The Constant Effort Sites Scheme
Full results for the 2008 season are now online in the latest CES News Download it here Information sheets and forms for CES ringers to download can be found here

The Constant Effort Sites (CES) scheme is the first national standardised ringing programme within the BTO Ringing Scheme and has been running since 1983.  Ringers operate the same nets in the same locations over the same time period at regular intervals through the breeding season at 120 sites throughout Britain and Ireland.  The Scheme provides valuable trend information on abundance of adults and juveniles, productivity and also adult survival rates for 25 species of common songbird.

2008 results

The final results for the 2008 season are now out, in the latest copy of CES News (download it here). The year was another poor one for many species, with productivity of 11 of the 25 core species significantly lower than the long-term average. The range of species affected was rather different though, and it looks like the timing of heavy rainfall in the breeding season was responsible for this. See CES News for more details...

2007 season

Whilst preliminary results for the 2007 season featured in major newspapers in the autumn (The Times and Telegraph are shown to the right), the final results for the season are now available. For many species this was the worst breeding season since the start of the CES Scheme, and the final results can be downloaded here. Of the 25 species regularly monitored by CES, 11 showed productivity significantly lower than the long-term average, with six also showing their lowest ever productivity (Reed Warbler, Whitethroat, Willow Warbler, Willow Tit, Blue Tit and Treecreeper). Productivity was only higher than the long-term average for Blackbird, Cetti's Warbler, Long-tailed Tit, Greenfinch and Linnet, with Long-tailed Tit productivity at its highest ever level. Measures of adult abundance showed eight species decreased on the 2007 figure, with seven increases. Adults of four species were at their lowest ever levels (Sedge Warbler, Lesser Whitethroat, Linnet and Reed Butning) with Robin, Blackcap and Treecreeper at their highest ever levels.

Why do we need CES?

We need to monitor bird populations through time in order to conserve them effectively. Firstly, we need to know whether numbers are stable or changing, whether decreasing or increasing. If there is a change in numbers, particularly a decrease, we need to know why. Conservation action can then be targeted appropriately.

The key things that we need to monitor are numbers (abundance), the number of births (breeding success or productivity) and the number of deaths, usually recorded as the number that do not die (survival). Once we have this information, we can calculate expected changes in numbers and look for the stage of the life cycle which is most affected by environmental change.  We can then find, or at least narrow down, the possible cause(s) of a decline.  This is the philosophy between the BTO Integrated Population Monitoring (IPM) programme.  The CES scheme uses comparisons of the numbers of birds caught each year to provide indices of population change for 28 species.  This information complements that from other BTO census schemes, such as the Breeding Bird Survey (BBS) and Common Birds Census (CBC), particularly for songbirds breeding in wetland and scrub habitats.  Changes between years in the proportion of juveniles in catches are used as an index of productivity for the 28 species, complementing information from the Nest Record Scheme (NRS).  Information on breeding success from CES is particularly valuable because it integrates success through the whole season (including the outcomes of multiple broods and early post-fledging mortality), whereas the NRS only monitors the results of single nesting attempts.  Between-year recaptures of individual birds in the CES scheme are used to determine adult survival rates.  This information complements that from general ringing from the BTO Ringing Scheme.  For the species monitored by CES, the scheme generally generates higher quality information on adult survival per unit ringing effort because of the low recovery rates from the latter.

What is CES?

The CES Scheme uses catches from standardised mist-netting to monitor key aspects of the demography of 25 common breeding songbirds. Around 120 sites are monitored through the breeding season, with twelve standard visits between May and August. Changes in the total number of adults caught provides a measure of changing population size, whilst the proportion of young birds caught forms an index of breeding success.  Retraps of adult birds ringed in previous years are also used to estimate annual survival rates.

CE Sites

The popularity of CES ringing is imncreasing again after a drop-off during 2001 (due to Foot and Mouth restrictions) and 117 sites operated in 2008.  Eight of these operated for the first time in 2008 including our most northerly site now in Scotland. The majority of CE Sites currently operated are in England (90 sites), but valuable contributions are received from Scotland (15 sites), Ireland (5 sites) and Wales (7 sites).  The geographical spread of sites is impressive, but still somewhat biased towards the south and east due to the higher densities of ringers. Poor weather in the north and west can also make regular ringing more difficult. The majority of CE Sites are in scrub (shown as circles in the map on the right (wet sites are blue and dry sites are green)) and reedbeds (light brown squares) with a small number of sites in deciduous woodland (dark brown squares).  The sites are selected by the ringers themselves as those that are are suitable for catching satisfactory numbers of birds at each visit in habitats where successional changes can be managed.  If the habitat were to change too dramatically, the results would be less meaningful because of changes in the chances of capturing individual birds.

Species monitored

The CES Scheme monitors 25 species of common passerines.  Of these, three are on the 'Red list' of the Birds of Conservation Concern (BOCC) document (Song Thrush, Willow Tit and Linnet) and five are Amber-listed (Dunnock, Whitethroat, Willow Warbler, Bullfinch and Reed Bunting).  The other species are Wren, Robin, Blackbird, Cetti's Warbler, Sedge Warbler, Reed Warbler, Lesser Whitethroat, Garden Warbler, Blackcap, Chiffchaff, Long-tailed Tit, Blue Tit, Great Tit, Treecreeper, Chaffinch, Greenfinch and Goldfinch.

