The Dutch Constant Effort Site (CES) programme has been operating since 1994 and is especially designed for the analysis of demographic parameters. Currently, it works with 40 active mist-netting sites and has a database with more than 250,000 records at its disposal. Here, we ask whether the effort invested in the programme is appropriate to detect temporal trends of productivity of passerine bird populations across The Netherlands. We specifically ask if less effort would produce the same results or if the number of CESs should be augmented. To evaluate these questions we used a resampling approach in which we decreased sample size in steps of five and iterated resampling 1000 times for each step, thus simulating CES programmes in which fewer sites were operated. For the twenty most abundant species and for each sample size, linear regressions of the productivity index (a logistic generalized linear model of the proportion of captured juvenile birds) on year were compared with the regression obtained using the complete records. The proportion of samples that yielded significant positive or negative slopes was determined, as well. For the majority of species without significant temporal trends (18), the proportions of significant slopes at smaller sample sizes were lower than 20%, indicating a moderate risk of committing Type I errors (detecting a negative or positive trend although productivity did not change over time). For the Garden Warbler Sylvia borin, which showed a significant positive productivity trend, the probability of committing Type II errors (not detecting existing trends) increased rapidly with decreasing sample size. We conclude that the Dutch CES programme works with a sufficient number of sites to detect reliable temporal trends for most of the more abundant passerine species. However, increasing the number of sites would allow for a more secure determination of productivity trends for those species that currently show ambiguous results (Reed Warbler Acrocephalus scirpaceus, Willow Warbler Phylloscopus trochilus, Willow Tit Parus montanus).
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