Prairie Rattlesnake (Crotalus viridis) Venomous

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Description: Pit on each side of head between and slightly below the eye and nostril; large rattle on tail; small scales covering most of top of head with one large scale over each eye; pattern of 30–55 dark gray or brown blotches on the back; dark bands on the tail. Head, body and tail are greenish gray to brown; tail bands similar in color to body blotches. Belly grayish or white. Young look like miniature adults.
Habitat: Prefers rocky canyons and open prairies with an abundance of small mammal burrows. Suns on southfacing hillsides with large rocks in spring and fall; roams open prairies in summer, when it is often nocturnal.
Diet: Feeds on lizards, rats, mice, gophers and young prairie dogs.
Size: Length up to 56 inches
Natural History: Females produce litters every other year; 5-18 young per litter, born in spring, summer or fall. Prairie rattlesnakes are very common found over most of Colorado with an elevation range up to 9500 feet.
http://www.coloradoherping.com/species-guide.html