We had 3 bantam chicks born Thurs/Fri. Were doing fine with momma. These are the first chicks born here. Yesterday one had a pasty butt so I cleaned it off carefully and put back with mom. Went out a few minutes ago and the little thing was dead. Another little one has same thing. Used warm water and it cleaned up easy. I put this one back with mom. Questions: What is it ? What causes it? How do I keep from losing the other two chicks?