Food webs on oceanic islands are often markedly different from continental food webs due to low species diversity and absence of key components of mainland ecosystems. Food webs and feeding habits are described from observations and scat, pellet, and gut analysis on the four islands of the Revillagigedo Islands, Mexico, when in their near-original state. Changes in food habits after introduction of exotics are described. Food webs increase in complexity with increase in island size, largely as a function of increase in habitat diversity seen with an increase in island size. Seabirds are the major components of food webs on the two smaller islands, and reptiles, passerine birds, hawks, owls, parrots, and doves are more important on the two larger islands. Some island vertebrates have food habits similar to those of their mainland counterparts, but others have become food specialists (e.g., the Socorro Red-tailed Hawk, in the absence of small mammals, eats mostly lizards and land crabs). Other birds have seasonally or permanently switched food (ravens and owls on Clarión eat cactus, towhees eat insects, red-tailed hawks eat plants). Some of this food switching may be due to presence of a harsh dry season and/or to the unique fauna and flora of the islands. Insectivorous birds on Socorro apparently reduce competition by utilizing different foraging heights, habitats, and feeding methods. Sheep introduced onto Socorro in 1869, whose numbers fluctuated over the years, destroyed some vegetation, but they have now been eradicated. Exotics introduced in connection with military garrisons and an airfield may have major effects on the ecosystem of these islands in the future.