EURING Newsletter - Volume 2, December 1998

The potential for co-ordinated projects carried on by large numbers of ringers over wide geographical areas represents one of the main features of the EURING community. In recent years, constant mist-netting has been shown to be a very useful tool to monitor bird populations on the basis of mark-recapture analyses. In this article, Dawn Balmer and Will Peach illustrate the aims, organisation and results of the British CES scheme. This very interesting approach has already been introduced in the States through the local MAPS project, and might soon become the basis for a larger EURING initiative.

CONSTANT EFFORT RINGING IN BRITAIN AND IRELAND

By Dawn Balmer (1) and Will Peach (2)

(1) BTO, NATIONAL CENTRE FOR ORNITHOLOGY, THE NUNNERY, THETFORD, NORFOLK, IP24 2PU, UK (Email: dawn.balmer@bto.org)
(2) PRESENT ADDRESS: RSPB, THE LODGE, SANDY, BEDS SG19 2DL, UK (Email: will.peach@rspb.org.uk)

Background

The British Trust for Ornithology's Constant Effort Sites (CES) Scheme uses standardised mist-netting at more than a hundred study sites to monitor the populations of a range of common passerines. The scheme was initiated in 1983 as a pilot project with a volunteer organiser. Following an evaluation of the scientific potential of the CES Scheme (Baillie et al. 1986) the BTO took over full responsibility for the project. The popularity of the scheme has continued to grow and in 1996, 120 sites were operated throughout Britain and Ireland.

The CES Scheme monitors:

• changes in adult and juvenile abundance
• productivity (proportion of young birds in the catch)
• adult survival rates (from between-year recaptures).

The CES Scheme forms part of the BTO's Integrated Population Monitoring (IPM) Programme, which incorporates the Common Birds Census, Breeding Bird Survey, Nest Record Scheme and national Ringing Scheme, and aims to identify the causes (natural and anthropogenic) of population changes affecting breeding birds in Britain and Ireland.

Methods

Each year CES ringers are asked to make twelve visits to their site spread evenly between early May and late August. A set of standard mist nets is erected in the same positions on each of the twelve main visits. The total length of standard netting is decided by the ringer and is typically 100-200m. Ringers are asked to operate their set of standard nets for at least six hours on each visit and to standardise their chosen duration. A typical regime would be to begin netting at dawn and continue until 1100hrs on each of the twelve visits. The scheme has some flexibility and permits additional nets to be used during main visits, and also extra visits.

Most constant effort sites are located in either wet or dry scrub, reedbeds or deciduous woodland. Sites in coniferous woodland are not accepted because rapid tree growth is likely to cause short-term changes in catching efficiency and the local bird community. Potential constant effort sites are proposed by volunteer ringers, and are generally accepted into the scheme as long as they are located in suitable habitats and are not considered to be undergoing major successional changes. A quantitative system of habitat recording was introduced in 1995 and all CES ringers are asked to record habitat in the first year they operate their site, and then at regular intervals thereafter.

Data collection and routine analysis

For each bird trapped on a CE site the following information is computerised: ring number, species, age, sex (in the case of adults), date(s) of capture and an additional net code to indicate whether the bird was trapped in a standard CES net or in an additional net. No information on biometrics, moult or brood patch is currently collected, although this may be collected by the ringer.

CES capture data are submitted either on paper forms or on computer disc. Paper submissions are computerised and checked by BTO staff. A package of computer programs for ringers (B-RING) enables ringers to computerise their ringing and recapture data and to carry out most of the paperwork required for administrative purposes. In 1996, data from 70% of CE sites were submitted on floppy disc.

Between-year changes in the catches of adults and young, and in the percentage of young birds in the overall catch are published annually in BTO News (Balmer & Peach 1997). The scheme's newsletter CES News, is sent out each year to all CES ringers and presents recent results and research.

Trends in adult and juvenile abundance

Using changes in standardized catch sizes we are able to measure long-term changes in the abundance of adult and juvenile birds. Long-term declines in catches of songbirds are of much greater conservation concern than annual fluctuations linked to particular weather events. For most species long-term changes in CES adult catches are very similar to changes in the numbers of territories counted on Common Birds Census plots, suggesting that standardised mist-netting is a reliable method for assessing extensive changes in songbird populations. In Britain & Ireland the largest increases in adult catches have been for Robin, Wren, Greenfinch and Long-tailed Tit and the largest decreases have been for Linnet, Redpoll, Spotted Flycatcher, Yellowhammer, Reed Bunting and Willow Warbler. Examples of trends in CES catches are presented for Willow Warbler and Reed Warbler (Fig. 1).

