The Santa Catalina Island Rattleless Rattlesnake.
In most rattlesnakes, the rattle is normally formed in segments. The first segment is attached to the base of the tail and additional segments are added when the snake sheds its skin. The degenerate nature of the rattle button in the Santa Catalina Island Rattlesnake causes the rattle segments to fall off after each shed.
It is thought that the secondary loss of the rattle is an evolutionary adaptation for hunting birds. This species has arboreal tendencies and can often be found a few feet off the ground climbing through the desert brush on the island. A fully developed rattle would make a considerable amount of noise as the snake climbed through the brush, which would scare off roosting birds. This species will often hang onto its prey, unlike most rattlesnakes. It is thought this behavioral modification is associated with the problems of tracking a bird after a bite is delivered.
http://www.sdnhm.org/fieldguide/herps/crot-cat.html