Temporal and spatial differences in three-egg clutch frequency of the African Black Oystercatcher

Abstract

African Black Oystercatchers Haematopus moquini produce a modal clutch size of two eggs and only rarely lay three eggs. A review of over 4 000 nest records from across their breeding range (dating back to the early 1960s) revealed that three-egg clutches have become more frequent, rising from 2% of recorded clutches before 1975 to >5% since 2000. Three-egg clutches are found predominantly in the south-western Cape and are clustered at specific sites in the region. They are produced earlier in the season compared to smaller clutch sizes and rarely occur after January. As this increase in three-egg clutch frequency is site specific, it might result from either a phenotypically plastic response to local resource availability, or a localised change in breeding system potentially resulting in more than one female laying in the same nest. While increased food availability might be the cause of changes in three-egg clutch frequencies, this is unlikely linked to the spread of the invasive mussel Mytilus galloprovincialis. Further research is needed to identify the cause of this phenomenon and evaluate how this change may affect the breeding success of this species.