The voltage that kills instantly is any voltage above 250 volts.
However even 100 volts to 250 volts can kill you near instantly if you're unable to let go.
If you were to touch a live wire at 7,000 volts or above such as a power line that is on the pole by your house then you would most likely be killed instantly or at least near instantly and not be able to let go.
Death can occur from voltages as low as 40 volts as well and any voltage over 50 volt AC or DC can be dangerous.
The amount of volts it takes to feel a shock is around 30 volts or higher.
Although if you stick your tongue to a 9 volt battery you can feel a shock from the 9 volts because of your tongue being wet.
You can't touch the wires in your 120V AC household electrical system without getting a nasty, dangerous shock, but you can touch bare wires carrying 12V DC in your car, even lay your hands across the positive and negative 12V battery terminals, without risk of electrocution.
The voltage coming into your home is 120 volts on each line or 240 volts for both lines.
The outlet voltage in a home is 12 volts and will certainly feel a shock from those outlets at 120 volts and it can even be deadly.
240 volts is even more deadly and the power line wires carrying electricity to the transformer on the poles carries around 7,200 volts which is even more deadly.
Birds don't get electrocuted on power lines because they are only in contact with the hot wire and not making contact with the neutral wire to ground themselves out.
For electricity to travel through the birds body the electricity needs a path to ground and since birds are sitting on only one wire even when it's a hot wire with electricity flowing through the bird is isolated and insulated from the ground.
If the bird were to sit on the hot electric wire and then make contact with the neutral or ground wire then they would be electrocuted.
The same would be true for a lineman who would be able to touch the hot wire while being insulated from the ground.
In order for a human to be electrocuted they have to also make contact with both the hot wire and the ground wire or the ground.
Lineman work on hot and live wires all the time although they also insulate the wires as much as possible in most cases and wear protective gloves to prevent them from potentially coming into contact with other wires that could cause them to get electrocuted.
Even squirrels that would walk across live electric wires do not get electrocuted because they are not touching the ground.
For electricity to flow and create a circuit it needs another wire which is the neutral to complete the circuit.
When a human gets electrocuted they are in contact with the ground and the electricity is seeking the path to the ground in the quickest way.
The body is a quick path to the ground and your body conducts electricity.
If the bird were on the ground and made contact with the live hot wire then they would also be electrocuted.
But because they are up on the wire and off the ground the bird is safe from electrocution.