I read somewhere that rabbits need to chew on hay as it gives them fiber to keep their GI tract moving properly. I'm a noob to rabbits as well as to chickens,I've had my rabbits maybe 4 months now. i feed them everythign and anything. I live in the Forest so theres all kinds of wild stuff going right now. my bunnies and chickens both LOVE fresh new growth wild grape vines/leaves. i mean love them lol. i have feed my chickens kale, grape leaves, collard greens, dog food, lizards crickets. earth worms. i rake up leaves and thro a big pile in there and they go crazy digging thru there eating bugs etc...
as for both rabbits and chickens i go with what someone else said, if they are out freeranging and find a citrus they probably gonna eat it lol.
so as for the rabbits i always keep food (pellets) in their feeders, i throw some coastal hay in about every 3 days for them. and usually give them some type of green daily.
have been giveing my chickens greens on a daily basis too