The process of transition from nesting on trees to electricity poles has been observed over the last decades in many European populations of the White Stork Ciconia ciconia, but the direct mechanisms behind this process have not been explicitly identified. The aim of this study was to identify selective forces responsible for transition to nesting on electricity poles using long-term data (1994–2011) collected for the central Polish population of c. 190 breeding pairs. We hypothesized that transition to nesting on electricity poles could be explained by two non-exclusive mechanisms. Firstly, the process could be driven by insufficient availability of traditional nesting sites in the vicinity of favourable foraging grounds. Secondly, nesting on electricity poles could be directly associated with fitness benefits resulting from different nest structure and microclimate. We found that the process of transition had started and proceeded at the highest rate in the good- and medium-quality areas of river valleys, where most intense competition for nesting territories and highest stork densities were recorded. By contrast, we found little support for direct fitness benefits associated with nesting on electricity poles. In fact, reproductive success (number of fledglings per pair) of storks nesting on poles was lower in comparison to those nesting on trees, but this relationship prevailed only in the areas of poor quality (dry agricultural landscape). The results strongly suggest that the process of transition to nesting on electricity poles in the White Stork is not facilitated by direct reproductive benefits, but could be driven by decreasing availability of natural nesting sites in the attractive breeding areas.
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Vol. 103 • No. 1