Cervical dislocation (pulling the neck to sever the spine) causes death from cerebral anoxia (interruption of the supply of oxygen to the brain) due to the cessation of breathing and/or blood supply to the brain though haemorrhages from damaged blood vessels in the neck. If there are only a small number of birds to be killed (i.e. up to 70 a day to prevent operator fatigue), and other methods of killing are not available, conscious birds can be killed using cervical dislocation but this is not recommended. The HSA strongly advise this method only for killing stunned birds. Birds should be monitored continuously until death to ensure the absence of brainstem reflexes. The killing of birds can be performed either manually (using own hands) if birds weigh less than 3 kg, or mechanically (assistance from a device) if birds are between 3 and 5 kg. The neck should never be crushed using equipment such as pliers as this does not provide a fast and humane death. Performing this method effectively and consistently requires strength and skill, so team members should be rested regularly to prevent fatigue. As this method does not guarantee immediate insensibility, it should be used in conjunction with a stunning method such as electrical stunning. As with the use of the non-penetrative captive-bolt, an effective kill will most likely result in involuntary wing flapping.