Evoluzione delle popolazioni di Laridae  e  Sternidae  nidificanti nelle Valli di Comacchio nel periodo 1977-2001

Brichetti P., Foscolo Foschi U.

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Population trends of Laridae and Sternidae breeding in the Comacchio Lagoon (NE Italy) between 1977 and 2001. The Comacchio Lagoon represents one of the most important breeding areas in the Mediterranean for Laridae and Sternidae and, at a national level, on area of concentration and propagation of several species of conservation interest. Censuses of nests were made on foot or by boat during peak colony occupancy, whereas the estimated size of the Larus michahellis population has been partly based on the count of adults showing reproductive behaviour. Between 1977 and 2001, 96897 breeding pairs were counted, with an average of 3876 pairs per year, of which 41636 belonged to four species of Laridae and 55261 to five species of Sternidae, excluding a single case of nesting by Hydroprogne caspia. In the first five years, there was a progressive increase in the Laridae and Sterindae population, followed by a long period of marked fluctuation which gave way to five to six years of clear decline, with evidence of a slight recovery in numbers in 2000-01. The negative trend, which was recorded as from 1993-95, may be related to environmental changes in the breeding habitats and to the marked increase and range expansion of Larus michahellis. Around 1991-92, a redistribution of the colonies occurred and, following this, the populations of some species, particularly Larus melanocephalus and Sterna sandvicensis, started to colonize new costal wetlands in the upper Adriatic Sea. Such changes may be directly related to the concomitant population increase of Larus michahellis. In the course of the last decade, the percentage in numbers of the local populations with respect to the national ones has overall decreased except for Larus genei and, above all, Larus michahellis, the only species to have shown a continuous increase from 1977-2001, which lead to the progressive colonization of the whole lagoon area.