I suppose you could take the coward's way out and write her a letter or email with a few pictures of people holding cute Silkie hens attached. Telling her in person may be more responsible, but if she reads it in a nice letter, that will spare you from the initial explosion of temper. I can guarantee that if she finds the chick before you tell her there will be screaming!
Try this opener on for size:
"Mom, I must have inherited your care taking instinct, because I just couldn't resist getting a little pet. I know I should have told you earlier, but I was scared that you would be mad at me."
By the way, when a mom raises her voice in anger, it is usually best just to admit you were wrong to diffuse the anger some. "I'm sorry, Mom, I should have told you. I can see why you are upset with me."
Make sure when you tell her, that the chick's enclosure is super clean and there is no smell in the room. Also, do your best to keep up on all your other responsibilities around the home and at school. Your mom probably has a short fuse partly because she is under a lot of pressure and has a lot of responsibilities including having a baby to take care of.