Purple Heron (Ardea purpurea) Science Article 2
The breeding population of Purple Heron Ardea purpurea in Mediterranean France showed a general decline since 1982 with a further reduction in number of breeding pairs in the early 1990s that has been reversed since 1995. In addition to this general decline, the population showed large annual fluctuations in size. We studied colony dynamics to determine which type of colony contributes most to the annual variation in total population size. Almost all (98%) annual variation in population size could be explained by the number and size of occupied colonies in three clusters with discrete dynamics, a cluster of large and stable colonies, one of intermediate-sized unstable colonies and a rest cluster of non-specified colonies. In general, colonies had high turnover rates and were very variable in size from year to year. Large colonies of stable presence played a pivotal role in the total population size. However, the hypothesis that when some local ecological factor caused a large site to be unavailable groups of birds moved to other, smaller and less reliable sites was not supported.
Deerenberg C. & Hafner H., ARDEA 87 (2): 217-226.