Calcium

George Fordyce, in his 1791 book "A Treatise on the Digestion of Food" observed that chickens were able to balance their nutritional needs, carefully choosing which kinds of stone to ingest alongside grains to aid in their grinding, and that they also swallowed different kinds of earth "for other purposes" (pages 24-26). He pointed out that when laying eggs, hens required "a quantity of calcerous earth, otherwise she is frequently killed by the eggs not passing forward properly", showing that if he kept one set of hens with any kind of old mortar (source of calcium), and another set without any source of calcium, then "many of these died, while the others, although otherwise exactly in the same circumstances, were none of them lost".

Bizarrely, on page 80, he comes to the conclusion that goldfishes can live on air and water, but that's another story.