For Birds in captivity, solely sure species of birds require grit of their food regimen for digestion. The use of gizzard stones seems to be a similarity between birds and dinosaurs, based mostly on trace fossils of gizzard stones referred to as gastroliths. Birds vary in measurement from the tiny hummingbirds to the large ostrich and emu. Depending on taxonomic viewpoint, there are about 8,800 to 10,200 living fowl species on the planet, making them probably the most various class of terrestrial vertebrates. Linnaeus, 1758OrdersMany – see part below.Birds are bipedal, warm-blooded, oviparous (egg-laying) vertebrates characterized primarily by feathers, forelimbs modified as wings, and a bony beak with out tooth.