they are among the most threatened habitats, and the species they support are among
the most endangered taxa (e.g. Millennium Ecosystem Assessment 2005; Tittensor
et al. 2014); therefore, further habitat degradation due to railways and other transportation infrastructures may have particularly negative consequences. Moreover,
wetlands are very productive habitats that are home to large concentrations of wildlife, particularly aquatic birds such as herons, egrets, waders, waterfowl and gulls,
among others, many of which are declining and are highly sensitive to habitat loss and
degradation (e.g. Kleijn et al. 2014). Despite its importance, there is very little
information on the eventual exclusion effects of railways crossing wetlands, with very
few studies focusing on aquatic bird species. In one of these studies, Waterman et al.
(2002) found significant negative effects of railways on densities of black-tailed
godwit Limosa limosa and garganey Anas querquedula, for noise levels above 45–
49 dB(A). In contrast, Owens (1977) observed that geese Branta spp. disregarded
trains passing nearby, while the single study using a Before-After-Control-Impact
(BACI) design found no exclusion effects of a new railway line on a diverse wetland
bird assemblage throughout the annual cycle (see Chap. 16). Possibly, the type and
magnitude of effects depend on a number of factors related to wetland and railway
characteristics, as well as the behaviour and life history traits of the species involved.
Therefore, the development of a number of case studies covering a wide range of
socio-ecological conditions would be needed to find general patterns.
In this study, we addressed the eventual exclusion effects on birds of a new railway
line crossing the upper sector of the Sado Estuary (Portugal). This estuary is a
“wetland of international importance” under the Ramsar Convention, and it is
included in a Natural Reserve and in the Natura 2000 Network of Special Protected
Areas and Sites of Community Importance. The Sado Estuary is inhabited by thousands of wetland birds, particularly from early autumn to spring, with large numbers
occurring in the area where the railway line started operating in December 2010
(Lourenço et al. 2009; Alves et al. 2011). The environmental impact assessment study
raised the possibility of the railway causing the displacement of these birds, thereby
causing potential losses in prime wetland habitat. The study was designed to clarify
this issue, aiming at: (1) describing the wetland bird assemblages occurring in the
vicinity of the railway, and estimating how wetland bird assemblages varied in
relation to proximity to the railway line, in terms of (2) species richness, (3) total bird
density, and (4) densities of the most common species. Results of our study were used
to discuss the potential exclusion effects of railways crossing wetland habitats.
Methods
Our study was carried out in the Natural Reserve of the Sado Estuary (Portugal), in
a 3-km section of the upper estuary crossed by the “Variante de Alcácer” railway.
Specifically, we focused in the area known as Salinas da Batalha (38° 24′N, 8°
36W), where the railway crosses the Sado River and adjacent wetland habitats
through a bowstring bridge (see details in Chap. 7). During the study period
11 Assessing Bird Exclusion Effects in a Wetland Crossed …
181
the most endangered taxa (e.g. Millennium Ecosystem Assessment 2005; Tittensor
et al. 2014); therefore, further habitat degradation due to railways and other transportation infrastructures may have particularly negative consequences. Moreover,
wetlands are very productive habitats that are home to large concentrations of wildlife, particularly aquatic birds such as herons, egrets, waders, waterfowl and gulls,
among others, many of which are declining and are highly sensitive to habitat loss and
degradation (e.g. Kleijn et al. 2014). Despite its importance, there is very little
information on the eventual exclusion effects of railways crossing wetlands, with very
few studies focusing on aquatic bird species. In one of these studies, Waterman et al.
(2002) found significant negative effects of railways on densities of black-tailed
godwit Limosa limosa and garganey Anas querquedula, for noise levels above 45–
49 dB(A). In contrast, Owens (1977) observed that geese Branta spp. disregarded
trains passing nearby, while the single study using a Before-After-Control-Impact
(BACI) design found no exclusion effects of a new railway line on a diverse wetland
bird assemblage throughout the annual cycle (see Chap. 16). Possibly, the type and
magnitude of effects depend on a number of factors related to wetland and railway
characteristics, as well as the behaviour and life history traits of the species involved.
Therefore, the development of a number of case studies covering a wide range of
socio-ecological conditions would be needed to find general patterns.
In this study, we addressed the eventual exclusion effects on birds of a new railway
line crossing the upper sector of the Sado Estuary (Portugal). This estuary is a
“wetland of international importance” under the Ramsar Convention, and it is
included in a Natural Reserve and in the Natura 2000 Network of Special Protected
Areas and Sites of Community Importance. The Sado Estuary is inhabited by thousands of wetland birds, particularly from early autumn to spring, with large numbers
occurring in the area where the railway line started operating in December 2010
(Lourenço et al. 2009; Alves et al. 2011). The environmental impact assessment study
raised the possibility of the railway causing the displacement of these birds, thereby
causing potential losses in prime wetland habitat. The study was designed to clarify
this issue, aiming at: (1) describing the wetland bird assemblages occurring in the
vicinity of the railway, and estimating how wetland bird assemblages varied in
relation to proximity to the railway line, in terms of (2) species richness, (3) total bird
density, and (4) densities of the most common species. Results of our study were used
to discuss the potential exclusion effects of railways crossing wetland habitats.
Methods
Our study was carried out in the Natural Reserve of the Sado Estuary (Portugal), in
a 3-km section of the upper estuary crossed by the “Variante de Alcácer” railway.
Specifically, we focused in the area known as Salinas da Batalha (38° 24′N, 8°
36W), where the railway crosses the Sado River and adjacent wetland habitats
through a bowstring bridge (see details in Chap. 7). During the study period
11 Assessing Bird Exclusion Effects in a Wetland Crossed …
181