Running trains kill birds, and this problem is foreseeable larger in high speed railways (HSRs). However, HSRs have been planned and built over years with scant knowledge of this impact, leading Environmental Impact Assessment and mitigation to be backed mainly on intuition or soft data. A special focus has been devoted to viaducts in valleys, where bird protection barriers are prescribed assuming without formal evaluation their high risk nature due to corridor effects and the elevated position of the railway there. Better knowledge of bird behavior around railways is thus needed to inform the environmental decisions for the expected increase in the extent and traffic of railways. In this context, we present results from a two-year study of bird flight patterns over an 8-km stretch and three viaducts (656m, 334m and 460m length) in the HSR line around León (NW Spain).The study is based on 10’ bird observation stations (N=2,342) devoted to the census of flying birds across 120m stretches of the railway (each flock or solitary bird denoted as a ‘crossing’). Since train-kill may happen when birds fly across the gap between the rail and the catenary, total frequency of bird crossings and the percentage of crossings through the risk area where our target variables. Our results show that bird crossing of the railway is frequent and rather dependent on very local conditions. Thus, in the study area bird flight across the viaduct over the Esla River (average 47.0 crossings/kmh) was less frequent than over other adjacent sections (69.6 crossing/kmh); and both these features much larger than those observed in the two viaducts over the Bernesga River (18.7 and 16.5 crossings/km*h) in the same railway. Regarding the proportion of flights across the risk area, it was higher where the railway runs over embankments (45.5%) than in any viaduct. The percentages of risk crossings were rather similar among these, though a bit lower (23.7%) in the one protected with 2m opaque screens than in the unprotected ones (32.2% and 27.5% respectively). Several common species (e.g. Columba palumbus, Fringillidae, Hirundinidae), but also others
of more conservation concern (e.g. Milvusmigrans, Circus aeruginosus, Buteo buteo), were found to cross frequently through the risk area in both the viaducts (with or without barriers) and the flat sections of the railway. We conclude that (i) many birds cross routinely the railway under the risk of being killed and (ii) this situation is very dependent on local conditions, forcing a detailed analysis of future HSR lines during planning and the development of extensive bird studies in order to properly evaluate the environmental impacts and locate and design the mitigation measures. Moreover, (iii) viaducts may not be always the most risky sites for birds, and (iv) bird protection screens should be taller than those routinely used for noise protection, since they do not dissuade birds to cross the gap between the rail and the catenary. Extensive basic studies like this one are also needed due to the huge gap of knowledge existent on railway effects on wildlife.
