The influence of nest location and the effect of predator removal on Cape Gannet Morus capensis egg predation by Kelp Gulls Larus dominicanus vetula

Abstract

The breeding range of the Cape Gannet Morus capensis currently extends to six of the ten islands formerly utilised by this species. The Cape Gannet is classified as an endangered species with a rapidly declining population. Since the mid-1950s, the global population has declined by 51% due to multiple causes, including egg predation by the Kelp Gull Larus dominicanus vetula. To assess the effect of this predation, we monitored 100 nests over an 11-week period in 2018 at the Lambert’s Bay colony in South Africa. To assess the effect of selective predator removal on the likelihood of predation, data were collected from 2006 and 2018. Our objective was to assess whether nest location and predator removal affected the likelihood of egg predation. The peripheral nests saw a higher level of egg loss (average 1.5 eggs per week) compared with the central nests (average 1 egg per week). Predator control of Kelp Gulls was implemented in 2015, 2017 and 2018. Between years, selective culling decreased the number of Cape Gannet eggs predated. However, within years, except for 2015, the predation rate on Cape Gannet eggs increased post-culling during the same year of implementation. In all three of the implementation years, predation started again (within 48 hours) after the culling commenced. Predation by this indigenous gull species is natural; however, certain human activities might have supported larger populations of Kelp Gulls leading to elevated predation intensity.