How many observation days are needed to reliably describe bird migration?

Abstract

The total number of migrating birds is needed, for exam-ple, to estimate collisions risk as part of the assessment process for wind-power parks. This estimation is often based on a small sample of observation days, making estimates uncertain. To determine the number of days needed to obtain reliable figures we used observations of migrating birds obtained from the Signilskär bird obser-vatory, Åland, Finland during the autumn seasons from 2009 to 2013. We sampled 1–14 days during each 14 day time window and used linear extrapolation and Poisson regression with weather covariates to estimate the total number of migrating birds and distribution of estimates. We calculated the head-wind and side-wind components according to the species’ most common flight directions. We found that observations should cover at least 90% of the migration period to obtain precise results and 70–90% to obtain results sufficiently reliable to use in practice. Linear extrapolation is the best model for small samples. The larger the proportion of days observed, the better is Poisson regression with weather covariates.