One of mine first went broody after a nice warm day in the yard (above zero) after 2 months of being cooped up due to snow on the ground and -10 to -25 every day!!!!
Im sure she got a "cue" that is was spring and two days later she went broody. Had to put her in the broody buster (didnt have a rooster so no fertilized eggs).... in February I got a rooster. In early March we had a few days of very warm weather (14 C ) thats like 60 i think..... so I let them free range all day hoping she would go broody. Sure enough - a week later she was broody! She is currently sitting on a clutch of 16 eggs- I candelled today (day 8) and all going strong!
I think I got lucky because she is an EE, which I don't think are known to be broody.
My Orpington hasn't gone broody yet, and they are supposedly more prone to it...so I am hoping for her to be next!