A vast amount of data has been gathered on the birds of Chew Valley Lake; not least on those for which Chew is most important - the waterbirds. Most of this data is scattered in all sorts of inaccessible places across nearly 60 years' worth of various old bird reports and journals, but now all the published counts are available in one place.
Click on the years below the Tufted Duck...
Waterbird counting at Chew Valley Lake
Ever since reservoir construction began at Chew in the early 1950s, regular counts have been made of the waterfowl using the lake. Unfortunately a lot of the counts made before 1960 are no longer readily available, but the brief accounts in the old Bristol Bird Reports usually gave a maximum count for the year and details of any rarities that turned up in the early days.
From the early 1960s the monthly wildfowl counts (now the Wetland Bird Survey - WeBS) give a much better idea of the numbers of ducks, geese and swans using the lake, and from the mid 1960s the personal records from an increasing number of birdwatchers added to the monthly data gathered by the duck counting teams. With local publications such as the Bristol Bird Report, the Proceedings of the Bristol Naturalist's Society and Somerset Birds now providing more data, we can also begin to pick out occasional counts for other waterbirds, such as grebes and Coot.
In the mid 1980s the WeBS scheme was expanded to include species such as grebes, Cormorants and Coot, so we have a more complete run of data. Also, through the mid 1980s into the 1990s, the amount of published data in the Avon Bird Report increased dramatically, giving us every peak waterbird count of each species using the lake, through a combination of WeBS counts and the numerous other records submitted by other birdwatchers.
Traditionally, WeBS counts were only made during the winter months, when bird numbers are at their highest. In the 1960s this would be from September to March, but in the late 1980s counts were made from July to March, which made sense as the peak months for many species were in the late summer. From 2005, counts were made in every month of the year.
The water level
The one single factor that has the biggest effect on waterbird numbers at Chew is the water level. A typical year sees the lake at, or near top level in January, remaining that way until the spring. A gradual drawdown will usually commence during April, continuing through the summer. A dry summer will normally see the level drop at a faster rate, when typically the lake will drop to around 50-60% capacity in late autumn (a drawdown of 2000 to 2500mm). But if we get a wet summer as we did from 2006 to 2008, the drawdown will be significantly less. From November the lake will begin to refill, usually reaching full capacity in the first few months of the following year.
The contour map above shows the shallow, natural topography of the lake bed, which means that a normal drop in the autumn reveals a large fringe of mud around most of the lake, except the deep water area between Woodford Bank, the dam and North Shore. This autumn drop is key to providing large areas of shoreline and shallow water for dabbling duck to feed, and although a number of other species' feeding requirements mean they will tolerate a higher level, it's during what we would regard as a 'low' autumn (-2500mm or more) that the biggest number of birds is likely to occur.
The annual tables are accompanied by a reservoir drawdown graph for each year since 1980, so providing a good indication of the feeding conditions during the year.
The annual tables
Click on the years below to show monthly species maxima for each year from 1960 to the 2012. You can navigate back to this page by clicking the 'CVL bird counts' banner at the top of the page, or the index page link at the bottom.
The tables are compiled mainly from published data, and follow the bird report style in showing the maximum count for each species made during the month in question. Inevitably the commoner species are normally only counted once each month as part of the WeBS scheme, but the scarcer species such as Smew and Garganey tend to be counted whenever they are present at the lake, so it makes sense to use maxima as not only is this the only available data in some cases, it ensures that short-staying rarities and high numbers not recorded on the WeBS counts are included.
It follows that the totals given at the bottom of each table (*and so used in the graphs on this page) represent the sum of monthly maxima for each species and not the total number of birds counted on any one date. Nevertheless, given the inherent margin of error when counting the lake and the fact that nearly all of the commoner species are only counted on WeBS day anyway, these totals can still be regarded as a fair representation of numbers present that month, and provide a like-for-like comparison with other years. A total is only given when most of the significant species have been counted that month.
I've omitted all those nasty hybrids and farmyard puddleducks, but included all escapes. As well as adding a bit of interest, an 'everything included' approach with escapes means that I don't have to concern myself with whether some of the more controversial ones should be included or left out.
In the tables, a count in red represents the all-time record count; a figure in italics means that due to incomplete source data, there is some uncertainty with either the accuracy of the count, and/or the month in which it was made. Older bird reports were a little sparing with counts for some of the non-wildfowl species, often giving only one or two for a particular year. Where such isolated figures have been published, I have included them for completeness, so it follows that a blank field means that no published data is available; no assumptions have been made as to whether the species was present that month or not.
The drawdown graphs shown with the annual tables from 1980 onwards are produced from spot measurements taken on 1st of each month.
It would be very remiss of me not to acknowledge the people who, over the years have carried out the thankless task of counting millions of birds at Chew. Not only does this fieldwork add considerably to our knowledge of birds at the lake, it also contributes to maintaining Chew's designations as an SSSI and Special Protection Area, and its status as an internationally important site for birds. Particular mention should be made of the late Bernard King, Roy Curber, Keith Vinicombe and Rupert Higgins.
I would also like to thank Andy Davis and Mike Bailey for providing additional references used to produce this online article, and I am also very grateful to Colin Hunt who has provided a wealth of invaluable reservoir data.