A History of Chew Valley Ringing Station

Ringing at Chew Valley Lake began on 4th June 1961 when eight Reed Warblers and one Reed Bunting were mist-netted. Further visits that year provided a total of 226 birds of 24 species, heralding the lake’s potential as a ringing site. The Mendip Ringing Group was formed in the following year and began operating at a number of sites, particularly the Saltford Sewage Works. Permission was given by Bristol Waterworks Company to ring at the lake and although ringing was held up by the severe winter of 1962/63 it restarted in April 1963.

The ringers tried various sites around the lake and by mid-summer had decided that the southern end of the lake was the most suitable area in which to base their activities. It was realised that the lake margins were an important breeding site and feeding area for warblers in the summer and autumn and emphasis on this aspect of ringing at the Chew Valley Lake continues to this day. It soon became obvious that the lake offered so much potential that ringing at other sites would have to be abandoned. It was therefore decided to re-name the group to fit its single-site status as the Chew Valley Ringing Station and the new name was officially adopted from the beginning 1964.

Ringing was carried out in the open or from the back of a car but in 1965 a 3m x 2m garden shed filled the triple-role of laboratory, equipment store and shelter. In 1968 a larger hut measuring 7m x 4m had four small rooms; laboratory, office, kitchen and lobby, plus a loft for the storage of poles. Services included electricity, Calor gas and running water. In 1988 Bristol Water provided a much larger hut (13m x 7m) and laid the breezeblock foundations. The hut then had to be moved in sections from Bristol, at Old Sodbury, and CVRS members took on the challenge of rebuilding it. £2,000 was raised by a sponsored birdwatch and donations from local natural history societies and CVRS members. Local firms were very generous in donating plywood sheets and preservative and the hut was operational by April 1989. A large Heligoland style trap was erected in 2007 with 50% of the funding coming from the Bristol Naturalists’ Society.

The catching effort at CVRS has been dominated by the use of mist nets although various traps have also been used. Circa 225,000 birds of 145 species have been ringed (by 2024), with an average of 3,600  birds per year. The highest annual total was 7,091 in 1995. 

Of course rarities, by their very definition, rarely turn up but nevertheless it does happen and surprises have included: Bluethroat  (1968), Little Bunting (1976), Savi’s Warbler (1986) and Marsh Warbler (1994). Other species such as Wryneck, Aquatic Warbler and Yellow-browed Warbler have even occurred on more than one occasion. However, it is the ‘common’ birds that are the mainstay of the ringing station’s activities. 

As a major reed bed site it is not surprising that Reed and Sedge Warbler are high on the list. In the mid-1960s there were more Sedge Warbler than the Reed Warbler, by the mid-1970s their numbers were approximately equal and from the mid-1980s Reed Warblers were more numerous. The ups-and-downs in the totals reflect both the conditions they face in their winter quarters in Africa and the variation in catching effort at the lake.  That the Reed Warbler now dominates the CVRS ringing totals is thought to be caused by the gradual development and expansion of the reed beds around the lake margins. This provides conditions that are ideal for Reed Warbler and less so for Sedge Warbler as their preferred fen-scrub-swamp habitat has become increasingly restricted due to the expansion of the phragmites. For both these species, especially Sedge Warblers, their high numbers are due to birds that have bred elsewhere but use the lake as a stopping-off and fattening-up site in late summer in readiness for migrating. The nest recorders might also admit that the Sedge Warbler nests are much harder to find!

Blue and Great Tits form a significant proportion of the overall total as a large number are ringed as nestlings in the spring and when coming to feeding stations in the autumn and winter months.

Although only a small number of Swallows are ringed locally during the early part of the breeding season large catches are possible as they come in to roost in the reed beds during July and August.

Blackcap, Chiffchaff and Willow Warbler are our other commonest summer visitors. Wren, Reed Bunting, Long-tailed Tit, Robin and Dunnock represent the common resident passerines at the lake.

The Canada Goose features high-up in our ringing totals list thanks to the annual Canada Goose roundup during their flightless period. Traditionally this takes place on a Tuesday, either at the end of June or early in July. By far the highest number of CVRS’s reported ringing recoveries is of Canada Geese with over 600 records (25% of the total).  A large majority of these are noted as being shot for crop protection in Devon and Somerset.

