Also whichever technique you use, make sure that the nib isn't frozen. I am always concious that the bubblewrap/sock technique etc can keep the water in the bottle liquid, but if ice crystals form around the ball at the bottom of the spout, the water still may not come out. Apparently the bottle snugs are meant to prevent this as well - my sale ones haven't arrived yet so I can't comment!
The other thing you can do is provide a bowl of water instead of or as well as a bottle, as it will freeze more slowly because it's inside the hutch/shed etc with the bunny heat, and bunny can still lick the top more easily than they can from a bottle. I don't do this with H&H though, as I've yet to find a bowl sturdy/heavy enough to stop them from picking it up and flipping it around :roll:
