Measuring passerine productivity using constant effort sites: the effect of missed visits

Site visits for the Constant Effort Sites (CES) ringing scheme are occasionally missed and the standard 12 scheduled annually (May–August) may not all be completed. To account for this, total annual adult and juvenile passerine catches may be adjusted for inclusion in CES productivity-indexing analyses. Adjustment methods and the inclusion of adjusted catches are thought not to generate unrepresentative measures of productivity, but we aimed to test this. Productivity-indexing analyses were carried out using CES data with and without adjusted catches, and long- and short-term changes in productivity were compared. Similarity between productivity indices (and also the precision of estimates) was consistently high between adjusted and unadjusted data, but greatest for species caught most frequently. The inclusion of data adjusted for missed visits increases the precision of measures of productivity by increasing sample sizes, although this improvement is likely to be exaggerated as no account is taken of the uncertainty in the data adjustment. We consider adjustment methods to be appropriate, despite a great potential for between-year variation in the seasonal pattern of catches on CE sites.