Florida cottonmouth snakes (Agkistrodon piscivorus conanti) inhabit insular as well as mainland habitat, and they occur in particularly large numbers on some of the Gulf coast islands of Florida. The success of this species on islands is attributable, in part, to a broad trophic niche that encompasses many prey items including carrion. Numerous cottonmouths living on the island of Seahorse Key consume primarily dead fish that are dropped from colonial nesting bird rookeries, but they also prey on rats (Rattus rattus) that are invasive fauna on the island. While alternative prey items (e.g. lizards) are available to newborn snakes, smaller individuals also scavenge for fish carrion at early ages. Field observations indicate there is social structure during foraging events that involve multiple snakes. Occasionally, insular cottonmouths scavenge for intertidal carrion and detritus that washes onto beaches. Numerous and diverse objects of appropriate size, shape, and odor are ingested by foraging snakes, including masses of marine algae that bear fish odors. The intensive scavenging behaviors of cottonmouths appear to reflect behavioral and physiological specializations that have evolved in response to insular resource limitations. Two important pathways by which allochthonous marine productivity enters the terrestrial food web to support these insular cottonmouths are (1) colonial bird rookeries and (2) shore drift of carrion and detritus. The importance of the bird rookeries as a source of marine fish is reflected in the observation that far lower numbers of cottonmouths occur on islands that lack a bird colony. We have no evidence that the insular cottonmouths feed on the colonial nesting birds, but they consume smaller passerine species.