Gils done it to strangersEver had one try to run up your arm yet? That's a mating thing too. One of my runners was doing that to me yesterday.
Head bob, then just a full scale up their whole arm. We just let them think she was trying to 'fight them for dominance'
She bobs her head at me if she's happy or super excited but rarely flirts with her human parents. If she does bob her head in a flirty way we know because she tries to bite down on us and we just push her off ad ignore her. She HATES being ignored so she learned not to flirt with us. Strangers however, she is pretty behaved with but just learning the last few manners 
So @2 many chickens yes, I think it can be an excitement thing although it usually is a mating thing. If they're looking at each other, they're flirting. If they're head bobbing at you they're probably just trying to look as cute as possible so you'll give them all that food you walked out with!
(Gil is an indoor pet duck)
