Zeetrekgegevens nader bekeken

During its more than ten years existence, the Club van Zeetrekwaarnemers (CvZ, dutch seawatching group) has collected an extensive archive of data about the migration of birds along the Dutch coast. In this paper a first attempt is presented to obtain a clearer picture of the differences in migration over the years. The numbers of birds observed are influenced by a complex of factors, of which weather conditions are an obvious example. A statistical method is described to correct the data for a variety of these factors, listed in chapter 2, so as to obtain a more fair comparison of the migration intensity between the years of study (1974-85). The migration intensity is expressed as average numbers per hour. The method has been applied to a selection of the data, concerning the migration of Velvet Scoter, Bar-tailed Godwit and Little Gull, during the period April 23-May 6 over the years 1974-85, observed at the Hondsbossche Zeewering (Noord-Holland province). The results show that, indeed, correction leads to more or less substantial changes in the average number per hour of the years considered. Especially correction of the data for the combination of windforce and winddirection yields a remarkable concentration of the numbers around the overall average number per hour (i.e. the average number per hour over all years). The remaining differences between the years are mainly due to other factors then those registered by the CvZ. We mention ecological factors. The corrected data will be useful for research into the relation between these type of factors and the migration along our coast. For example, one may use these data to investigate what can be told about fluctuations of the population of the various species.