Citizen science data confirm that expanding non-breeding distributions of goose and swan species correlate with their increasing abundance

Abstract

We combined data from two independent Danish citizen science time-series to describe changes in (1) abundance and (2) distribution of 12 wintering populations of geese and swans and tested the hypothesis that increases in national abundance since 2003 correlated with an expansion into formerly unoccupied winter farmland habitat. Five populations showed significant increases in national abundance, two declined and the remainder showed no significant trend over the same period; nine populations (including the five nationally increasing and two nationally declining) showed significantly positive correlations between annual abundance and distribution range size, including one stable population and one showing only local expansion. These results support our prediction that the five key increasing goose and swan populations benefiting from farmland exploitation are showing the greatest correlation between changes in abundance and wintering range in Denmark. This implies that continued growth in abundance in these populations will lead to continued expansion in their ranges, which will not only increase the potential for agricultural conflict with increasing abundance, but importantly also the geographical extent of such conflicts.