My sister raised jungle fowl, the precursor of our chickens. They were seasonal egg layers. There are egg breeds, meat breeds, show breeds, dual purpose breeds, etc. Kind of like dogs.
At the end of "Pottenger's Cats" by Pottenger Jr. he suggests eating "fertile eggs of hatching quality because they contain natural estrogenic substances." Pottenger did a ten year study on cats that showed cats fed cooked meat and pasteurized milk were destroyed physically, could no longer breed, and died early of all kinds of diseases by the third generation.
Just ask the people you buy your eggs from if they have a rooster with the chickens. Yes they should be fertile, no they are not. Roosters are so noisy if your source is near a city they probably don't have one.
I raised ducks for eggs. They are richer than hen eggs. A Khaki Campbell ducks lay more than chickens in warm climates. Drakes don't crow.
The climate will effect the laying of birds. They get stressed out in extreme heat and cold. Egg factories are in the south. I was in one once. The chickens were in little cages with only enough room to stand up, two to a cage. The eggs roll down to a conveyor belt. The food comes in on a conveyor. They never see the light of day. The farmer has calculated just how little feed he can use and still get an egg. The chickens only last about a year in these conditions. The yokes are pale yellow, flat, and break easily. You can tell a free range egg by the orange yoke that stands up.
I eat egg nog every morning. 2 raw free range eggs whipped with raw milk yogurt. Cocoa powder and ground flax makes it more nutritious.