(truenature)

Decoding the Egg

Biologists have long wondered why some bird eggs hatch and others don't. Recent studies reveal that an embryo's development depends as much on its surrounding environment as it does on its mother.

By T. Edward Nickens

The flawless curve of its nacreous horizon, the shimmering gloss of its textured surface: nothing in nature is more perfect than a bird's egg. An egg holds all the makings for life without any of the messiness to come: the blood, the hunger, the scraggly wet down feathers of the newborn chick, the insistent screechings

for food, the white splashes of feces around the nest. An egg is pure potential, life compressed into perfect compactness and elegance.

At least that's how it seems when you happen upon a bird's nest one spring morning. It can happen almost anywhere. Unlike soft-shelled amphibian eggs, which generally must be laid in wet places to avoid desiccation and extreme temperature swings, bird eggs can tolerate an astonishing range of conditions. They can be found from the hottest, driest deserts, where the brooding lesser nighthawk merely covers her eggs with her body on the bare ground, to places as raw as Antarctica, where emperor penguins incubate their eggs through the depths of the dark, cold winter.

But, in fact, eggs are as closely bound to the struggle between life and death as any breathing creature. Like the chicks that will hatch from them, they exist in delicate balance with their environment, their parents, and other organisms. Recent studies have shown that eggs have surprisingly intricate means of interacting with their surroundings—a lesson that may have some downright practical implications when it comes to working toward the survival of many endangered species.

Scientists have long mused about eggs. Aristotle believed, wrongly, that male chicks hatch from rounded eggs, and female chicks hatch from pointed eggs. When a craze for collecting birds' eggs, especially rare or strange-looking ones, hit Europe and North America in the 19th century, it contributed to the decline of many species—and perhaps to the notion that eggs are more or less independent of their surroundings. For my guide to bird reproduction I found a researcher with more firsthand field experience: Steve Beissinger, a biologist at the University of California at Berkeley (and a member of National Audubon's board of directors). He began studying green-rumped parrotlets in Venezuela more than 20 years ago.

The science of incubation might be of only academic interest were it not for the modern-day importance of captive breeding.

At first Beissinger focused on the species' demographic and population dynamics. But then he realized that the parrotlets' nesting habits raised a question. “The intriguing thing about them is that they lay a fairly large clutch for tropical birds—seven or eight eggs—and they begin incubating soon after laying the first one,” he says. “It often takes two to two and a half weeks between the hatching of the first and last chicks. Oftentimes the last-hatched and often the second- to-last don't hatch.” Why, he wondered, do parrotlets make a reproductive investment that so often doesn't pan out? Why do they begin incubating so early if it means that the final eggs will often be wasted?

To understand the importance of these questions, it helps first to know a bit about egg physiology. A bird egg consists of a hard shell, made up primarily of calcium carbonate, that surrounds both the albumen, or egg white, and the yolk, upon which the growing embryo sits. Though the shell appears solid, it is in fact pocked with tiny pores—at least 7,500 in a typical chicken egg—that allow gases to move between the embryo and the atmosphere.

The embryo needs to be heated to at least 77 degrees Fahrenheit to grow and does not grow properly until it is at about 95 degrees Fahrenheit. In most climates that means heat needs to be provided by an adult. Below 25 degrees the embryo is at what's called “physiological zero,” meaning that it is effectively in a state of suspended animation. By not incubating, many birds keep their eggs at physiological zero until most or all have been laid; that allows all the eggs to hatch and the nestlings to fledge at about the same time. Species such as parrotlets and many owls, instead, begin incubating with the first or second egg. As a result, the eggs hatch asynchronously. Ornithologists had an explanation for this behavior: The range of nestling ages spreads out the food demands of the entire clutch, potentially allowing more chicks to survive.

In fact, eggs are as closely bound to the struggle between life and death as any breathing creature.

But Beissinger found that didn't apply to parrotlets, as he'd noticed that adults had no trouble finding food for all the young. He wondered if they might have another reason for beginning incubation early. By placing freshly laid parrotlet eggs in an unattended nest box, and then returning them to incubating adults at different time intervals, he soon had an answer. “After about three days the hatching success of unincubated eggs would decline markedly,” he says. “After five days only half would hatch, and after nine days only about 10 percent. So delaying incubation too long was clearly a problem.”

Beissinger thought the explanation for that was pretty simple. In Venezuela temperatures often rise to between 77 and 95 degrees during the day and sink again at night. Every day unincubated embryos were heated enough to begin growing, but not sufficiently to grow evenly. The resulting developmental problems caused most of the embryos to die.

But maintaining unincubated eggs at temperatures below 77 degrees did not lead to survival, either. Beissinger next began a three-year study in Puerto Rico. By examining eggs of pearly-eyed thrashers at different elevations—and, hence, different temperature ranges—he discovered that unprotected eggs were equally subject to failure in cool temperatures as in warm. The researchers noticed the eggs that failed tended to stink. Microbes, they realized, were to blame.

