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BBWC Home > Contents > Methodology > Constant Effort Sites

2.6 Constant Effort Sites Scheme

The Constant Effort Sites (CES) Scheme uses changes in catch sizes across a network of standardised mist-netting sites to monitor changes in the abundance and breeding success of common passerines in scrub and wetland habitats. At each constant effort site, licensed ringers erect a series of mist nets in the same positions, for the same amount of time, during 12 visits evenly spaced between May and August. Year-to-year changes in the number of adults caught provide a measure of changing population size, while the ratio of young birds to adults in the total catch is used to monitor annual productivity (breeding success). By monitoring the abundance of young birds between May and August, the CES method should integrate contributions to annual productivity from the entire nesting season, including second and third broods for multi-brooded species, but will also include a small component of mortality during the immediate post-fledging period. Between-year recaptures of ringed birds can also be used to calculate annual survival rates of adult birds, although this requires specialised analytical techniques (e.g. Peach 1993) and is not considered further here. Further details of the CES Scheme are presented by Peach et al. (1996) and methods of analysis are detailed in Peach et al. (1998) for abundance measures and Robinson et al. (2007) for productivity measures.

The CES Scheme began in 1983 with 46 sites and now has around 120. The distribution of CES sites tends to reflect the distribution of ringers within Britain and Ireland. The majority are operated in England, and there are small numbers in Scotland, Wales, Northern Ireland and the Republic of Ireland. The CES routinely monitors the populations of 25 species of passerines in scrub and wetland habitats.

Data analysis

Smoothed trends in the abundance of adults and young are separately assessed using a generalised additive model (GAM), with 85% confidence intervals calculated by bootstrapping ( Fewster et al. 2000). At sites where catching effort in a year falls below the required 12 visits, but eight or more visits have been completed, annual catch sizes are corrected according to experience during years with complete coverage, by incorporating an offset into the GAM (see Peach et al. (1998) for full details). Sites with fewer than eight visits in a given year are omitted for the year in question.
Annual indices of productivity (young per adult) are estimated from logistic regression models applied to the proportions of juvenile birds in the catch, the year-effects then being transformed to measures of productivity relative to an arbitrary value of 100 in the most recent year. As above, catch sizes are corrected where small numbers of visits have been missed. It should be noted that these indices are relative, and are not estimates of the actual numbers of young produced per adult (Robinson et al. 2007).

Data are presented graphically with the smoothed trend in blue and their 85% confidence limits in green. A caveat is provided for 'Small samples' when the average number of plots per year is between 10 and 20.

Next section – 2.7 Nest Record Scheme

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This report should be cited as:
Baillie, S.R., Marchant, J.H., Leech, D.I., Joys, A.C., Noble, D.G.,
Barimore, C., Grantham, M.J., Risely, K. & Robinson, R.A. (2009).
Breeding Birds in the Wider Countryside: their conservation status 2008.
BTO Research Report No. 516. BTO, Thetford. (http://www.bto.org/birdtrends)

Pages maintained by Iain Downie, Mandy T Andrews and Laura Smith: Last updated 18.02.2009