A bird perches on the high …

According to our conventions,Potential of earth is considered to be zero.When a bird perches on the live wire only,it does not get any shock,but if it on other hand touches both the live wire and neutral wire at the same time,it gets a complete shock because live wire has high potential and neutral wire has low potential due to which a potential difference is created and thus current flows giving the bird a shock.While the same case is applicable to a man standing on the ground.When a man stands on a ground(zero potential) touching the live wire(high potential),current flows through him(as human body is a good conductor of electricity) giving him a fatal shock. ••• Note that if the man too touches the live wire as the bird does(ie,not on the ground but up in the air),he won't get any shock.