Occasionally, we use a thermal imager to look for wildlife in the garden at night. The imager uses sensors that detect heat radiation (in the electromagnetic spectrum) that allows the operator see in the dark. It is a remarkable device. Mammals can be detected from great distances as they radiate a lot of heat. Birds on the other hand are trickier because they are so well insulated.
So much heat is radiating off of a sleeping Fox Squirrel that you can detect it through the walls of this Barn Owl box.
Preening Great Horned Owl