Do you have 11-- 1 week old ducklings?
Or theya re 11 weeks old?
Ducklings can go outside easily with no heat by 4-5 weeks old. Some larger breeds even earlier.
They do very well if there is enough of them to keep each other warm.
I have never heard of waiting till 15 weeks old---- im guessing thay was a typo?
My ducklings last spring went out at 3 weeks old. And nights dipped into low 30s and they did great. There were 6 of them. And they were pekins, swedish, buffs and runners.
My 2 and 3 week old ducklings are being brooded in the outdoor coop right now and the heated corner is 70-75 and they sleep on the outside edges of it. And play all over and never act cold. And its still getting into the 30s at night here .