Mother Knows Best

(Mothers' Day issue...)

Of all the routes that reach motherhood, birds take one of the most elegant journeys. Like us, bird mothers provide their embryos with food and water. But once she lays her eggs in their protective shells, the mother hen can only offer warmth and protection during 2-4 weeks of incubation.

Why do mother birds lay their eggs in a nest in the first place? What goes on inside a bird egg? And just how does a baby bird grow bones from a liquid egg?

It's a complicated case of caching and recycling.

A baby bird's route to hatching is complicated from beginning to end. Between ovulation and hatching, a bird-to-be (an ovum) must take a 24-hour journey to laying (an egg). To hatch out, it must survive weeks of growth and transformation while isolated inside of a hard shell.

To begin with, the hen's internal body temperature is too hot for fertilized eggs to survive. Adult bird body temperatures are 104-108F (40-42C), but embryos can't survive above 104F (40C). So each egg must be quickly "moved" to a cooler environment and then incubated externally.

Food is the first thing mother caches for her young. Almost a week before the embryo's journey begins, yolk (food) is added to the microscopic ovum in the ovary, in alternating day/night layers. The layers can be counted like tree rings; 4-5 layers for perching birds ("passerines"), 10-13 layers for gulls, and so on. As more yolk layers are added, the ovum swells in size and eventually bursts free of the ovary.

This marks the start of a day-long trip down the convoluted "oviduct."

Just as the ovum bursts free, it gets swallowed by the upper end of the oviduct (the "infundibulum"), where it gets fertilized and becomes an embryo. This only lasts 18-20 minutes, as peristaltic contractions begin pushing the embryo farther along its journey.

Twenty minutes in, the fertilized embryo is provisioned with food (yolk) but still needs a water supply to carry it through weeks of incubation and growth. Consisting of 90% water, the egg-whites or "albumen" serve this purpose. Four layers of jelly-like albumen are secreted around the bird-to-be during its 3-4 hour passage through the middle region ("magnum") of the oviduct -- at a scorching pace of 0.09 inches (2.3 mm) per minute.

One of these jelly layers (the "chalaza") grows into a pair of coiled springs that suspends and rotates the yolk sac to always keep the embryo on top when the parents turn the eggs over during incubation. Pheasants turn their eggs about once each hour, while Kestrels turn their eggs about three times more often. No matter, the embryo always stays right-side-up inside its shell.

Once supplied with food and water, the embryo now needs protection. So mother hens also provide flexible membranes and a rigid egg shell.

Two flexible membranes are added during the embryo's one-hour passage through the lower ("isthmus") region of the oviduct -- at a more leisurely pace of 0.05 inches (1.4 mm) per minute. A thin, inner membrane surrounds the embryo and its food and water supplies. A thick outer membrane will attach to the egg shell (which arrives momentarily). These membranes manage the exchange of gasses into and out of the egg -- oxygen moving in, carbon dioxide moving out.

Within the inner membrane, the embryo grows in a self-contained environment. But its own metabolism produces toxic wastes that obviously can't escape through the hard shell. So the excreted amonia is converted into nontoxic uric acid (in non-soluble crystalline form) for storage. The egg-bound embryo is elegantly separated from its wastes.

One final level of protection is the familiar egg shell. Egg shell, and shell pigmentation, are added during a 19-20 hour stay in the lower end ("uterus") of the oviduct. About 12% of the calcium that is stored in the hen's bones is rapidly released to form each shell. Egg shells are often formed overnight, while mom rests in the nest.

All bird egg shells were probably once white, like reptile eggs. Bird species that hide or cover their eggs still lay white eggs, including cavity nesters (kingfishers, woodpeckers), those that start incubation with the fist egg (loons, herons, eagles), and those that cover their eggs with down (some ducks, geese). But natural selection has also produced a palette of colored and camouflaged eggs. A few examples: pheasant eggs are olive-green, grouse eggs are spotted brown, and Red-winged Blackbirds lay blue eggs with purple or black scrawls.

Pigments are secreted from the uterine wall while a shell slowly forms around the egg. Pigmentation helps to hide an open nest from larger predators, while the hard shell helps to defend the embryo from smaller soil invertebrates and microbes. Egg shells also provide structural support against the weight of an incubating adult, while at the same time remaining delicate enough to allow a chick to break out.

But that's just the exterior. What's going on inside is even more elegant.

