This is a very interesting observation ans frequently asked by many persons.
To understand the answer of your question you will have to know one basic concept of Electrical Engineering which I have explained below:
There are two types electrical power generation and supply through metal wires.
One of the types of electrical power is Alternating Current(A.C) and the other is Direct Current(D.C).
In A.C power is distributed to consumer points through 4 numbers of distribution wires out of which 3 numbers of wire carry 3 phase current and the 4th one is neutral wire. All the phase and neutral wires have specified clearances between them to avoid short circuits between phase and neutral wires as the phase wires are in high voltages.
Similarly in D.C power system the power is transmitted and distributed through 2 wires i.e positive and negative. The positive wire is in high voltage and negative wire is earth at the supply point and both are kept with specified clearnce to avoid short circuit.
When a bird seats in a single wire it does not come in contact with other wires, therefore short circuit between wires through the bird can occur.
At the moment of flying if it touches adjacent wire the bird will be electrocuted immediately as the electrical circuit is formed through bird.
This is a very good question and the answer is simple. Electric shock can occur only if a flow of current exists through the body. For current to flow a potential difference has to exist between two points along with a path between them.
When a bird is sitting on the wire, its feet are resting on the wire and its head is in the air. Air cannot conduct electricity and hence there is no closed path being formed through the bird. Also since air is not a neutral point, there is no potential difference between the bird's feet and head. As such current cannot flow through it and it doesn't suffer from a shock.
For the same reason we are asked to wear slippers while handling electrical equipment. Slippers are normally made of an isulating material and cannot conduct current. When you wear slippers and touch any electrical point like a plug, the slippers are preventing a closed path from being formed and thus blocking the flow of current through us.
On the other hand, if we touch the plug with bare feet, our feet are in contact with the ground which is at 0 potential whereas the plug has some potential. This creates potential difference and since our body doesnot oppose current flow through it, a closed path is also formed. Thus we suffer from electric shock.
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due to this reasonbirds may sit on the wires of power lines and be safe. but every time they will not be so lucky if they touch the ground while touching power line. in this condition ,A bird, or any living thing, which touches a wire and also touches the ground creates a path to allow travelling of electricity through that body and into the ground (with no voltage).Whenever it occurs the bird can die.
The birds do not get the current because:
1] The flow of current is complete when the positive & negative meets,either in the way of passing the current from one wire to the another or from positive wire to the another terminal[earth].
2] The bird sitting on one wire,could not get the shock,due to incomplete circuit,while during flying,if the wings are touched to another wire,the bird get the electric shock,resulting to death.
why don't the air in which it hangs act as a conductor?
Air in contrary to earth (good conductor and a pool of +ve and -ve charges) is a very bad conductor of electricity. It is termed as the perfect dielectric and the dielectric constant of air is one. It is taken as the reference value.
This dielectric property also explains why earth and the things on earth can sustain so heavy mega voltage lightnings and thunders. It is said that one lightning can provide the electricity for the world for few hours.
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Birds are small species. Also have you seen any pattern in their sitting and flowing away. They don't touch 2 wires at a time if they do it then they will get electrocuted. Also the scaly skin of the bird provide it sheathing effect making its skin a bad conductor of electricity. Also did you notice that they don't hang with 1 leg on the wire they will clasp both the legs on the wire at a time and while flowing away they do the same. If they do not do this then there is acute chance of getting electrocuted. Also birds have sense of magnetic and seismic activity. So whenever a bird flies to a electric pole it must be sure that the wire on which it is sitting is not a live wire . They sense the magnetic field around the live wire hence preventing then getting electrocuted.
One more point is the legs will not be spaced much from the each other and body so a low resistance would not affect the flow of current when the legs are at a distant from each other and the possible high resistance path makes them get electrocuted.
Notice this. A wet bird would never sit on a electric wire wetness makes the resistance to increase hence opposing the current flow. Thus making the bird get electrocuted.
Also see here air is a conductor of electric charges. But the dielectric constant of air is less. As moistness increases the dielectric constant also increases. Hence moist air acts as conductor. But this concept has nothing to do with bird getting electrocuted.