Carrion tends to be an unreliable resource in any one location or point in time. This has hindered the evolution of strict or obligate scavenging behavior, except in a few notable species like vultures or certain flying insects. Vultures in particular have several traits that facilitate specialization as scavengers. First, they have large broad wings that enable them to expend minimal energy by soaring over vast areas to locate carrion. They have sharp eyesight and a keen sense of smell. They also are able to consume carcasses very rapidly once they have been discovered. Even so, they represent a very small faction of the range of species that scavenge.
Most scavenging is facultative; in essence a dietary supplement. It is undertaken by a broad range of species. Most notably carnivores of all stripes, including the majestic birds of prey such as eagles, hawks and falcons but also other birds such as ravens and magpies; canid, felid, ursid, and hyenid mammal predators; snakes, lizards, and spiders all consume fresh carrion when it is found. After all, it does not make evolutionary sense (in terms of improving individual survival and reproduction) to pass up a free meal, one that effectively differs little from a hunted prey item, whenever it is encountered. The proportion of the diet that comes from carrion, however, can vary widely among carnivore species, making some species like hyenas and ravens seem close to being obligate in their scavenging.
Many seemingly unlikely species such as herbivores also scavenge. For example, on islands in Lake Michigan, white-tailed deer consume large quantities of dead alewives, herring-like freshwater fish that undergo annual mass die offs and wash onto shore in spring. It has been estimated that alewives comprise 30-54% of the daily diet of individual deer during the spring period. Such purposeful scavenging appears to provide the deer with an important dietary supplement during a period when terrestrial food resources are in critically low supply after lengthy winter browsing. Alewives have higher protein, fat, energy, and mineral (especially salt) contents and are more easily digested than the heavily browsed plants on the islands. Other herbivores including grasshoppers and hippopotamus also readily engage in scavenging.
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