Ice Up Close

I had to hack out the ice so that the chickens could drink. I need to find a way of stopping their water from freezing. I'm not a chemist but I'm wondering whether adding a bit of vinegar might lower the point of freezing without harming the hens?

And speaking of ice - DotBag headed off to archery this evening with Explorers (older scouts). She was outdoors for 2 hours and wore a t-shirt and a fashion coat. She declared emphatically that it's just not cold out there. ???