baby chickwith large bubble on neck

[thomas mynhier]

I have a 2.5 month old chick and noticed today that she has a large bubble on her chest. She is eating fine and doesn't seem to be bothered with it. She has not gained an ounce and her sisters have tripled in size. What is this and what can I do to help her.

 

[hrhta812]

Can you post a photo, please? Hope chick is still doing okay...