Unlike other birds where you find partners, in chickens there is no courtship or romance. A rooster just forces himself on the hen. So what happens is when a rooster mates a hen, his injected semen is stored in numerous sperm storage tubules (SSTs) located in the area where the hen’s uterus joins the vagina. But this only happens provided the hen likes the rooster. If she doesn’t, she can squirt out the semen to avoid hatching his offspring. Therefore, a hen may still proceed to lay eggs that are not fertile despite even mating with a rooster every day.

This podcast is also available as a blog post here: https://zaszambia.wordpress.com/2022/02/04/interesting-facts-of-chicken-egg-fertility-that-you-may-not-know/