenish egg with large pale brown blotches.  Robins are not the only birds with blue eggs.  Wood thrushes, starlings, and bluebirds share the blue hues.  Cardinals lay grayish eggs that are dotted, spotted, and blotched with browns and grays.  Orioles, grackles, and blackbirds all have pale bluish green eggs with brown, purple, gray or black pigments scrawled, blotched, spotted, and drizzled all over. (This is starting to sound like a breakfast order at the Waffle House, “I’ll have my eggs over-easy, dotted, spotted, and drizzled!”).

Most birds lay their eggs, pointed end first, during the early morning hours.  It takes only a few minutes for an egg to emerge through the cloaca and out of the vent.  After five or six days, and five or six eggs, incubation begins.  Some birds, like owls, begin incubating immediately with the result that the last egg will hatch nearly a week after its oldest sibling hatches.  This is bad news if your older brother or sister gets hungry!  Other birds, such as ducks and fowl, don’t begin to incubate until all the eggs are laid.  Each nestling or chick hatches within minutes of each other and the entire brood runs around peeping behind their parents.

So the next time you buy a carton of chicken eggs at the grocery, reflect upon all those other creatively exquisite eggs laid in nests in your yard and garden.  Just don’t tell the chicken; she’ll be jealous.

Birding Adventures, Inc.

Georgann Schmalz