The Constant Effort Sites Scheme

2009 results

Following the two very poor breeding seasons in 2007 and 2008, this summer saw many species have a much more productive year. The preliminary results were released in December, and can be seen here.

The headlines results included:

Adult numbers

• 18 species showed a decline vs 2008 (this is a consequence of the previous two poor breeding seasons)
• Significant decline for Robin, Reed Warbler, Blackcap, Long-tailed Tit, Blue Tit, Great Tit and Linnet
• Lowest ever adult numbers for Blue Tit and Linnet
• Significant increase only for Whitethroat and Bullfinch

Productivity

• Significant increases (vs 2008) for 18 species
• Significant increases (vs long-term) for 15 species
• Highest ever productivity for Reed Warbler and Chaffinch
• Significant decrease only for Willow Tit (and lowest ever)

2007 and 2008 seasons

The previous two years were the worst we have seen since the start of the CES scheme, with productivity significantly lower than the long-term average for 11 of the 25 core species. The range of species affected was rather different in the two years though, and it looks like the timing of heavy rainfall in the breeding season was responsible for this. The results for the 2007 season featured in major newspapers in the autumn (The Times and Telegraph are shown to the right) and we hope the better news in 2009 will be equally popular! Full results for the last two years can be found in the relevant copy of CES News which can be downloaded here.

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 behind 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, higher quality information on adult survival per unit ringing effort is generated, compared with the low recovery rates from the general ringing.

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 increasing 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, 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  very 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 (being of 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, with productivity also in long-term decline.

Long-Tailed Tit

The population fluctuations shown by Long-tailed Tit are likely to be due 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 with the population increasing, though this has levelled off in recent years.

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, 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 are also able to produce an index of productivity (a measure of breeding success) from CES data. Information on individual nesting attempts from the Nest Record Scheme allows 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 (and 'whole season' productivity).  Productivity measured by CES integrates success (or failure) across the whole breeding cycle, including all breeding attempts and early post-fledging mortality.  We currently produce trends in productivity for all CES species 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.

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

If you are considering starting a new CES please first read the instructions and complete a registration form. For further information about the CES Scheme or setting up a new site, please contact . A newsletter, CES News, is produced on an annual basis and is sent to all ringers involved with the scheme. 

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.

Back to Ringing Home Page