Hi, I have 4 3-week old runner ducks, they currently live in a dog cage in the house. On days when I am home and the weather is ok the tray comes out heir cage, it becomes a run and they spend the day outside on the grass (with their heat plate). What age can they go outside permanently -assuming they still have a heat plate and a secure house attached to a secure run? I have read 10 - 12 weeks, but is that without a heat source?