Wintering Waders – an update of GPS-tracking studies of Oystercatchers and Curlew
Ryan Burrell, Projects Officer, Devon and Cornwall Wader Ringing Group
The Exe Estuary holds an internationally important community of winter waterfowl and waders. The mudflats, salt marshes and shellfish beds of the estuary itself, alongside the damp wet grasslands surrounding it, provide crucial foraging and roosting sites for a whole range of waders. Two key species within the Estuary are its Oystercatchers and Curlew.
Oystercatchers are easily identifiable by their black and white plumage and often noisy piping “kleep-kleep call”. Curlews are often more secretive but can be found at many sites during the winter and are easily recognised by their large size, brown plumage, and distinctive, downwardly curving bill.
Unfortunately, the Exe wintering Oystercatcher population has declined by 43.5% compared to a national decline of 24% (1991/92 – 2017/18: WeBS Online). Similarly, wintering Curlew in the UK have declined by 33% between 1992/93 to 2017/18 (WeBS Online).
The Exe Oystercatcher project started in 2018 to answer questions on the importance of and fidelity to, sites within the Exe for Oystercatchers and to gain a better understanding of survival and foraging. In early 2020, this work was expanded to include a new study on Curlew, following similar objectives.
So far, 24 Oystercatchers and 5 Curlew have been fitted with GPS-tags (Figure 1 & 2). These tags record the position of each bird every 1-2 hours and later transmit this information to a remote basestation or via the mobile phone network. To date, we have collected over 63,500 GPS positions, which alongside detailed foraging observations have provided a huge amount of data to analyse.
The majority of the tracked Oystercatchers foraged at Lympstone and Cockle Sands, during at least part of their tracking (Figure 3). These sites are in the centre and south of the estuary within the Special Protection Area (SPA) boundary and are dominated by mussels, cockles and oysters.
Although the Covid-19 pandemic limited fieldwork, we managed to tag 5 Curlew in 2020 in a regular winter curlew roost on RSPB Exminster Marshes. This study remains it is early stages, but the home ranges created by the tracking data suggests interesting differences between the movements of individuals, with birds visiting sites throughout the estuary, including mudflats, saltmarsh, and wet grassland fields. One bird regularly visited wet grassland fields east of Exton and outside the SPA area, others utilised RSPB Bowling Green marsh, suggesting an initial link between these sites and a useful finding for the RSPB site managers.
Although GPS-tracking tells us a great deal about habitat use and the importance of different sites, its ability to tell us information on the survival and foraging of the wider population is limited. For this we have used colour-ringing, where each individual is fitted with a uniquely coded or unique number of coloured leg rings (Figure 4 & 5).
These rings allow us to individually identify each bird and generate data on individual habitat use and behaviour. So far, we have marked 283 Oystercatchers generating over 5500 resightings and 16 Curlew producing 160 resightings. In addition, we are starting to gather a picture of where Exe birds breed, resightings during the breeding season have revealed Oystercatchers from the Exe breeding in Scotland, Norway, Northern England, Iceland, and the Netherlands.
In 2020, we know that 5 of our colour-marked Curlew summered on the Exe (non-breeding), 1 bred in the Netherlands and 1 in the Peak district. An additional, 6 wintering Curlew marked by other projects are known to come from breeding sites in Wales, Scotland, and Eastern England.
The Exe Oystercatcher Project and our recently started studies on Curlew have provided some fascinating and important insights into wader behaviour and site use with the Exe. Covid-19 stunted our progress in 2020, however, we still have further tags to deploy in winter 2020/21. The data collected by the GPS-tracking, colour-ring resighting and behavioural studies will used to produce scientific papers with the first of these recently submitted for publication. Finally, if you are out and about in the Estuary, do keep an eye out for colour-marked waders and submit them to http://www.dcwrg.org.uk/
We are extremely grateful to our partners and landowners for supporting this research. Further thanks are extended to everyone who has contributed resightings of colour-marked waders within the estuary, but in particular the excellent re-sighting work undertaken by the Dawlish Warren Recording Group.