Electrocution

White stork got killed by electrocution. Photo: Langgemach, LUA.

Above all, overhead power lines are a danger to birds. Every year several thousand birds die from electric shocks or collisions with power lines. Birds are electrocuted when their bodies happen to touch two areas on the power lines which have different voltages. If a bird perches on a power line, and then, almost at the same time touches another part of the power line carrying a different voltage, the current will pass through its body causing electric shock resulting in severe burns or death. Such accidents happen primarily with large birds on medium voltage power lines. 

Birds landing on power lines

Many bird species are at risk of colliding with power lines. The two dimensional way in which birds see makes it difficult for them to actually catch sight of the lines. In particular, the earth wires at the very top, hanging individually and especially thin, are a great danger when birds try to fly over the more visible bunch of conductors. Particularly nocturnal birds and migratory birds are unable to get used to the presence of individual power lines, and are therefore most at risk of collision. Specific markers attached to the earth wires make the power lines more visible for birds and can significantly reduce the collision risk.

Link:

NABU: 'Caution: Electrocution! - Suggested Practices for Bird Protection on Power Lines'