Each year CES ringers are asked to make a standard number of visits to their site spread evenly during the breeding season, broadly early May to late August although this differs between countries, to account for different periods of breeding and migration. The same set of mist nets are erected in the same positions on each of the main visits. The total length of standard netting is decided by the ringer, but is typically 100-200m. Ringers choose the duration for which nets are operated but should maintain this same effort across years, although effort may vary across the season, as dawn gets earlier. A typical regime (in Britain) would be to begin netting at dawn and continue until around 1100hrs on each of twelve visits. Some schemes permit extra flexibility to erect additional nets during these main visits, and also extra visits.
Most constant effort sites are located in either wet or dry scrub, reedbeds or deciduous woodland, where catching rates are generally reasonable (although catching on some days can still be very slow!); in some countries, gardens are also used. Sites with vegetation that is likely to change substantially across years, and hence cause changes in catching efficiency and the local bird community are generally discouraged, and if sites change substantially between years (for example, because of habitat clearance or alteration) they should be re-registered as a new site. The aim should be to have a number of sites with a good geographical spread across the country (although the number of sites varies between countries depending on the number of ringers). In most schemes there is some level of habitat recording, so that some idea of the representativeness of sites can be formed, and successional (and other) changes in vegetation structure and composition can be quantified.
Data collection and routine analysis
A certain minimum amount of information should be collected for each bird: ring number, species, age, sex (where appropriate) and, preferably, breeding status (for example extent of brood patch in females). Recaptures of individuals both between visits in a season, and between seasons are vital and must be recorded; multiple recaptures on the same visit can permit additional analysis but are not routinely recorded by most schemes. Additional information on biometrics, moult or other indicators of bird condition can usefully be collected and broaden the scope of possible analyses.
Using changes in standardized catch sizes we are able to measure long-term changes in the abundance of adult and juvenile birds. Long-term changes in catches of songbirds are of much greater conservation interest than annual fluctuations, which may be linked to particular weather events, for example. The indices of abundance and productivity generated by CES ringing have been compared to those other schemes, especially in Britain where many years data are available, to assess their validity. For most species long-term changes in the number of adult birds caught on CES sites are very similar to changes in the numbers of territories counted on Common Birds Census (CBC) plots, suggesting that such standardised mist-netting is a reliable method for assessing extensive changes in songbird populations (Crick et al. 2004). Similarly, 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).
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, at least across sites, mark-recapture data from individual sites are often quite sparse and survival estimates for single sites often lack precision (Peach 1993).
An R package, cesr, is avaialble to assist in the analysis of CES data.