Monitoring of some species of colonial nesting Ardeidae at Lake Trasimeno (Central Italy).

During 1993-2010 we have monitored at Lake Trasimeno the population of colonial nesting herons, with the exception of Ardea purpurea. Two distinct heronries have been found, placed 9 km away from each other and composed of the following species: Nycticorax nycticorax, Ardeola ralloides, Bubulcus ibis, Egretta garzetta, Ardea cinerea, Casmerodius albus (the last perhaps not breeding). We have usually carried out a total census of the nests and assessed the proportions of the different heron species on a sample of visible adults. Nycticorax nycticorax and Ardeola ralloides have decreased, whilst Ardea cinerea and Bubulcus ibis have shown positive trends. The cumulated breeding population has shown a strong decrease, followed by a recovery. The population size of the last year (298 pairs) is of great importance in Central Italy, accounting for 10% of the heron population of the whole region.