Grey Heron (Ardea cinerea) Expansion Promotes the Persistence of Mixed-Species Heron Colonies

Abstract

Range expansion of native as well as exotic species often causes effects on the original species; however, not all range expansions are negative for breeding colonies consisting of mixed species. The Grey Heron (Ardea cinerea) has increased and expanded its distribution in Japan in the last 20 years. Its solitary and early nesting habits may facilitate establishment and persistence of colonies of mixed heron species. To examine whether the addition of Grey Herons has affected colony persistence and the number of established and abandoned colonies, 16 years of monitoring data from before and after the expansion of Grey Herons around Ibaraki Prefecture in Japan were analyzed. Most newly established colonies were without Grey Herons before their expansion, but more than half of the established colonies contained Grey Herons after the expansion. The number of colony abandonments decreased after the Grey Heron expansion and most of these occurred in colonies that were without Grey Herons both before and after the expansion. Persistence periods were longer in colonies with Grey Herons than in colonies without Grey Herons. The expansion of Grey Herons into mixed heron colonies has promoted the persistence of mixed heron colonies, and local populations of colonial herons seemed to gain the benefit of colony sustainability.