Not the most helpfull reply but close to the truth, anything a chicken makes rather than a human. That bantam hen Cheepy in the picture I posted earlier with ten chicks features in this article.
https://www.backyardchickens.com/ar...e-make-a-sitting-hens-job-so-difficult.74389/
The article also shows what is not a good hatching nest; fine for just laying an egg in though.
The hen needs to be able to form a hollow in which she lays the eggs. The hollow has a number of functions.
a) the eggs can't roll out of it.
b) a deep hollow helps maintain the temperature and humidity in the nest because the hollow sides shelter the eggs and the hen sitting on top can create a dome in effect over the eggs by her sitting position and the pancake shape they create on the nest.
c) the eggs should touch each other
d) the hollow means when the hen turns the eggs the eggs don't roll away from the other eggs and/or out to the edges of the nest.
e) the best hollows I've seen are dug into moist earth, surround by grass/vegitation.
The deepest hollow I've measured has been 70cm.
Hens do manage to hatch in all kinds of nests but in order to have the best chance of a maximum hatch rate the hen needs to be able to control the position of the eggs. This is almost impossible on a hard flat surface.
Edit,
This article goes into some depth about hatching.
https://www.scielo.br/j/rbca/a/ZFYLhJkZ8VSVpXZSJmCcKvr/?lang=en
