Having a step parent is never easy and add the fact that he didn't respect your space makes it much worse. Here is the deal though. Unless he is hurting you, he is the one in charge because he is an adult in your home. If your mother allows him to have that position, there is nothing you can do but accept your fate at least until you can move on.
I have loved chickens all my life too but now as a grown up, I don't have to worry what anyone says (except the city of course). The city is like MY step dad cause they are always telling me what to do with my chickens. At first, we went to war with each other but after we both lost in court, I finally accepted my life and we made a compromise. Do you think you and your step dad could come to some agreement?
If you can, that would make your life easier until you are grown up!!!
Hope this helps.
Dave