This is a strange one. First thing to say is don't blame yourselves! It can't have been the cold. Cold chicks huddle and they don't jump around and interact as you have described. It can't have been a germ that you introduced. If they were healthy when you got them then it takes longer for an infection to kill them, and you would have seen signs of illness before they died. You don't say whether they were eating and drinking and poohing before they died? Did you check their backsides for 'pasty bottom'? I lost a chick at 2 weeks from this. However, it's unlikely that all 7 would die at the same time from this. If the brooder was well-ventilated then I do not believe Co2 build-up could be the problem, especially because they were in your house. There are 4 options. Either they were not drinking, or eating, and that would kill them all at the same time. Secondly, they were infected when you bought them. Thirdly, there was a pathogen in the food you gave them. Fourthly, they did not know how to drink. Chicks born out of incubators have no mother to show them. I normally 'teach' chicks out of incubator to drink. They know how to peck the crumbs, but they may need to learn where the drinker is. Half an inch of water in a shallow bowl, and stick their beaks into it. They soon get the idea.