Exceptional individuals

There are two birds that deserve special mention and both were almost certainly born at CVL. Cetti’s Warbler N482152 was ringed as a juvenile male on 20th June 1999 and retrapped 28 times until it was last trapped on 18th October 2008. A couple of years after initial capture he moved from the east side of the reserve and settled in the area of our constant effort site in front of the ringing station having lived for at least 9 years, 3 months and 28 days. He holds the UK longevity record for this species.
 
Reed Warbler F088114 was caught as a juvenile on 17th July 1989 and sexed as a male by cloacal development in subsequent years. He was recorded once or twice every year until last seen on 13th May 2001 having lived for at least 11years, 3 months and 15 days. He remained in approximately the same area of the reserve throughout his life except that remarkably he must have made 12 return trips to Africa!

Catching Effort

Clearly, if there is to be any means of calculating changes in bird populations by general ringing then it will be necessary to adjust for catching effort. Three measures were proposed by Roy Smith published in the CVRS 6th Report covering 1976–1978 pp 20-25. These were ‘Operational Days’, ‘Ringer Days’ and ‘Net Foot Hours’. Of these the net foot hours have proven to be the most useful. 

Constant effort ringing

To help address the question of the variability of catching effort the ringing station takes part in the BTO’s Constant Effort Scheme (CES). This has been in operation since 1983 with the first four years being used to test and validate the scheme. Ringers operate the same nets in the same locations over the same time period at regular intervals through the breeding season at over 140 sites throughout Britain and Ireland. The Scheme provides long-term trend information on the abundance of adults and juveniles, productivity and also adult survival rates for 24 species of common songbird. Of these, two are on the ‘Red list’ of the Birds of Conservation Concern (BOCC) document (Song Thrush and Willow Tit) and four are Amber-listed (Dunnock, Willow Warbler, Bullfinch and Reed Bunting). The other species are Wren, Robin, Blackbird, Cetti’s Warbler, Sedge Warbler, Reed Warbler, Whitethroat, Lesser Whitethroat, Garden Warbler, Blackcap, Chiffchaff, Long-tailed Tit, Blue Tit, Great Tit, Treecreeper, Chaffinch, Greenfinch and Goldfinch. All, apart from Willow Tit, occur annually at our CES sites at Chew Valley Lake and are included in the national figures for population trends.

Computerisation

The use of computers by CVRS began in the mid- 1980’s with ringers experimenting with various home computers and databases. However, no real progress was made until 1993 when the British Trust for Ornithology produced a standardised program called B-RING. This was originally based on the BBC B home computer and could be used for sending records to their headquarters on a disc and subsequently by email. Despite the setbacks of two burglaries and theft of the computer the data group which, for some unknown reason, became known as “The Tufty Club” held regular meetings on Tuesday evenings for data entry. By 2003 two computers were in operation and a switch was made to using the BTO’s IPMR (Integrated Population Monitoring Recorder). This involved not only entering the current data for submission to the BTO but work also began on catching up with historical records back to 1976; a rather monumental task that was completed by 2007. The CVRS database now holds records for circa 275,000 birds. This includes 95,000 recaptures that provide some of the most informative data for analysis. Currently the ringing scheme is running a new system called DemOn (Demography On-line) which allows data to be input directly via the internet into the BTO’s central database.

The CVRS Year

Apart from the general mist netting and training a pattern of activities has emerged. In the winter months the lake level usually rises to such an extent that much of the reed bed is inaccessible and there is a greater use of feeding stations. By April nest recording gets underway. The 200 boxes need to be checked once per week and searching for open nest is undertaken by a small band of very dedicated recorders. CVRS contributes approx 650 nest records annually to the BTO’s Nest Recording Scheme. The constant effort scheme begins on May 1st and runs through until the end of August. The two CES sites are operated from 6.00 am until 12 noon with three sessions per month at approximately 10 day intervals. The end of June is the ‘traditional’ time for the Canada Goose roundup and the BTO sponsored ringing course have taken place at the end of July. In the autumn and winter more time is taken for habitat management, sometimes helped by groups of volunteers. In between these activities the ringing station plays host to various interested groups to demonstrate and explain the rationale behind the bird ringing process. 

Bristol Water

And finally, no account about the history of Chew Valley Ringing Station is complete without a special mention of our relationship with Bristol Water. From the outset Bristol Water has always been supportive of our activities and in so many ways. Not only supplying our present accommodation but every year with donations for various items such as rings, producing reports and help with reed bed management. Above all, though, allowing us access to one of the best reed bed ringing sites in the country!