Trends in productivity

The percentage of juveniles in the catch is a good indicator of overall breeding success. Several intensive studies have shown that annual catches of young birds in CES mist-nets do accurately reflect local breeding success (e.g. du Feu & McMeeking 1991). Long-term trends in the proportion of juveniles in CES catches are presented for Long-tailed Tit and Dunnock (Fig. 2).

Poor weather during May 1996 resulted in the poorest breeding season ever recorded by CES ringing, particularly for Dunnock, Robin, Chiffchaff, Long-tailed Tit, Great Tit and Blue Tit.

Adult survival rates

Many passerines show strong site fidelity to breeding sites in successive breeding seasons, so regular sampling of breeding birds can be an effective means of generating between-year recaptures which can be used to estimate survival (return) rates of adult birds. Mark-recapture data from single CE sites are often quite sparse and survival estimates for single sites often lack precision (Peach 1993). Recent advances in analytical techniques allow mark-recapture data to be pooled across CE sites which usually leads to considerable improvements in precision. Examples of adult survival rates from combined datasets are shown in Table 1.

Table 1. Adult survival rates calculated from retraps at CE sites.

Using CES data we have been able to demonstrate clear relationships between survival rates of some long-distance migrants and annual rainfall in the African winter quarters (Peach et al. 1991) and between survival rates of resident species and winter weather (Peach et al. 1995a).

Perhaps the best example of how retrap data from CE sites can be used is for the Willow Warbler (Peach et al. 1995b). Until the late 1980s the Willow Warbler was one of the most stable breeding species monitored by the BTO Common Birds Census. However, during the early 1990s there was a dramatic reduction in the numbers of breeding birds counted on CBC plots. The decline was much more severe in southern Britain (47% decline) than in northern Britain (7% decline). A detailed analysis of nest record cards showed that there were only minor changes in the breeding success of Willow Warblers during this period, and these were far too small to account for the huge reduction in breeding pairs. Annual survival rates of adult Willow Warblers were calculated from retraps at 18 constant effort sites and one colour-ringing study. The results showed that in northern Britain, survival rates of adult birds averaged 39% and did not change during the period 1987-93. But in southern Britain survival rates of adults crashed from a healthy 45% in 1988 to just 24% in 1992 (Fig. 3). This huge decline in survival coincides with, and probably caused, the population decline in southern Britain. When survival rates improved in 1993, the number of breeding birds on CBC plots stopped declining.

Increased mortality amongst adult Willow Warblers suggests that problems in the African winter quarters have probably caused the large decline in breeding birds.

Other schemes

CES-type ringing projects are now well established in France (Vansteenwegen 1997), Finland (Haapala & Saurola 1995), The Netherlands and particularly in North America where there are currently more than 400 (MAPS) sites (Burnham 1995). We believe the scientific credibility of constant-effort mist-netting is now well established and we urge other ringing schemes to seriously consider setting up similar monitoring projects.

A national scheme would not need 100 sites to be useful; 15-30 sites could provide high quality information for a range of species.

Further information on the BTO CES Scheme (instructions, articles and newsletters) are available from Dawn Balmer at BTO, The Nunnery, Thetford, Norfolk, UK, IP24 2PU. (e-mail dawn.balmer@bto.org).

Further Reading

BALMER D. & PEACH W. 1997. Population changes on Constant Effort Sites 1995-96. BTO News 208.

BURNHAM K. 1995. North America goes CES. BTO News 199.

DU FEU C. & MCMEEKING J. 1991. Does constant effort netting estimate juvenile abundance? Ringing & Migration 12: 118-123.

HAAPALA J. & SAUROLA P. 1995. Constant Effort Sites Schemes in Finland 1992-94. Linnut 3: 32-33.

PEACH W. J. 1993. Combining mark-recapture data sets for small passerines. In: Lebreton, J.-D. & North, P.M. (Eds.) Marked Individuals in the study of bird Populations, pp. 107-122 (Birkhauser Verlag, Basel, Switzerland).

PEACH W.J., BAILLIE S.R. & UNDERHILL L. 1991. Survival of British Sedge Warblers Acrocephalus schoenobaenus in relation to West African rainfall. Ibis 133: 300-305.

PEACH W., DU FEU C. & MCMEEKING J. 1995a. Site tenacity and survival of Wrens and Treecreepers in a Nottinghamshire wood. Ibis 137: 497-507.

PEACH W, CRICK H. & MARCHANT J. 1995b. The demography of the decline in the British Willow Warbler population. Journal of Applied Statistics: 22, 905-922.

VANSTEENWEGEN C. 1997. Monitoring breeding bird populations by capture in France. In The use of mist-nets to monitor bird populations (eds C.J. Ralph & W.J. Peach). Proceedings of a technical workshop held at Point Reyes Bird Observatory, California. US Forestry Service. In press.

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