"Thanks! I do have power at the coop via an extension cord. How warm does it get inside your coop with that pet warmer under the straw?" Stinky Acres
At 13 watts it isn't designed to heat, though some warmers are larger and draw a higher wattage. (an electric blanket uses 100-200 watts). The instructions that come with these warmers say it's common to not feel any heat at 1st. If there's a pet or a pet bed on top of the warming pad, you should be able to feel a slight warmth with your hand after 20 minutes. In our house, kept about 73f, our cats get so warm laying on 'em, they get up and go elsewhere to cool down. Out in freezing weather, the warmers only cut down on the cold radiating from the ground and warms bellies

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It's one possibility: if used in an enclosed coop, if the temps don't get extreme, and if you own a small number of ducks so they all can sleep on the pad.
Great topic. Let us know if your ducks beat up that rat!