Most dogs when out beating won't be tempted or motivated by treats, some will take the treat from you out of politeness but will spit it out.
Treats are fantastic for teaching and reinforcing recall in a park environment or similar. But when live game is involved and the dog is switched onto hunt mode, physical and verbal praise are generally received better in my experience.
Repetition of the long line method and general desensitizing the dog to rabbits running around, combined with fun dog/handler focused games and retrieves in rabbity environments are the main stays for producing steadiness in dogs.
Good luck! It does pay off
I promise you that theres nothing more wonderful than owning the only spaniel that hasn't decided to give chase to the wayward rabbit that's popped up in the middle of a big pheasant drive
Best wishes
Mary
PS: Yes the rabbits are wild rabbits. My dogs work along the dry stone walls and once they smell the rabbit sitting in the wall they stop and sit. I then remove a loose stone from the wall, pop my hand in and grab the rabbit (poachers have been doing this for years to get a rabbit meal without using a gun or other trapping method) I then place the stone back into the wall with the rabbit tucked under my arm. It probably is quite stressful for the rabbit, but I do handle them very gently and ultimately I'm using them to teach my dogs not to chase them, so in the long run I'm hopefully minimising the stress that my dogs will cause any wild game because they won't chase anything. The rabbits up on our land are all rather tame now anyway
because of the regular handling they get!!