Some information has been provided by Stefan and Els – we give you below a short translation of the news in the link sent by Sergio from AHSA:
Last May there were lots of Slender-billed Gulls (200 pairs) and Common Terns on El Pinet. The Slender-billed Gulls started nesting but at the end of May more and more nests were abandoned. On the 30th only 22 nests were occupied. A Yellow-legged Gull landed in the neighbourhood of the nests, remarkably no response from the breeding birds followed. Young birds were not seen. Yesterday all the Slender billed had left the place, an occasional over flying one was chased by the remaining Avocets. The Common Terns on that island (17 pairs) had also left, and with them the most of the about 30 pairs on the islands near the parking site. There also were seen two bodies of Common Terns. Several explanations are given – low water discouraged starting the breeding process; Yellow-legged Gulls can be very glutinous with young birds as was observed at La Mata in the past breeding seasons, though no direct attacks have been observed in the El Pinet case; possibly a dog attacked the birds near the parking site, footprints of a dog were found at the waterside. The dead Terns are now being investigated by the Centro de Recuperación de Fauna. AHSA will present a letter to the Consellería about what has happened, and ask for measures after the current breeding season. The good news is that Slender billed Gulls on the Island at La Mata are doing very well this year!