Comparison of population indices derived from theFinnish Constant Effort Site and National BirdMonitoring Scheme data

Piha M., Valkama J., Väisänen R.A., Saurola P.

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We studied the concordance of population trends and indices between data from (1) the Finnish Constant Effort mist-netting Scheme (CES) and (2) line transect and point count based National Bird Monitoring Scheme (NMS). Population indices of nine common passerines between 1987-2006 were calculated with log-linear Poisson modelling (TRIM). The concordance of trends and population indices between the two monitoring schemes was studied by including the scheme as a covariate in the models. In general, the overall 20- year linear trends were rather similar between CES and NMS data across species, although the magnitude (but not sign) of trends differed in two species. On the other hand, when the short-term fluctuations were included in the comparisons, time-series of fve species (out of nine) showed signifcant differences between the two monitoring schemes. These fndings highlight that although long-term trends may usually be coherent, two data-sets may give different results if subjected to detailed analyses of between-year changes.