Depends on the voltage innit.
Sitting on a single wire at lower voltages = no problem because the bird does not form part of any circuit through which a current could flow.
Sitting on a single wire at higher voltages = capacitive current through the air plus a bit of leakage current from ionisation of the air = probably won't kill the bird but unpleasant enough that they don't seem to do it. You don't see birds sitting on the phase conductors of a supergrid line, only on lower voltage lines.
Bridging two adjacent conductors with their wings = potential electrocution. At high voltages the separation of the conductors is too big for this to hapoen, as it is for domestic (smaller) birds at lower voltages. But it can hapoen with large birds (probably in foreign parts).
Hitting a wire = obvious (mechanical) problem for the bird. Particularly with some larger water birds - swans are often mentioned though I am not an expert - their take off path is marginal to start with - they are operating close their maximum capability just to gain any height at all, they have nothing to spare for avoidance manoeuvres, so by the time they see the wire it's probably too late for them to avoid it.
Bird diverters on wires: usually only at low voltages. At high voltages, the diverters would become a source of corona (breakdown of the air under the enhanced electric field) and hence crackling. We have put orange balls or the like on the earth wire at the top but not to my knowledge on the phase conductors for high-voltage lines. Low-voltage lines - I'm betting the OP is 400 V - is no problem, the electrical stresses are much lower.
Biggest problem with birds sitting on wires or pylons: forget anything about electrical engineering, it's the accumulation of bird shoot underneath
.
