Various bird species use power line structures as perching or roosting place during the day & at night as they are often higher and more suitable than the surrounding natural vegetation In the rainy day, the branches used for nesting may meet the lead wire and cause earthing accident.
If the birds sit directly above the live conductors there is a chance that their excreta will fall directly onto the conductors and insulators. This excreta can build up over time and compromise the insulation, leading to electrical faults.
Some species have a watery excreta which is ejected in long conductive streams called streamers & Streamers can bridge the air gap between the earthed and live components which may cause a fault.
Short circuit and electrical tripping due to perching on the electrical installation may cause abrupt electrical failure hence lead to halting of trains, disruption on the traffic signals, hospital operations and various sensitive installations. This may lead to major accident and disasters.
SOLUTION
The best way to solve this problem is to prevent the birds from perching in the critical areas on the pylon or tower structure. This is known as "Perch management" and is done with the use of BIRDSCAREŽ which are plastic spikes that block birds from perching. The aim is not to prevent the birds from using the particular tower altogether, as this may just shift the birds to the next tower and displace the problem. Sufficient perching space should be left for the birds.
Various bird species use power line structures as perching or roosting place during the day & at night as they are often higher and more suitable than the surrounding natural vegetation In the rainy day, the branches used for nesting may meet the lead wire and cause earthing accident. 
