I've seen a billy collar mentioned before, I'm guessing it's one a billy has worn?
both my adults have been in season, for various reasons I've missed the chance, I think one of the yearlings has been on, but it was the same day as her mum so can't be sure, they all acted a bit silly. Billyboy is coming back tomorrow so that should help.
Never thought of lights on, I always thought it was temperature based, seasonal light levels seem obvious now you've said it.
I leave a low light on till about midnight (about 6w,other side of barn, would that affect things?) and sometimes main light is left on till about 10pm, after milking. May have a rethink on leaving lights on.