What time of year do hens go broody?

Online Answer
We say a hen has "gone broody" when something in her biological clock kicks in and she starts sitting on a nest of eggs. It usually happens in the spring or early summertime but I've had hens suddenly go broody in September. The most obvious sign of broody hen behavior is she won't get off the nest.
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12-15 eggsIf the broody is a large fowl breed, she can handle 12-15 eggs of the size she would ordinarily lay, more if they are bantam eggs. Much more about acquisition of and handling of the hatching eggs, here..
When you keep chickens, a hen's eggs may be fertilized or unfertilized. The broody hen is the name for a dedicated mother bird that has begun to sit on and incubate the eggs day and night, leaving only once daily to eat, drink, and poop. If you try to remove her from her eggs, she may hiss at you and peck you.
You can encourage a hen to go broody by leaving artificial eggs in the nest for an extended period. (You can buy artificial eggs from the Omlet Shop here.) You should place several in the nest and leave them for an extended period. Your hen might move on and off the nest for longer and longer periods..
If she's already been sitting on eggs, you can try moving them with her but I tend to have a little better success rate when I notice I have a broody hen, move her and get her settled in a chick-friendly spot, and then give her eggs to sit on. But you WILL need eggs on the destination nest to keep her interested.
The advantages of separation are to protect the broody hen and her eggs (or chicks) from the rest of the flock. A broody hen that stayed with the flock will very likely be disturbed more, at greater risk of broken eggs or an infestation and may be bullied because she has withdrawn from the flock.