Nesting phenology and breeding performance of the Slender-billed Gull Chroicocephalus genei in Sfax salina, Tunisia

Abstract

In 2010, nine colonies of Slender-billed Gull Chroicocephalus genei bred at Sfax salina in south-eastern Tunisia totalling 7 912 breeding pairs. This amounted to >50% of the breeding population for the entire Mediterranean region. Colony initiation and egg laying occurred between late April and late May, with two distinct nesting peaks. Colony size ranged from 21 to 5 850 breeding pairs and was not correlated with the area of the breeding site or with laying date. The modal clutch size was three eggs for early colonies and two eggs for later ones. Egg volume was 37.5 cm3 on average, with first-laid eggs generally larger than those that followed in the same clutches. Hatching success, estimated by the Mayfield method, was lower in early colonies (37%) than later ones (59%), but birds nesting early fledged 0.85 chicks pair−1 compared to 0.45 chicks pair−1 by late breeders. Breeding performance was negatively affected by nest and chick predation, mainly by stray dogs Canis familiaris and Yellow-legged Gulls Larus michahellis.