How old is the feed? After six months, I wouldn't bother using it for anything but fertiliser or pig feed. (Think graham crackers. Now think stale graham crackers.)
My sister's rabbit feed did compare pretty favorably with the layer feed when I looked once, but you'd definitely need to supplement calcium. (oyster shells free-choice is the preferred method, I think.)
Pull up the feed tag on your rabbit feed and compare it with the nutrient tag on your chicken feed, but I think you'd be alright. Protein's about the same, rabbit calcium is definitely low, and rabbit feed fat is a little high, in most feeds. Sodium is really high, comparatively, in rabbit feed.
You might mix it, rather than feeding it outright. Most rabbit pellets are alfalfa/soy/legume based and will give chicken yolks a nice, deep orange color. Still, chicken feed is the ideal.
EDT: Definitely do not feed rabbit feed to chicks!