In the context of an extensive study on bird migration in the Austrian Alps, we compared data from a fixed-beam radar with data collected by visual observation to estimate the intensity of migrating birds up to a height of 150 m above the ground. Migration traffic rates calculated from visual bird counts and radar measurements were strongly correlated. Using resampling techniques, we calculated a minimum observation effort for counting birds during a 5-week period of peak migration. We chose ±20% of the real mean as the criterion for a reliable estimation. Our results showed that at least 19 observation days are necessary to assess the mean number of migrants passing within this low altitudinal range.
