Hematophagy (sometimes spelled haematophagy or hematophagia) is the practice of certain animals of feeding on blood (from the Greek words αἷμα haima "blood" and φάγειν phagein "to eat"). Since blood is a fluid tissue rich in nutritious proteins and lipids that can be taken without enormous effort, hematophagy has evolved as a preferred form of feeding in many small animals such as worms and arthropods. Some intestinal nematodes, such as Ancylostomids, feed on blood extracted from the capillaries of the gut and about 75% of all species of leeches (e.g. Hirudo medicinalis), a free-living worm, are hematophagous. Some fish, such as lampreys and Candirus and mammals, especially the vampire bats, and birds, such as the vampire finches, Hood Mockingbirds, and oxpeckers, also practise hematophagy.