The Aquatic Warbler is a scarce inhabitant of marshy areas and sedge beds in eastern Europe and has the dubious distinction of being the most threatened passerine in Europe. The bulk of the breeding population, estimated at 12 – 14,000 pairs, is centred on eastern Poland, southern Belarus and northern Ukraine. Small and critically endangered populations exist in western Siberia and northern Poland / northern Germany.
Post breeding the many migrate westwards through the Low Countries and northern France before turning south towards their winter grounds in Senegal, Mali and Mauritania. The main wintering area appears to in Djoudj National Park in northern Senegal where between 20 – 60 % of the world population may occur.
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The purpose of this posting is to draw attention to this declining species and to highlight Stour Ringing Groups involvement in monitoring its migration.
Although always scarce, the above map shows how much the range has contracted in recent years due to habitat destruction. In addition places like Djoudj National Park in Senegal have had water diverted for agricultural development resulting in a huge loss of wintering habitat.
Much of the extensive wetlands in Djoudj National park, Senegal have been lost since this photo was taken in 2005.
Breeding Aquatic Warblers can easily be seen in eastern Poland in the Biebzra marshes or in Hungary on the Hortobagy. in the UK birds there was once a reasonable chance that juvenile birds could be found at Lodmoor RSPB or at Marizion in Cornwall in the last ten days of August. However in recent years very few have been seen in the field.
Aquatic Warbler – Holland – photo by James Lidster.
A similar drop in numbers ringed has occurred in recent years. In the 70’s birds were regularly ringed at Radipole with 22 trapped in one year. This relatively high capture rate led to the species being removed from the British Birds Rarities Committee list in the mid 80’s.
Stour Ringing Group ringed at Lodmoor from 1979 until 1982. Our first Aquatic was trapped in August of that year. As a new trainee I was ringing with Trevor Squire on 22/9/79 when the second was caught. It was made clear that I was extremely lucky to be allowed to ring such a great bird so early in my ringing career. A further 11 birds were trapped between then and 1982 when the RSPB took over management and withdrew our ringing permission.
Aquatic Warbler Lodmoor 1979 – image taken from scanned 35mm slide
Aquatic Warbler Lodmoor 1979 – image taken from scanned 35mm slide
In August 1983 I obtained my permit and started regular ringing at Lytchett Bay. on 23/8/83 I trapped an Aquatic Warbler one evening. This remains the only Aquatic caught away from the early morning period and was one of only five or so birds trapped that evening.
In the late 80s I did some ringing at Keysworth near Wareham. Realising the potential of the site, the whole group got involved and a considerable amount of manpower and netting was deployed. Also tape lures were used for the first time. This proved very productive and the following were ringed 20 in 91, 13 in 92, 7 in 93, 17 in 94, 5 in 95, 4 in 96, 11 in 97 and 4 in 2000. The low numbers in 95 and 96 and the blanks in 98 and 99 were due to reduced coverage. At the end of this period there was a change of land ownership and we were unable to continue. As well as the expected first year birds we trapped a number of highly bleached and abraded adults. These were a creamy colour with chocolate brown streaking and tails so worn that only the shafts remained.
This is sight that is unlikely to be repeated for a long time. Keysworth, Wareham August 1994. Image taken from a scanned print.
From 2001 to today ringing effort has returned to Lytchett Bay, a mere 4km north-east of Keysworth. In spite of considerable effort including the use of tape lures and signing up to the Europe wide investigation into Aquatic Warbler distribution, not a single bird has been trapped up to 2010
What is the reason for this huge decline?
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Is Lytchett that much poorer for the species that Keysworth? maybe, the reed bed is smaller, we use fewer nets, but you would have expected at least a few to be caught at Lytchett.
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Has the population crashed since 1997? the decline continues, but the change could not have been that rapid. Numbers ringed on the continent have not reduced that much during the period concerned.
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Is the weather to blame? Probably yes. During this period the number of field sightings have dramatically declined, with few records from Lodmoor, Christchurch Harbour, Portland or West Bexington / Cogden. For the last decade August has been dominated by westerly winds. Aquatic Warblers that migrate to northern France / the Low Countries may arrive here by ‘drift migration’ if the wind is in the south or east.
This August for the first time for years the wind was easterly at the critical time and an Aquatic was trapped at Lytchett Bay on 20th August (regrettably I was in Derby doing decorating at the time)
Aquatic Warbler Lytchett Bay 20th August 2011 and the 95th to be ringed by SRG. Photo by Shaun Robson
Aquatic Warbler Lytchett Bay 20th August 2011. Photo by Shaun Robson
In conclusion: 1) choice of ringing site and ringing effort has reduced the numbers trapped by Stour Ringing Group, 2) breeding and wintering habitat destruction has produced a slow decline in the world population but 3) weather conditions seem to have caused the enormous reduction of birds seen or ringed in Dorset over the last decade.
The ongoing ringing of this species Europe wide allows continuing monitoring of the migrant population and the retrapping in the UK of several Polish ringed birds proves the origin of our birds. However only the protection of the vulnerable wetland habitats in eastern Europe and the oasis of Djoudj National Park and similar areas in Senegal, will ensure long-term survival of this enigmatic species.
For further information including a correlation of arrival dates with wind direction see an article I wrote in the Dorset Bird report for 1995 and for up to date information on this species see http://www.aquaticwarbler.net/sar/
gryllosblog 
Excellent review Ian. Further evidence to support the “weather theory” is provided by Bruno Bargain who is a leading ringer at a site in Finistere, Brittany, France. Data from that site shows that the numbers of Aquatics trapped and ringed has remained broadly stable over the period 1988 to 2007. This information is reliable in terms of this debate as it is correlated to the lengths of mistnet deployed. Interestingly 2003 (c180) and 2004 (c120) produced above average numbers. Meanwhile in Dorset there were only 2 records in 2003 and 3 in 2004. Having invested considerable effort at Lytchett Bay during the period 2004 – 2010 I can confirm that the number of days which dawned with suitable conditions during this period could be counted on the fingers of two hands. 2011 has been slightly better with small windows of winds between east and south during the key period. This led to the capture of the bird on the 20th and of 2 birds at Marazion, Cornwall in the following two days.
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