LG8 It's easier to go from jacks to bunnies than the other way around.
Also reasonably obvious once you think about it.
(Years later): Well, I take it back. Sort of. There is much to be said for training your hawk on cottontail to learn good footwork and gain confidence in rabbit-sized quarry. At the same time, redtails are pretty good at geometry. They know far better than eyases how big things are, or aren't, and given a choice will likely pass up a jack for something that doesn't fight as hard. So a balance is in order.
A friend of mine (Ed Ng) once recommended, "If you're going to bag your bird on a dead jackrabbit - use just the head." In other words, don't let your bird know that there are any other parts, the head is the only thing they should be concerned about. You don't want your bird grabbing any other part, or thinking that the back end is as legitimate as the front.