The nesting season for birds shifts into high gear in May. Geography, weather, and each species’ biology affect the precise timing of the nesting season, but one constant is the role of the incredible egg.

Think of an egg as an avian gestation chamber, or the functional equivalent of a mammalian uterus. It’s where a fertilized egg develops into an embryo and then transforms into a chick mature enough to hatch.

The wonder of embryonic development is how fertilized eggs transform into chicks. Typically, females sit on a clutch of eggs and after a period of time, the eggs hatch. The key ingredient is heat. Incubation is all about heating eggs to a temperature warm enough for embryonic development to proceed.

This might seem an impossible task because birds are covered with feathers, one of nature’s best insulators. Feathers protect birds from winter cold and summer heat. So how do birds raise the temperature of their eggs?

The answer is quite simple. After a clutch of eggs is complete, whether it’s the single egg of a penguin in Antarctica or seven chickadee eggs in your backyard, females warm their eggs by pressing their warm skin directly to the eggs. Body heat is transferred directly from the female’s belly to the eggs.

And by the way, females of most species do all or most of the incubating. Monogamous males defend the nest, protect their mates, and provide food; polygamous males shirk all parental duties and move on to acquire the next in a series of mates.

Triggered by seasonal hormonal changes, the development of a highly vascularized, engorged, featherless patch of skin on females’ bellies makes incubation possible. Even to a human the “brood patch” is noticeably warm. When placed directly on eggs, the transferred heat raises the temperature of the egg to enable embryonic development.

During incubation, a female songbird, for example, spends most of the 14-day incubation period sitting on the nest warming the eggs. On average she spends about 45 to 50 minutes each hour on the nest. On cold rainy days she takes shorter breaks so the eggs don’t chill while she’s away eating, drinking, and getting a little exercise.

After each break, mom returns to the nest and settles on to the eggs. Then she quivers to move her belly feathers to the side so her brood patch can touch the eggs. In this way incubating females raise the temperature of their eggs from 93 to 100 degrees F. Periodically while incubating, she reaches down with her bill and turns the eggs so they warm evenly.

Visit an active nest cam to watch an incubating female for 20 to 30 minutes to watch her settle down on her eggs and reach down occasionally to turn them. Any species will do. Watching something is always more compelling than simply reading about it. I must, however, caution that when you search for a “live bird nest cam” you might get more ads to buy nest cams than sites to actually watch nests. Try the Cornell Lab of Ornithology’s “cams.allaboutbirds.org” to get started.

Finally, after about two weeks, females can turn their attention to sheltering, warming, and feeding their broods.

Generally, the length of incubation is related to the size of the bird. Bigger birds lay bigger eggs that take longer to incubate. Small songbirds such as chickadees and song sparrows incubate their eggs for 12 to 14 days.

A bigger bird such as a blue jay takes 17 to 18 days. Canada geese, turkeys, and great blue herons take about 28 days. And bald eagles and great horned owls incubate their eggs for about 35 days. If the weather turns cold during incubation, these times can be extended by several days.

Another factor affecting incubation is clutch size. Smaller birds usually lay fewer eggs than larger birds. And fewer eggs are easier to warm than more eggs. Small backyard birds usually lay four to six eggs per clutch, while larger game birds and waterfowl have clutches that can exceed 10 eggs. Keeping 10 eggs warmed evenly is obviously more difficult than warming five or six eggs.

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Send questions and comments to Dr. Shalaway at sshalaway@aol.com or 229 Cider Mill Dr., Apt. 102, Hendersonville, NC 28792.