The use of constant effort mist-netting to measure between-year changes in the abundance and productivity of common passerines

The Constant Effort Sites (CES) Ringing Scheme of the British Trust for Ornithology aims to provide annual measures of change in the abundance and productivity of common breeding passerines in scrub and wetland habitats in Britain and Ireland. Changes in the sizes of the annual catches, from a set of standard mist nets operated during 12 summer (May-August) visits, are combined across sites to produce estimates of the percentage change in adult and juvenile numbers. The proportion of juveniles in the catch is used as a relative measure of breeding productivity. Methods are presented for calculating standard errors of between-year changes in both adult and juvenile catches, and changes in the proportion of juveniles. Present levels of precision are summarized and predictions are made concerning likely improvements in precision from a larger CES Scheme. For most of the species considered there was little evidence that between-year changes in catches of adults at CE sites have differed between habitats, regions or according to coastal proximity. For several species there was evidence of consistently higher percentages of juvenile birds being captured at sires in wet habitats (mainly reedbeds and wet scrub) compared to sites in dry habitats (dry scrub and woodland). However, between-year changes in the percentage of juveniles caught were generally homogeneous across habitats and regions. There was some evidence that between-year changes in catches of Whitethroats Sylvia communis and Bullfinches Pyrrhula pyrrhula differed between coastal and inland sites. Implications of these findings for future analyses and interpretation of results from the CES Scheme are discussed.