Despite these potential problems, our study suggests that the mortality risk for
birds was higher in railway sections crossing wetlands than elsewhere, as underlined by the concentration of bird kills in the Sado and, to a lesser extent, the São
Martinho wetlands. However, in contrast to expectations, most mortality involved
small passerines, particularly white wagtails, while there were only a few aquatic
birds killed. This may be a consequence of the high productivity of wetland
habitats, which attract large numbers of birds that are then exposed to collision with
circulating trains and railway structures during their daily movements. In the
Mediterranean region, these wetlands are particularly important in autumn and
winter, when they are used by large numbers of passerines from northern latitudes
(Finlayson et al. 1992), which may be associated with the higher mortality observed
during these seasons. Aquatic birds may be less exposed to collisions because they
fly higher than small passerines (see below), which may explain the small number
of kills recorded in our study. However, this may vary in relation to ecological
conditions and railway characteristics, because apparently larger mortalities of gulls
and waterfowl have been found in other study areas (van der Grift and Kuijsters
1998; Heske 2015).
As expected, we found a large number of aquatic birds crossing the bridge over
the Sado River, particularly from October to February, and following a path along
the river. The birds observed most frequently included a number of waterfowl,
waders and other aquatic species, which are abundant both locally and across the
estuary (Alves et al. 2011). The flight patterns taken by these birds suggest that they
were very rarely at risk of collision with circulating trains or bridge structures. We
found that only a very small proportion of crossings (%1%) were within the height
of collision risk with trains, while most birds (%52%) crossed above the bridges’
arcs. Actually, the presence of the arcs may have helped deflect birds away from the
collision risk area, as we often observed birds avoiding these by changing flight
paths. This is in line with observations showing that individual birds and bird flocks
change flight paths in response to the presence of anthropogenic structures such as
pole barriers (Zuberogoitia et al. 2015).
Overall, our study contributed to clarifying the bird mortality risk due to collisions in railways, adding to just a few previous studies addressing this issue (SCV
1996; Peña and Llama 1997; van der Grift and Kuijsters 1998; Schaub et al. 2010,
see also Chap. 2). Our results, together with those previous studies, suggest that the
risk associated with railways may be relatively low compared to other anthropogenic sources of mortality (Loss et al. 2015), although differences are difficult to
judge precisely, because of the variation in methodological approaches and ecological contexts. Nevertheless‚ it is worth noting that there were many more collisions where railways crossed wetland habitats, suggesting that the planning of
new railways should strive to avoid these habitats as much as possible. Where this
is unfeasible or undesirable, monitoring programs should be implemented to
evaluate the risks and provide information to design mitigation measures if necessary. Despite the relatively low numbers of dead aquatic birds recorded in our
study, we believe that they should not be disregarded in monitoring programs
because mortality patterns may depend on local ecological and railway
7 Bird Collisions in a Railway Crossing a Wetland …
113
birds was higher in railway sections crossing wetlands than elsewhere, as underlined by the concentration of bird kills in the Sado and, to a lesser extent, the São
Martinho wetlands. However, in contrast to expectations, most mortality involved
small passerines, particularly white wagtails, while there were only a few aquatic
birds killed. This may be a consequence of the high productivity of wetland
habitats, which attract large numbers of birds that are then exposed to collision with
circulating trains and railway structures during their daily movements. In the
Mediterranean region, these wetlands are particularly important in autumn and
winter, when they are used by large numbers of passerines from northern latitudes
(Finlayson et al. 1992), which may be associated with the higher mortality observed
during these seasons. Aquatic birds may be less exposed to collisions because they
fly higher than small passerines (see below), which may explain the small number
of kills recorded in our study. However, this may vary in relation to ecological
conditions and railway characteristics, because apparently larger mortalities of gulls
and waterfowl have been found in other study areas (van der Grift and Kuijsters
1998; Heske 2015).
As expected, we found a large number of aquatic birds crossing the bridge over
the Sado River, particularly from October to February, and following a path along
the river. The birds observed most frequently included a number of waterfowl,
waders and other aquatic species, which are abundant both locally and across the
estuary (Alves et al. 2011). The flight patterns taken by these birds suggest that they
were very rarely at risk of collision with circulating trains or bridge structures. We
found that only a very small proportion of crossings (%1%) were within the height
of collision risk with trains, while most birds (%52%) crossed above the bridges’
arcs. Actually, the presence of the arcs may have helped deflect birds away from the
collision risk area, as we often observed birds avoiding these by changing flight
paths. This is in line with observations showing that individual birds and bird flocks
change flight paths in response to the presence of anthropogenic structures such as
pole barriers (Zuberogoitia et al. 2015).
Overall, our study contributed to clarifying the bird mortality risk due to collisions in railways, adding to just a few previous studies addressing this issue (SCV
1996; Peña and Llama 1997; van der Grift and Kuijsters 1998; Schaub et al. 2010,
see also Chap. 2). Our results, together with those previous studies, suggest that the
risk associated with railways may be relatively low compared to other anthropogenic sources of mortality (Loss et al. 2015), although differences are difficult to
judge precisely, because of the variation in methodological approaches and ecological contexts. Nevertheless‚ it is worth noting that there were many more collisions where railways crossed wetland habitats, suggesting that the planning of
new railways should strive to avoid these habitats as much as possible. Where this
is unfeasible or undesirable, monitoring programs should be implemented to
evaluate the risks and provide information to design mitigation measures if necessary. Despite the relatively low numbers of dead aquatic birds recorded in our
study, we believe that they should not be disregarded in monitoring programs
because mortality patterns may depend on local ecological and railway
7 Bird Collisions in a Railway Crossing a Wetland …
113