Adult numbers

A full analysis of changes in abundance measured by CES has been carried out for the years 1983-1995.  Catches of most insectivorous resident species either increased or remained stable, while catches of resident thrushes, small finches, buntings and some trans-Saharan migrants declined.  The largest increase in catches of adult birds were recorded for Robin, Wren, Greenfinch, Long-tailed Tit and Chaffinch, while the largest decreases in adult catches were recorded for Linnet, Redpoll, Spotted Flycatcher, Yellowhammer, Reed Bunting and Willow Warbler (Peach, W.J., Baillie, S.R. & Balmer, D.E. 1998. Long-term changes in the abundance of passerines in Britain and Ireland as measured by constant effort mist-netting. Bird Study 45, 257-275)

As an example, here we show the trends in the abundance of adults for Linnet, Long-tailed Tit and Wren.

Linnet

Adult numbers of Linnets on CE sites show a  worrying decline, generally consistent with those shown by other BTO schemes; for example, the Breeding Bird Survey (BBS) shows that the breeding population has been declining since 1977.  Linnet is already on the Birds of Conservation Concern Red List (high concern).   Overall, Linnets numbers underwent a large decline between the mid-1970s and mid-1980s, probably due to a loss of weed seeds, but may have recovered since that time, perhaps due to the beneficial effects of set-aside. More recently, oil-seed rape has probably helped to compensate for losses of traditional foods.  But CES catches of both adults and juveniles suggest that the decline of Linnets in scrubland continues.

Long-Tailed Tit

The population fluctuations shown by Long-tailed Tit are likely to be due mainly to variation in  winter weather conditions.  The trend produced from the Breeding Bird Survey shows the effects of the very cold winters in the 1970s and early 1980s.  CES information from 1983 onwards shows a period of recovery following a series of mild winters.  In recent years,  the population has tended to increase, perhaps driven by increases in breeding success.

Wren

Like the Long-tailed Tit, winter weather is the major determinant of population changes in Wrens in Britain.  Even a short spell of winter weather that is severe enough to prevent efficient feeding results in high mortality.  Throughout the 1980s and early 1990s, catches of adult Wrens have fluctuated markedly, suggesting  recoveries from cold winters until reduction again in the next hard spell.

Breeding success

We have recently started work to develop rigorous methods of indexing productivity (breeding success) from CES data.  CE sites provide unique information on productivity, and until now these data have not been used to the full.  Information on individual nesting attempts from the Nest Record Scheme permits a detailed investigation of success at various stages of the breeding cycle for many species, but cannot provide information on the number of breeding attempts.  Productivity measured by CES integrates success (or failure) across the whole breeding cycle, including all breeding attempts and early post-fledging mortality.  We are currently producing trends in productivity for all CES species for the period 1983 to 1998 using generalised linear modelling methods.  These will allow influences of short-term weather variation on productivity to be controlled, so that true long-term trends are revealed.  The results should be available by mid-2000.

Song-Thrush and Blackbird

The long-term trends in productivity for Blackbird and Song Thrush, two ecologically similar species, are very similar.  Productivity was low for both species in 1984, 1986, 1988-90 and for Song Thrush only in 1995.  The summer weather in these years was characterised by high temperatures and drought conditions in some areas (see graph).  Productivity tended to be better in the wetter summers (eg 1985, 1991 and 1993).  Sustained dry summer weather might reduce the availability of earthworms and other important foods of young thrushes.

Survival

The Sedge Warbler is a common breeding species throughout most of Europe, wintering south of the Sahara Desert in West Africa (see Migration Atlas).  In Britain, numbers of Sedge Warblers breeding in farmland and riparian habitats fluctuate markedly from year to year but declined by about two-thirds between the mid-1960s and mid 1980s.  Survival rates of adult Sedge Warblers were estimated for the period 1969-1984 using mark-recapture data collected at two long-running CE sites in southern England (Marsworth Reservoir, Hertfordshire and Wicken Fen, Cambridgeshire).  (Peach, W.J., Baillie, S.R. & Underhill, L. 1991. Survival of British Sedge Warblers Acrocephalus schoenobaenus in relation to west African rainfall. Ibis133:300-305). The results of the investigation showed that fluctuations in the population levels and annual adult survival rates of British Sedge Warblers since the late 1960s are strongly correlated with indices of wet season rainfall in the West African winter quarters, the higher the rainfall in West Africa the greater the adult survival rate (graph).  This suggests that the continued expansion of the Sahara Desert due to droughts, land drainage and over-grazing is a threat to our Sedge Warblers, as well as to populations of many trans-Saharan migrants.

How to get involved

A newsletter, CES News, is produced on an annual basis and is sent to all ringers involved with the scheme.  For further information about the CES Scheme, please contact the at the BTO.

The Constant Effort Sites Scheme is funded by a partnership of the BTO, the JNCC (on behalf of Natural England, Scottish Natural Heritage, the Countryside Council for Wales, and also on the behalf of the Environment and Heritage Service in Northern Ireland), Duchas the Heritage Service - National Parks and Wildlife (Ireland) and the ringers themselves.

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