Laske V. & Helbig A. (1987) Influence of atmospheric turbidity on counts of visible migration and a method of correction. ARDEA 75 (2): 245-254

geen Nederlandse samenvatting beschikbaar op internet This study is based on records of a long term investigation of visible autumn migration at a topographically uniform inland site in Northwest Germany. The influence of varying horizontal visibility on systematic counts of diurnal migration is described quantitatively by means of hyperbolic functions. With the help of these functions correction equations are derived which only differ by a species-specific coefficient: the ratio of two parameters determined empirically for 15 different species. Results confirmed the assumption that counts of large birds or typical flock migrants are considerably affected by reduced horizontal visibility, whereas this effect is moderate to negligible in small and singly migrating species. Correction equations should apply to counts of visible migration in any other location except in places characterized by strong leading lines like sea coasts or on isolated islands and high mountain passes.