One of my chickens has a large soft swollen 'sac' above his breast. I've never seen this before and am not sure what to do about it. I'm afraid he may have eaten something bad for him that blocked him up and seems like all his food is just sitting in this 'sac'??? Does that make sense? He's still eating everything in site and acting normal but this big sac hangs down from his neck/breast like a water balloon. One of the kids took a styrofoam cooler outside and the chickens ATE IT before I realized it was even out there. Could this have damaged my chicken to cause this sort of issue? I'm at a complete loss as to what to do.