Breeding parameters of Yellow-legged Gull Larus michahellis in the largest Moroccan colony: Natural versus urban nesting sites

We investigated the main differences in the breeding ecology of Yellow-legged Gulls Larus michahellis nesting in urban (city of Essaouira) and natural (Mogador Island) environments on the African Atlantic coast of west-central Morocco during the 2018 and 2019 breeding seasons. This Yellow-legged Gull population is the largest in the southern portion of this species' range, making it excellent for research on the breeding biology of marginal populations. Over two years, we collected data on nest density, chronology, clutch size, egg dimensions, and hatching success from nests across the city and island. Results showed that nest density averaged 1.26 ± 1.03 nests/ha (126 ± 103 nests/km2) in the urban areas and 442 ± 254.56 nests/ha in the natural areas. The laying period was significantly longer at Mogador Island, lasting more than 20 weeks on average and occurring in two egg-laying peaks. Hatching success (number of hatched eggs per nest) in the whole study area averaged 86.04% ± 23.01% and was similar between seasons and habitats. Fledging success (number of chicks surviving over 40 days per hatched egg) and breeding success (number of chicks surviving over 40 days per laid egg) in the urban area averaged 54.98% ± 23.93% and 46.52% ± 22.01%, respectively. In both natural and urban habitats, the high hatching success rates, which are the highest values reported among North African colonies, may explain the rapid growth of this population.