Background Power lines are amongst the main causes of mortality for birds globally. Electrocution drives the population dynamics of several threatened species of raptors, at local and global scales. Among the many solutions that have been tested to minimize this threat are supplemental perches; however, their efficiency has rarely been assessed.

Methods We designed 43 transects in 4 districts in mainland Portugal to gather data from birds perching on pylons with or without supplemental perches. From 2015 to 2018, transects were surveyed by car at least once. We analyzed the factors driving the use of these supplemental perches, and we analyzed if there were differences in the perceived risk (calculated from measurements and not from field surveys) depending on whether the perching was on pylons with or without supplemental perches.

Results We recorded 548 perches of 14 species. Weather conditions seemed to play a role in birds' choice of pylons with supplemental perches versus pylons without supplemental perches. Models also indicated a strong influence of observational conditions. The use of models showed us an important specific effect in the selection of supplemental perches where available: there are some species with a greater tendency to perch on supplemental perches, even when they use both pylons with and without supplemental perches. For most of the analyzed species and species groups, perceived risk was higher in pylons without supplemental perches compared to pylons with supplemental perches, but there were differences between species.

Conclusions Supplemental perches may be a useful and efficient tool for mitigating raptor electrocution. However, there are many influential factors affecting their success, and their effectiveness with different species groups is not homogeneous. Some studies show higher electrocution rates for certain species and devices and thus, their efficacy must be validated. We propose a two-step validation process, first in controlled conditions and then in the field. A common protocol should be established to enable comparisons between studies.