Here's the recipe for nesting success in the bird world: Lay an egg, heat it up — anywhere from 12 days to five weeks — and then stand back as an infant bird breaks through and tumbles out of its shell.
But as with most simple-sounding recipes, the full story is a bit more complicated than that. And because not many birds (other than crows and a few others) get to watch their parents raise a brood, these instructions are imprinted in their brains.
The nest itself, which will hold the eggs and later the chicks until they're ready to fledge, is built almost entirely by the female bird. This makes sense, because she's the one who'll be spending most of her time in it, so she turns and twists inside as the walls are constructed to make them conform to her shape. The task of keeping the eggs warm, known as brooding or incubation, falls to the female in most species. To allow her body heat to reach her eggs, a mother bird pulls her belly feathers aside to expose an area of warm skin.
In many bird species, the female also develops something called a brood patch on her belly, essentially an area full of blood vessels that helps maintain the eggs at a steady temperature between 98 and 100 degrees F. This heat is the vital force that transforms a small mass of yolk and egg white into a hatchling. (Unusual in the avian world, a bald eagle pair shares incubation duties, and both genders develop this brood patch.)
There's another vital ingredient in the recipe for a successful nesting season, one that surprises most people: Eggs need to be turned on a regular basis. A mother bird gently nudges the eggs with her beak several times a day, or even more often. She usually does this after returning to the nest after feeding herself, rotating each egg about 180 degrees to ensure that all are warmed equally. This also prevents embryos from sticking to the shell wall, which would lead to deformed chicks. (This may remind you of recipes for cookies or cakes, where the baker is instructed to rotate the pans halfway through the baking cycle to ensure even browning.)
'Mom, we're cold'
Too much heat is more dangerous to developing chicks than too little, so the female's brood patch has sensors that tell her if the eggs need a cool-down period. Few birds sit on their eggs continuously and in fact, the warmer the weather, the less time is spent on incubation. If nearly-ready-to-hatch chicks start to feel chilled they'll peep from inside their shells to tell their mother.
Heat from a parent bird triggers the start of embryo development, so many species delay egg-sitting until the last (or second to last) egg has been laid. This ensures that all the young birds hatch within hours of each other, thereby easing the job of feeding and cleaning up after them. (If their ages varied by several days, they'd require different diets and brooding times, etc.)
Breeding season is a harried time for adult birds, but the time spent on her eggs is a rare period of repose for the female before young birds break out of their shells and begin to noisily nag their parents for food and care.
St. Paul resident Val Cunningham, who volunteers with the St. Paul Audubon Society and writes about nature for local, regional and national newspapers and magazines, can be reached at firstname.lastname@example.org.
A female bluebird turns her four eggs each time she returns to the nestbox until they hatch after 12 to 14 days.
A female cardinal incubates and turns her two to five eggs in a cup nest for about 12 days.
Female mallards take short breaks, then turn their 12 or more eggs and shift those in the center to the outer edges for even heat.
Bald eagles incubate and turn their two to three eggs for about 35 days before they hatch.