Moving to Venice and settling there: a synthesis on the biology of Sandwich Tern (Thalasseus sandvicensis) in the largest Mediterranean wetland during the 1995-2023 years

In 1995, the first colony of Thalasseus sandvicensis settled in a saltmarsh islet in the Lagoon of Venice, NE Italy. The number of nesting pairs rose from 202 (1995) pairs to 3503 (2023), with the arrival of immigrants peaking in 2014 (+887 pairs) and 2023 (+1884). From 1995 to 2023, the number of breeding pairs showed a moderate increase (p<0.01), with a yearly rate of +5.7%. Two kinds of colony sites were used: i) salt marsh islets, where birds nested mostly on windrows. These sites were usually in well-secluded area, with low levels of human disturbance; ii) man-made sites such as dredge islands, with nests placed on the bare ground. These man-made sites, in one case located at about 200 m from the town of Venice, were used only since 2014 and now support a large fraction of the breeding population. Overall, 73 colonies were found (361±423 pairs, range: 5-2762; median: 203). Each year there were one to six colonies; 21 colony sites were used at least once. On average, a site was used for 3.5±5.4 years (range 1-25), but the first site was used 25 years out of 29. The turnover rate was overall 41.4%. Over the study period, the biggest cause of clutch loss was flooding during extreme high tides, which are becoming more and more frequent and responsible for more than 90% of losses until 2022. In 2023, an H5N1 avian influenza outbreak annihilated the productivity of the whole nesting season. Clutch size was 1.7±0.5 (n=1338) without differences between saltmarsh and dredge islands: 1.64±0.5 vs 1.81±0.45 eggs per clutch. At the colonies, the association between T. sandvicensis and Larus ridibundus was strong and moderate with Sterna hirundo; instead, a negative and moderate correlation was found between T. sandvicensis and Sternula albifrons. The persistence of the largest Italian population appears threatened by several factors, among which the multiple effects of climate change are the most dangerous.