African Oystercatchers on Robben Island, South Africa: The 2019/2020 breeding season in its two decadal context

The African Oystercatcher Haematopus moquini is distributed along the coastline of southern Africa. Robben Island, near Cape Town, South Africa, holds about 8% of the global African Oystercatcher population. During the 2019/2020 austral summer (November–March), we monitored the breeding of oystercatchers on Robben Island at target intervals of six days. We counted 550 oystercatchers and we estimated there were 133 breeding pairs. We found a total of 158 clutches, of which 47% produced hatchlings; of a total of 288 eggs laid, 38% hatched. We estimated the earliest incubation start date was 5 November, and 90% of nests started incubation between 17 November and 10 February. The egg-laying activity had five peaks with a median interval of 16 days. Nests were found along the entire shoreline of the island, mainly in three clusters. Of all successful nests, 61% were located along the north side of the island, adjacent to the Kelp Gull Larus dominicanus breeding colony, while 61% of unsuccessful nests were in the southern part of the island, where 18 nests were predated by Mole Snakes Pseudaspis cana. This fieldwork was part of a long-term study that stretches over two decades. We consider how predation pressures have changed through time and concluded that Mole Snakes are now the dominant predator on oystercatcher nests. Hatching success, however, seems stable. The oystercatcher population on Robben Island has increased fourfold and the number of nests has almost doubled since 2001, fueled by the invasion of the Mediterranean Mussel Mytilus provincialis. The timing and the duration of the breeding season have not changed notably over the two decades. We recommend that there is no need to undertake any control of Mole Snake or Kelp Gull populations; however, ongoing monitoring of the breeding of African Oystercatchers on Robben Island is needed.