Jumping is a natural part of chicken behavior. In the wild, they might jump to get a bug, etc., but domesticated chickens have been bred to be heavier, which means jumping can put too much strain on their legs. Jumping needs to be done in moderation. Don't put things too high or make them do it too often. Understand your chicken's breed as well. Lighter breeds and banty chickens are safer to jump than heavier breeds such as Orphingtons or meat birds. Just as you could jump a little to grab an apple off of a tree and would be fine, but doing a high jump could hurt you, chickens can jump for treats but only in moderation. Keep their body type in mind.