Bird diverter devices were developed to improve power line visibility for birds and reduce their risk of collision. However, differences in efficacy between types of devices, and in some cases conflicting results, place in question the ability of these devices to reduce collision risk to birds. Here, we investigated the efficacy of three types of flight diverters in reducing avian collision with power lines: yellow spiral, orange spiral, and flapper, additionally we used unmarked spans as a control. We recorded bird collisions and estimated removal rates of bird casualties by scavengers in three different 400 kV transmission lines comprising 133 spans in southern Spain. A total of 131 dead birds from 32 species were found. The power line and the type of marker significantly affected avian mortality. The flapper flight diverter was responsible for a 70.2% lower mean avian mortality rate (95% Confidence Interval: 50–90%), followed by the orange spiral (mean = 43.7%, CI = 15.8–71.6%) and the yellow spiral (mean = 40.4%, CI = 2.8–78%), compared to control spans. Flappers were the only marker that showed greatest reduction in relation to non-marked spans. The flapper flight diverter showed the highest reduction in mortality and the narrowest confidence interval when tested in different environmental conditions, and thus may serve as a better alternative to the more commonly used spiral flight diverters.