“Microbes can get through the pores of the shell,” Beissinger says. “It's not easy, but they can do it. Moisture on the eggshell surface helps transfer bacteria and fungi through the few pores that are open, and it also promotes their growth.”

The experiment suggested that many tropical species have to begin incubating early. In warm conditions they need to incubate to prevent the developmental problems caused by midrange temperatures. In cool conditions they need to begin incubating early both because their body heat, by drying the eggshells, keeps fungi at manageable levels, and because that heat promotes the activation of the egg's own antibiotic enzymes.

“Egg whites have a lot of antibacterial properties,” Beissinger says, “but they need to be warmed a bit to function properly.” He's now examining bluebirds and swallows in California in order to learn whether similar dynamics are at work in temperate-zone birds.

The science of incubation might be of only academic interest were it not for the modern-day importance of captive breeding. From the California condor to the Mauritius kestrel to a variety of parrot species, many birds owe their sheer survival to breeding in captivity, which often requires people to act as surrogates for avian parents. A range of new insights into how incubation works is allowing bird handlers to play that role more effectively.

Of course, people have been incubating birds—especially chickens and other domestic fowl—for a long time. Most traditional incubators use forced hot air to keep eggs warm. That works well enough when the number of eggs is large and their overall survival rate not critically important. But for rare species whose every egg represents invaluable genetic material, the process has its shortcomings, because hot air simply can't replicate the subtleties of what an adult bird does.

“The neat thing about incubation is that there seems to be a really close correlation between hatchability and the type of environment the bird is in,” says Paul Reillo, director of the Rare Species Conservatory Foundation in Loxahatchee, Florida. The foundation has raised many individuals of the highly endangered red-browed Amazon parrot, a native of Brazil's Atlantic forests. “It's not rocket science,” Reillo says, “but there are plenty of unknowns. Different species require different rates of turning. Even atmospheric pressure can make a difference.”

In an effort to boost hatching success rates for this and other rare species, increasing numbers of breeders are turning to artificial incubators that use plastic or rubber membranes filled with hot water or air to closely mimic what parent birds do. These incubators are based on the research of scientists like Scott Turner, a biologist at the State University of New York at Syracuse who has extensively studied how heat is transferred between adult birds and eggs.

“The egg's not just a shapeless lump,” Turner says. “Early models of incubation used unrealistic assumptions about how heat flows from parent to egg; they were based on an egg sitting in air. In fact you have a parent bird pressing its brood patch against the egg. That sets up an entirely different way for heat to flow.”

The myriad tiny blood vessels in an adult bird's brood patch—a featherless area of highly vascularized tissue on its breast—can quickly radiate heat to eggs. The patch is extremely sensitive to temperature variations, so the adult can tell when to turn its eggs or when to stop incubating for a while. There are even tantalizing suggestions that the beating of an embryo's heart, or chemical signals traveling from an egg to an adult, can affect heat transfer from the brood patch, though the exact workings are not clear. “What's unknown is the extent to which the embryo can exercise control over the parent,” Turner says.

However it perceives its eggs' needs, an adult bird has to know exactly how much heat to provide and when to turn its eggs so the heat is distributed properly. It doesn't need to know this only at the onset of incubation. As it grows, an embryo begins to produce some of its own metabolic heat, but that doesn't free it from reliance on its parents. “The old assumption was that as the embryo grows, the parent has to put less heat in,” Turner says. “But as circulation gets more vigorous inside the egg, it actually demands more energy from the parent.”

Heat, after all, makes everything happen in an egg. One of the reactions it provokes results in the secretion of carbonic acid, which works to break apart the shell's calcium carbonate. The microbes Beissinger learned about may also help in many cases, as they slowly eat away at the shell pores and enable the chick to hatch.

In captivity, the natural breakdown of eggshells may or may not function in the same way. Reillo has at times resorted to drilling small holes into eggs toward the end of their incubation period. Some parrots lay eggs that have very hard, calcified shells, especially those laid early during the nesting season. Without the helping hand of Reillo's drill, those shells might not permit proper respiration. As a result, the developing chicks could drown inside or lack the strength to peck through the shell.

It's a helping hand, but even the most skillful breeders acknowledge they can't quite replicate the work that parent birds do. “Hatchability is never as good in an artificial incubator as under the hen,” Reillo says. “We're still very much in the infancy of this field. We're fumbling around and learning as we go. We learn through trial and error, and let nature teach us as we go. And it's the connection to in situ conservation work that makes what we do have some value.”

That's a good reminder that, just as eggs don't exist in some idealized isolation, neither do species. They need their habitat. However skilled our techniques for husbanding various species, to keep them solely in captivity is no answer to conservation problems. The true perfection in the design of a bird's egg, it turns out, lies not in the egg by itself but rather in its delicately nuanced interactions with its surroundings. A

Peter Friederici is a writer in Flagstaff, Arizona. He is currently working on a book about ecological restoration.