Long-term declines of wader populations at the Tagus estuary, Portugal: a response to global or local factors?
Bird Conservation …, Jan 1, 2011
Migratory wader populations face global threats, mainly related to increasing rates of habitat lo... more Migratory wader populations face global threats, mainly related to increasing rates of habitat loss and disturbance driven by human activities. To a large extent, the long-term survival of these populations requires the conservation of networks of sites along their migratory flyways. The Tagus estuary, Portugal, is among the most important wetlands for waders in the East Atlantic Flyway. Annual winter wader counts have been carried in this wetland since 1975 and a monthly roost-monitoring programme was implemented in 2007. Wintering populations of three out of the five most abundant species, Dunlin Calidris alpina, Grey Plover Pluvialis squatarola and Redshank Tringa totanus, showed significant population declines over the past three decades, which are most likely due to the loss and degradation of roost sites as a result of increasing human activity. The situation is unlikely to improve, as a high proportion of the wintering waders use roost sites that are situated in highly urbanised areas with no legal protection. The use of different roost sites by waders is highly variable both temporally and spatially, thus emphasizing the need for a network of good quality roost sites. Additionally, during migration, 60–80% of all the waders of the Tagus estuary concentrate at a single refuge, thus increasing the risk for wader populations during these periods.
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Papers by José Alves
Throughout the range of this migratory shorebird, there is no evidence of large-scale sex differences in distribution during the non-breeding season. However, the sexes differ in their selection of prey types and sizes, which results in small-scale sexual segregation within estuaries. The scale of sexual segregation therefore depends on the scale of variation in resource distribution, which, in this system, is primarily within estuaries. Sexual segregation in within-site distribution and resource use means that local-scale anthropogenic impacts on estuarine benthic prey communities may disproportionately affect the sexes in these migratory shorebirds.
Aims: To quantify the extent to which two distinct breeding populations of a migratory shorebird, the Black‐tailed Godwit Limosa limosa, overlap spatially, temporally and in their use of different habitats during winter.
Methods: We use mid‐winter counts between 1990 and 2001 to identify the most important sites in Iberia for Black‐tailed Godwits. Monthly surveys of estuarine mudflats and rice‐fields at one major site, the Tejo estuary in Portugal in 2005–2007, together with detailed tracking of colour‐ringed individuals, are used to explore patterns of habitat use and segregation of the Icelandic subspecies L. l. islandica and the nominate continental subspecies L. l. limosa.
Results: In the period 1990–2001, over 66 000 Black‐tailed Godwits were counted on average in Iberia during mid‐winter (January), of which 80% occurred at just four sites: Tejo and Sado lower basins in Portugal, and Coto Doñana and Ebro Delta in Spain. Icelandic Black‐tailed Godwits are present throughout the winter and forage primarily in estuarine habitats. Continental Black‐tailed Godwits are present from December to March and primarily use rice‐fields.
Conclusions: Iberia supports about 30% of the Icelandic population in winter and most of the continental population during spring passage. While the Icelandic population is currently increasing, the continental population is declining rapidly. Although the estuarine habitats used by Icelandic godwits are largely protected as Natura 2000 sites, the habitat segregation means that conservation actions for the decreasing numbers of continental godwits should focus on protection of rice‐fields and re‐establishment of freshwater wetlands.
Throughout the range of this migratory shorebird, there is no evidence of large-scale sex differences in distribution during the non-breeding season. However, the sexes differ in their selection of prey types and sizes, which results in small-scale sexual segregation within estuaries. The scale of sexual segregation therefore depends on the scale of variation in resource distribution, which, in this system, is primarily within estuaries. Sexual segregation in within-site distribution and resource use means that local-scale anthropogenic impacts on estuarine benthic prey communities may disproportionately affect the sexes in these migratory shorebirds.
Aims: To quantify the extent to which two distinct breeding populations of a migratory shorebird, the Black‐tailed Godwit Limosa limosa, overlap spatially, temporally and in their use of different habitats during winter.
Methods: We use mid‐winter counts between 1990 and 2001 to identify the most important sites in Iberia for Black‐tailed Godwits. Monthly surveys of estuarine mudflats and rice‐fields at one major site, the Tejo estuary in Portugal in 2005–2007, together with detailed tracking of colour‐ringed individuals, are used to explore patterns of habitat use and segregation of the Icelandic subspecies L. l. islandica and the nominate continental subspecies L. l. limosa.
Results: In the period 1990–2001, over 66 000 Black‐tailed Godwits were counted on average in Iberia during mid‐winter (January), of which 80% occurred at just four sites: Tejo and Sado lower basins in Portugal, and Coto Doñana and Ebro Delta in Spain. Icelandic Black‐tailed Godwits are present throughout the winter and forage primarily in estuarine habitats. Continental Black‐tailed Godwits are present from December to March and primarily use rice‐fields.
Conclusions: Iberia supports about 30% of the Icelandic population in winter and most of the continental population during spring passage. While the Icelandic population is currently increasing, the continental population is declining rapidly. Although the estuarine habitats used by Icelandic godwits are largely protected as Natura 2000 sites, the habitat segregation means that conservation actions for the decreasing numbers of continental godwits should focus on protection of rice‐fields and re‐establishment of freshwater wetlands.