Warning markers, covers and blankets are usually Hi-Visibility ideal for visual identification of low hanging electrical conductors, overhead wires and cables. Used in construction zones to alert drivers of machinery (tippers, cranes, trucks, etc) or low flying agricultural aircraft of cabling. They also can deter birds and wildlife. Various models are available as stand alone rotating 3D units with reflectors, flags with reflectors, covers (also known as Torapoli Pipe or Tiger Tails) come in yellow/black or red/black stripes - these can be electrically tested for low voltage brush contact protection, and lastly insulating blankets and covers - usually yellow/grey that drape over the lines again these can be electrically tested for live working operations.
Alternate terminology: Powerline Warning Marker, Overhead Cable Flag, Bird Diverters.