when d bird sits on d wire how does d body potential of d bird rises and becomrs equal to d wire??????????

Good question.

When the bird sits on a wire, the potential difference between it's feet is very little because of the low resistance of the current carrying wires. So, due to this low potential difference(0.011 V in this case), the bird will be a high resistance connected in parallel across the wire (between A and B). A very low current will pass through the bird which would have negligible effect on it.

But, if the bird is connected between the wire and the ground (0 potential) the potential difference between the feet of the bird and the ground would be very high (1000-0 = 1000 V). this will cause a large current to flow across the body of the bird and would prove to be fatal.

I hope this helps!