The Sado Estuary is one of Portugal’s most important wetlands – a key link in the chain of sites connecting Africa and the Arctic, on the East Atlantic Flyway. In a paper in Waterbirds, João Belo and colleagues analyse changes in numbers of waders wintering in this estuary over the period 2010 to 2019, with a focus on roost sites. These results are interpreted in regional and flyway contexts. The team find serious declines in numbers of Avocet, Dunlin and Ringed Plover.
Lost roost sites
The Sado Estuary became a nature reserve in 1980 and has since been classified as a Ramsar Site, a Special Protection Area (SPA – Natura 2000) and an Important Bird and Biodiversity Area. The Estuary lies about 30 km south of the much larger and more famous Tagus (or Tejo) Estuary, which is threatened by a new international airport (see Tagus Estuary: for birds or planes?). A suggestion that the Sado could provide mitigation habitat for damage done to the Tagus prompted scientists from the Universities of Aveiro and Lisbon to review the current state of the estuary.
João Belo and his colleagues used monthly data from a programme of wader surveys, conducted largely by volunteer birdwatchers. These took place between January 2010 and December 2019. Roosting birds were counted at high tide along the northern shores of the Sado estuary and any habitat changes were noted.
During the ten-year survey period, 21% of the available high-tide roost area was lost. These changes were associated with the commercial abandonment of saltpans (with consequent increases in vegetation) and the conversion of others for fish farming (often with netting, to keep out fish-eating birds).
Results of the survey
In their paper, João Belo and colleagues focused on total numbers of waders and counts of the six most commonly-encountered species: Avocet, Dunlin, Black-tailed Godwit, Redshank, Ringed Plover and Grey Plover. They compared winter (Dec, Jan and Feb) counts in 2019 with those from 2010. This is when peak numbers of Avocet, Redshank and Grey Plover occur. Higher counts of Dunlin are made in spring, as schinzii birds returned from Africa, with peak counts of Black-tailed Godwit and Ringed Plover occurring in autumn.
The key findings are:
- There was a strong decrease in the overall number of waders wintering in the Sado Estuary. This trend is mostly driven by steep declines in three of the six most abundant species: Avocet, Dunlin and Ringed Plover.
- Avocet numbers were 42% lower in 2019 than they had been in 2010.
- Dunlin numbers dropped by over half, with 2019 counts being only 47% of those in 2010. These are mostly dunlin of the alpina subspeciesthat breed between Northern Scandinavia and Siberia.
- Ringed Plover numbers dropped by 23% between 2010 and 2019.
- Redshank increased significantly between 2010 and 2019, while the population of Grey Plover was relatively stable, and it was not possible to derive a population trend for Black-tailed Godwit.
It is interesting to look at these patterns alongside data collected in Britain & Ireland, over the same period. As discussed in Do population estimates matter? and Ireland’s wintering waders, there have been major changes in wader numbers, with most species currently in decline.
- Winter Avocet numbers have increased massively in Britain & Ireland. It is possible that young birds are more easily able to settle in these northern areas, now that winter temperatures are generally warmer. Declines in Portugal may reflect a northwards shift of the winter population, driven by new generations of birds.
- Numbers of Ringed Plovers in Britain & Ireland did not change over the period 2010 to 2019 but had dropped a lot in the preceding twenty years.
- For Dunlin, the size of the declines in Britain & Ireland are consistent with those on the Sado. It has been suggested that more young birds might be settling in areas such as the Wadden Sea, closer to Siberian breeding areas, something that may have become more possible given the reduced intensity and occurrence of freezing conditions along the east coast of the North Sea.
Regional and Flyway patterns
As discussed in the blog Interpreting changing wader counts, based on research led by Verónica Méndez, local changes in numbers are usually reflective of broader changes in population levels. Individual waders are unlikely to seek alternative wintering sites unless habitat is removed, so birds do not re-assort themselves into the ‘best’ areas when population levels decrease. Instead, there is general thinning out across all sites as populations decline. In this context, it is unsurprising that the trends in the Sado Estuary are similar to those found elsewhere in Portugal and in other Western European wintering areas.
The survey data collected between 2010 and 2019 form a useful backdrop against which to monitor what might happen when (or perhaps if) a new international airport is constructed within the nearby Tagus Estuary. If some birds are displaced to the Sado, increases in numbers might be expected.
Displacement is not cost-free, as has been shown in a well-studied population of Redshanks on the Severn Estuary in Wales. When Cardiff Bay was permanently flooded, as part of a major redevelopment, colour-marked Redshank dispersed to sites up to 19 km away. Adult birds that moved to new sites had difficulty maintaining body condition in the first winter following the closure of Cardiff Bay, unlike the Redshank that were already living in these sites. Their survival rates in subsequent winters continued to be lower than for ‘local’ birds, indicating longer-term effects than might have been predicted.
These three papers are essential reading for anyone interested in the consequences of displacements caused by development projects.
- Burton, N.H.K. 2000. Winter site-fidelity and survival of Redshank at Cardiff, South Wales. Bird Study 47: 102–112.
- Burton, N.H.K. & Armitage, M.J.S. 2005. Differences in the diurnal and nocturnal use of intertidal feeding grounds by Redshank. Bird Study 52: 120-128.
- Burton, N.H.K., Rehfisch, M.M., Clark, N.A. & Dodd, S.G. 2006. Impacts of sudden winter habitat loss on the body condition and survival of Redshank. Journal of Applied Ecology 43: 464-473
Given that the Sado has multiple conservation designations, including as a Ramsar site, and that this study has shown a clear loss of available roosting areas, perhaps it is time to identify a high-tide refuge that can be fully protected and managed in ways which create a range of suitable habitats for use by long-legged and short-legged waders. A nature reserve such as this has a potential to attract birdwatchers too, with prospective increased income from tourism.
Curlews don’t get a mention in this paper but the The Sado provides a neat link to a 2022 blog, A Norfolk Curlew’s Summer. This tale focuses on ‘Bowie’, a male Curlew that breeds in Breckland (Eastern England) and has been tracked to The Sado Estuary. In the blog, Bowie’s story stops in The Tagus but he subsequently headed further south to The Sado, where he spent the winter. At the time of writing (13 Feb 2023) he is still there but hopefully he will heading north soon.
The Sado is not only important in the winter, of course. As mentioned earlier, it is a spring stop-over for birds such as schinzii Dunlin, heading north from Africa to Siberia, and a moulting/staging site for waders heading south in late summer. Tracking and colour-ringing are telling more of these stories, with links to countries as far north as Canada and Siberia and as far south as South Africa.
The Sado story could not have been written without the work of volunteer counters who collect monthly data during the winter months, on the Sado Estuary, across Portugal and on the wider East Atlantic Flyway. These monitoring efforts are essential when attempting to track changes in wader populations, especially when extra information can indicate links to habitat changes, as is the case in the Sado. The international picture is painted using Flyway information generated using January counts that are developed by the Institute for Nature Conservation and Forests (ICNF).
Here is a link to the paper:
Synchronous declines of wintering waders and high-tide roost area in a temperate estuary: results of a 10-year monitoring programme. João R. Belo, Maria P. Dias, João Jara, Amélia Almeida, Frederico Morais, Carlos Silva, Joaquim Valadeiro & José A. Alves. Waterbirds. doi.org/10.1675/063.045.0204
WaderTales blogs are written by Graham Appleton (@GrahamFAppleton) to celebrate waders and wader research. Many of the articles are based on published papers, with the aim of making shorebird science available to a broader audience.