catenary lines, where there is a risk of collision with trains (Fig. 7.3). Although
most birds crossed the bridge above the train collision risk zone (91.3%), 39.1% of
the crossings were recorded below the top of the bridge arcs (Fig. 7.3).
Most aquatic birds observed crossing the bridge were gulls (lesser black-backed
gull and black-headed gull (Chroicocephalus ridibundus), greater flamingos and
glossy ibis (Plegadis falcinellus) (Table 7.2). The proportion of crossings within
the train collision risk height were low ( 3%) for most species, with relatively
high values (>10%) recorded only for dunlin (Calidris alpine) and black stork (C.
nigra) (Fig. 7.4; Table 7.2). In the case of black storks, however, this value is based
on just two individuals observed flying at a low height.
Discussion
Our results suggest that bird mortality in the “Variante de Alcácer” railway was
relatively low during the study period, although there were sharp peaks in the two
sectors where it crossed wetland habitats. Nevertheless, there were very few aquatic
birds found dead, suggesting that they were not particularly susceptible to collisions
with circulating trains or the railway structures. This was in line with the observations of aquatic birds flying across the railway in the area of the bridge over the
Sado River, with only a very small proportion of individuals using heights that
expose them to collision risk. Overall, we suggest that this railway is unlikely to
cause significant mortality to aquatic bird species, although it remains uncertain
whether this result can be extrapolated to other wetlands and railway lines.
This study had some limitations and potential shortcomings, but it is unlikely
that they have significantly affected our key findings. A potentially important
Table 7.2 Aquatic bird species observed most frequently crossing the railway bridge over the
Sado River, indicating the total number of birds crossing and the number of crosses within the train
collision risk zone (between the deck and the catenary lines)
Species
Total
crosses
Collision risk
zone
Lesser black-backed
gull
Larus fuscus
8432
20 (<1%)
Black-headed gull
Chroirocephalus
ridibundus
7036
91 (1%)
Great cormorant
Phalacrocorax carbo
1982
10 (1%)
Glossy ibis
Plegadis falcinellus
1596
10 (<1%)
White stork
Ciconia ciconia
1380
25 (2%)
Little egret
Egretta garzetta
1074
17 (2%)
Western cattle egret
Bubulcus ibis
693
19 (3%)
Mallard
Anas platyrhynchos
205
1 (<1%)
Dunlin
Calidris alpina
99
15 (15%)
Black stork
Ciconia nigra
16
2 (13%)
7 Bird Collisions in a Railway Crossing a Wetland …
111
most birds crossed the bridge above the train collision risk zone (91.3%), 39.1% of
the crossings were recorded below the top of the bridge arcs (Fig. 7.3).
Most aquatic birds observed crossing the bridge were gulls (lesser black-backed
gull and black-headed gull (Chroicocephalus ridibundus), greater flamingos and
glossy ibis (Plegadis falcinellus) (Table 7.2). The proportion of crossings within
the train collision risk height were low ( 3%) for most species, with relatively
high values (>10%) recorded only for dunlin (Calidris alpine) and black stork (C.
nigra) (Fig. 7.4; Table 7.2). In the case of black storks, however, this value is based
on just two individuals observed flying at a low height.
Discussion
Our results suggest that bird mortality in the “Variante de Alcácer” railway was
relatively low during the study period, although there were sharp peaks in the two
sectors where it crossed wetland habitats. Nevertheless, there were very few aquatic
birds found dead, suggesting that they were not particularly susceptible to collisions
with circulating trains or the railway structures. This was in line with the observations of aquatic birds flying across the railway in the area of the bridge over the
Sado River, with only a very small proportion of individuals using heights that
expose them to collision risk. Overall, we suggest that this railway is unlikely to
cause significant mortality to aquatic bird species, although it remains uncertain
whether this result can be extrapolated to other wetlands and railway lines.
This study had some limitations and potential shortcomings, but it is unlikely
that they have significantly affected our key findings. A potentially important
Table 7.2 Aquatic bird species observed most frequently crossing the railway bridge over the
Sado River, indicating the total number of birds crossing and the number of crosses within the train
collision risk zone (between the deck and the catenary lines)
Species
Total
crosses
Collision risk
zone
Lesser black-backed
gull
Larus fuscus
8432
20 (<1%)
Black-headed gull
Chroirocephalus
ridibundus
7036
91 (1%)
Great cormorant
Phalacrocorax carbo
1982
10 (1%)
Glossy ibis
Plegadis falcinellus
1596
10 (<1%)
White stork
Ciconia ciconia
1380
25 (2%)
Little egret
Egretta garzetta
1074
17 (2%)
Western cattle egret
Bubulcus ibis
693
19 (3%)
Mallard
Anas platyrhynchos
205
1 (<1%)
Dunlin
Calidris alpina
99
15 (15%)
Black stork
Ciconia nigra
16
2 (13%)
7 Bird Collisions in a Railway Crossing a Wetland …
111