Changes in habitat suitability for three declining Anatidae species saltmarshes on the Mersey estuary, North-West England

Abstract: Saltmarshes are areas of coastal grassland that are regularly flooded by seawater. They support a large number of resident and migratory bird species, both overwintering and breeding. The Mersey Estuary Site of Special Scientific Interest (SSSI) is situated in North West England and it supports nationally significant numbers of overwintering wildfowl and waders. Decline of three overwintering bird species’ populations (pintail, teal and wigeon) were more marked than national trends, so changes in local factors were believed to play a part. The aim of this study was to determine whether habitat suitability for the three target species changed between 2002 and 2012 in saltmarshes on the Mersey estuary. One habitat suitability model per species was built using a variety of source data (elevation, vegetation and macroinvertebrate surveys, aerial photography, questionnaires) and expert judgment to evaluate the relative importance of factors. Models uncertainty was estimated using “bounding maps” representing the most extreme plausible scenarios for each factor contributing more than 10% to the total value of the habitat suitability index. Saltmarsh area decreased between 2002 and 2012 in the Mersey Estuary, reducing available habitat for all three target species. Vast areas of pioneer zone disappeared during the study period, especially affecting species like pintail, which preferentially feed on pioneer zone species. Evidence suggested that although recreational disturbance is likely to be an issue in parts of the study area, its intensity did not change notably during the study period. Wildfowling, however, significantly affected the suitability of the study site for all species. The western part of the site had no shooting disturbance in 2002 but by 2012 a clay-pigeon shooting club had opened nearby. Although this activity does not cause direct damage to birds, the shooting noise is likely to make birds avoid the area. In addition, wigeon was also negatively affected by the increased abundance of an invasive bird species in the eastern part of the study site. The observed decline in pintail, teal and wigeon numbers overwintering in the Mersey estuary could only partly be explained by habitat changes. While suitable habitat for pintail drastically decreased and could fully explain this species’ decline, the decrease in suitable area for teal and wigeon was not as marked as the decline in bird numbers. For these two species other factors may be involved, such as habitat improvements in nearby estuaries or changes in land use in the area functionally linked to the Mersey saltmarshes. The possible management measures identified by this project include altering the Mersey estuary dredging regime to reverse saltmarsh erosion and addressing the lack of a wildfowling sanctuary area in the study site.