The scarlet smear is no ornament. The stain flashes against pale down. It is a crop secretion, pushed from the upper digestive tract of both parents into a chick's bill, long before that chick can gather enough food for itself.This is biology's blunt workaround. Mammary glands are unnecessary because prolactin drives the crop epithelium to proliferate and shed nutrient-filled cells, forming a dens...More