Yorkshire pudding is made with eggs, flour and milk. The batter is whipped or blended and placed on greased pans that many years ago were lined with suet. Suet is beef or mutton fat that collects around the loins and kidneys.
My Dad was a butcher, and in January Mom would request portions of suet for birds when the temperatures plummeted.
Due to its high energy content, suet was also used by cold weather explorers to supplement high daily energy requirements needed in extreme cold. Often suet was added to food rations to assist dog-sled traveling.
When my grandfather laced the suet to a Maple tree, woodpeckers, cardinals, wrens and starlings invaded the mesh. Neighborhood dogs enjoyed it too if winter storm winds blew it from the tree.