Science seeks to explain the natural world and its explanations are tested using evidence from the natural world. Birds and lizards are known to exist in nature and therefore fall within the scope of science. Elves and gnomes are great fun to read about and even to enjoy as statues in our gardens, but they do not dwell in the natural world. That means they are not appropriate for scientific study. The basis of any scientific understanding is information gleaned from observations of nature.
Science assumes that we can learn about the natural world by gathering evidence through our senses and extensions of our senses. A flower or a rock can be directly observed with no special aids. But using technology, we can expand the realm of human senses to observe such invisible phenomena as electricity and magnetic fields, and objects such as bacteria and faraway galaxies. Dreams, apparitions and hallucinations, on the other hand, may seem real but they do not arise from our senses and are not even extensions of our senses. The ultimate test of any conceptual understanding exists only in real materials and observations. Evidence is the basic stuff of science. Without evidence there is only speculation.