Well, how do they?
Viviparous animals get oxygenated blood via their umbilical cords. Are eggshells porous enough to sustain the chicks via diffusion? But what happens when mummy sits on them?
I remember doing science experiments that showed eggs to be gas permeable. I expect the oxygen they need just diffuses in there.
The shell is very permeable. As the embryo develops in to a chick, an air sac slowly develops within the egg. When the chick is ready, it uses it’s egg tooth to release this air and begin to breath. This is when the chick begins to peep inside the shell. Cite.
Actually, I would say the the air sac is there very early.
Most eggs have a “fat/rounded” end and a pointed end; the air sac is on the fat end. Some eggs look look the same on both ends, though, and if they are incubated the wrong (non-air cell) end up, the hatchability really goes down.
Candleling (illuminating the egg with a light source) is one way to tell the corrcect orientation when placing eggs in an incubator–which is how commercial eggs are “set”.
Commercial eggs aren’t incubated by the mother hen, but in a incubator.
Mother hens don’t smother their eggs in a natural setting-- they have to eat, drink, and poop and pee–and just move around, so the eggs are moved with the hen’s movements. Incubators rotate eggs to approximate this.
DeVena gave a great link to visualize the hatching of a chick–but notice it was in artificial setting. That wasn’t the farmer’s henhouse or the free range.
Of course observing the hatching in a controlled setting is what lets us understand it. Penn State has a pretty good poultry science department going.
I have an incubator.
Its 21 inches in diameter.
It has a 6 1/2 inch water pan inside that provides enough moisture for chick hatching.
50% during incubation. I add a jar lid with water to increase moisture to 65 % during hatching.
Ducks need 65% moisture during incubation and 75% at hatch.
I spray the eggs several times a day with warm water.
Geese need 75% and 80 % at hatch. Haven’t tried hatching them yet.
justwannano–great point–humidity is a big factor in hatchability.
I worked in a college poultry science department and helped ostrich eggs hatch once. The chicks (and eggs) were huge, but they needed a lot of misting with water to hatch. Makes me wonder about the hatchablity in the wild.
Permeability, yeah. Which means it’s possible smother eggs.
In the Milwaukee area, Canada geese abound to the point of nuisancy, taking over parks and beaches.
Since protesters won’t allow the city to just shoot 'em, they go around and coat the eggs with oil, which prevents them from hatching.
The geese continue setting them, whereas if you just destroyed the eggs they’d just lay more.