Cicadas call from varying heights on trees. Where multiple species occur, the species may use different heights and timing of calling. While the vast majority of cicadas call from above the ground, two Californian species, "Okanagana pallidula" and "O. vanduzeei" are known to call from hollows made at the base of the tree below the ground level. The adaptive significance is unclear as the calls are not amplified or modified by the burrow structure but it is thought that this may avoid predation.