Hi! I have three flying mallard ducklings, about 4 weeks old now! They have been outside during the day time since about week 3, but come in at night. I am curious on when can these guys stay out permanently?! Our weather is kind of wacky, they have been off of their heat lamp since around 2 1/2 - 3 weeks, and they have feathers on their tummies. Thoughts?!