In the city of Osnabruck, Lower Saxony, Germany, and its surroundings, at least four different rain call dialects are found. Up to now, two dialect borders have been surveyed by mapping, recording of calls and analysis by means of sonograms. The dialect borders have proved to be soft. On the borders two strategies of call patterns have been found: 1. Males switch between two different dialects. These males are called alternative mixed callers. 2. Males combine characters of two different dialects into one call. These males are called combined mixed callers. In a transect crossing a dialect border, the share of one dialect continuously increased while the share of the other dialect decreased to the same extent. This pattern could be produced by repeated learning every year in adult male Chaffinches according to their acoustical surroundings. This question is subject to further investigation.