Abstract

Mediterranean Gulls Ichthyaetus melanocephalus usually position their nests close to water, just above the high-water mark, and are thus vulnerable to flooding caused by extreme water levels and tidal surges at coastal locations. This short paper describes long-term productivity and causes of failure in relation to habitat choice of Mediterranean Gulls during their colonisation of the Venice Lagoon, northeast Italy, in the last decade. During the study period, huge colonies of nesting Mediterranean Gulls have occurred in the area in some years, while there have been no nesting pairs at all in others. Overall, we found nine colonies at three sites in seven different years. The mean colony size was 618 (±926) pairs (range 1–2,512). Most colonies (7 out of 9, 78%) were found on marshy islands, while two were found on artificial islands. Overall fledging success was low, with 650 chicks fledged from 4,329 nests during the ten-year study period, which corresponded to 0.15 fledglings per nesting attempt. This was due to catastrophic failures, caused by extreme high tides flooding most nests/clutches. Our work provides evidence of the risks that waterbirds nesting on saltmarshes are facing due to unprecedented increases in sea-levels and extreme weather events, and shows that these are leading to serious declines in reproductive success in consecutive breeding seasons. Introduction The breeding range of the Mediterranean Gull Ichthyaetus melanocephalus is restricted to the Western Palearctic, from the Atlantic to the Urals. During the second half of the twentieth century, the population increased exponentially in Europe (Rudenko 1996; Ardamatskaya 1999), with the species spreading westwards and colonising central, northern and particularly western Europe (Bekhuis et al. 1997). The European population was recently estimated at 128,000–203,000 individuals (BirdLife International 2021). Despite the range expansion and increase in numbers in Europe, the global population is decreasing overall. In Italy, the species colonised wetlands along the Adriatic coast in 1975, rising to 25 pairs in 1978 and then showing rapid population growth, reaching 2,500–4,000 pairs in 2013–18 (Brichetti & Fracasso 2018). The species was first recorded breeding at the Venice Lagoon in 1996, when one pair settled within a Sandwich Tern Thalasseus sandvicensis colony (Scarton et al. 1996). In the following years, breeding by isolated pairs was irregularly recorded at different sites across the lagoon, until 2012, when a large colony of hundreds of pairs settled on the site that had been used for the first breeding attempt in 1996 (Scarton & Valle 2015).