Embryos need to breathe. So all bird egg shells are permeated by thousands of microscopic pores, which allow gas exchange while preventing water loss. The number of pores is a balancing act that varies between species and habitats. A common chicken egg has 6,000 to 17,000 pores that allow the developing embryo to passively breath.

Just like the membranes, egg shells also have two layers. The outer shell layer is made of calcium carbonate (in the form of calcite crystals). The inner shell is a thin layer of magnesium and phosphate. At first, the developing embryo grows a skeleton of soft cartilage. But as it ages, the bird-to-be removes calcium from the outer shell layer and uses it to turn its soft skeleton into hard bones.

In essence, the mother hen recycles some of her bone calcium into egg shells, and her young recycle some of their egg shells back into bones!

(The magnesium in the inner layer facilitates this recycling program, but pesticides can interfere with the shells of some bird species. Some pesticides increase the amount of magnesium in the inner layer, and this inhibits the crystal formation in the outer layer, which leads to thinner shells that can break under the weight of the incubating parent. That's why we banned DDT use.)

Thinning egg shells is normal, near the end of development. As the young develops and recycles egg shell back into bird bone, the shell gets progressively thinner and more porous at the same time that the young bird's respiration needs are increasing. A thinning shell is also easier to break out of when the chick is ready to hatch.

One or two days before hatching, the chick punctures the flexible, inner membrane. Within a few hours, it also chips through the outer membrane and shell, and breathes fresh air for the first time. Canada Goose goslings will finally emerge from the egg about 24 hours later (see bottom photo), but chicks of some species won't emerge for several days.

That's because there are two different hatching strategies. Hatching can be a race to win or loose, or it can be a synchronized tie.

Some bird species, like Bald Eagles and Robins, start incubation when the first egg is laid. Hatching is staggered, and their young will stay in the nest and continue growing. These species are called "altricial," or in older textbooks, "nidicolous" (nest hugging). Often, only the older, bigger chick survives unless there is abundant food that summer. (As the youngest of seven sons, I can testify to the difficulties!)

But in other species, incubation does not start until the last egg is laid, and all of the eggs in a brood will hatch at about the same time. These species are called "precocial," or "nidifugous" (nest fleeing). A brood of 11-13 Mallard eggs that are laid over a period of 1-2 weeks will all hatch within two hours of each other. Precocial ducklings can swim and catch food as soon as they hatch. Usually, mom leads them away from the nest the following morning.

But how do the eggs of precocial bird species know when to hatch? Yet another elegant solution -- the unhatched chicks talk to each other to synchronize their break-out efforts.

During the last few days of incubation, the egg-bound chick pecks weakly, but regularly, at the inside of the shell while using its legs to rotate counterclockwise. This behaviour, called "pipping," creates a circular series of fractures from inside the shell. Some chicks also have a small, upward-pointing "egg tooth" on top of their bill, and their neck muscles swell up for 1-2 days to help with the job.

While working on their egg shells, these precocial chicks keep track of each other's progress by vocally clicking. Older chicks click relatively slowly (1.5 to 60 times per second), causing their younger siblings to speed up. And younger chicks click more rapidly (more than 100 times per second), causing the older ones to slow down, delaying their emergence by up to 33 hours. When all are ready, the jarring of an adjacent egg by the first one to hatch is the signal for the rest to follow suit.

After the chicks hatch, their neck muscles immediately shrink and their egg tooth falls off in 1-3 days (or, gets resorbed in songbirds). The yolk sac is now the stomach, and the remaining yolk will supply the chick with food for a few more days: 4 days for starlings, 6 days for quail, 7 days for mallards. This explains why some baby birds look, well, pudgy.

The chicks survived the weeks-long transformation. Mother hen packed in food and water, encased them in protective shells, and then provided gentle warmth during incubation. And, she used the egg shells as a medium to transfer bone calcium to her young when they were still microscopic.

Pretty complicated, and rather elegant. As you knew all along, mother really does know best.

Behind the bird: If you are the recycling type, you can help mother birds out. Some birds eat the broken egg shells or feed the shells to their chicks. You can recycle your breakfast egg shells to provide your local birds with calcium, especially during nesting. Simply wash and dry the shells, bake them at 200-300 degrees until they just start to turn brown. Then crush them up and serve them in your bird feeder or on the ground. Mother hens will thank you.