It may not be necessary, but, I installed 2 plant seedling heat mats under my duck’s nest boxes. The heat mats are attached to a timer which turns them on at dusk and off at dawn. The boxes, which are food storage tubs, are filled with pine shavings and hay. I have used this setup for the past two years and I get clean, non frozen eggs. I thought I would share the idea as I got it from a post.