Collisions and electrocutions on power lines are known to kill large numbers of birds annually on a global scale. We conducted comprehensive research focused on bird mortality caused by 22 kV and 110 kV distribution power lines in 13 Special Protection Areas in Slovakia. In the period between December 2014 and February 2016, 6,235 km of power lines were inspected twice during two periods (12/2014–03/2015 and 04/2015–02/2016) of field survey. In addition an intensive study was conducted during the second field survey at one-month intervals on power lines identified as the most dangerous for birds to collide with. As a result, 4,353 bird carcasses and bird remains representing 84 bird species and 14 orders were identified. Electrocution was suspected for 76.72% and collision for 23.28% of fatalities. Raptors were associated with 40% of all identified victims of electrocution. Two peaks of incidence were recorded, the first in March with a high rate of electrocutions as well as collisions of swans, pheasants, common blackbirds, ducks and herons, and the second in September predominantly featuring electrocution of raptors, magpies and corvids. We were unable to quantify seasonal patterns of mortality due to the limited sample of repeated mortality surveys resulting from the large grid of inspected power lines. We conducted comprehensive statistical analysis of more than 100 configurations of pylons and calculated their potential risk towards birds. Strong spatial correlation was revealed in the data set. Metal branch pylons and corner pylons with exposed jumper wires passing over the supporting insulators above the cross arms were the most dangerous configuration, accounting for 34.72% of total recorded electrocution fatalities (0.13 carcass/pylon). Cases of electrocution were also recorded for two bird species of major conservation concern in Slovakia: saker falcon (Falco cherrug) and eastern imperial eagle (Aquila heliaca). The results of this study may substantially improve conservation management and policies needed to reduce bird mortality.