Was poor breeding productivity of African Black Oystercatchers on Robben Island in 2004/05 caused by Feral Cats, Kelp Gulls, Mole Snakes or the Sumatra tsunami?

Although the breeding population of African Black Oystercatchers on Robben Island, South Africa, increased from 2001/02 to 2003/04 and past breeding productivity there has been good, nesting success in 2004/05 was disastrously low. Sixty-one pairs laid 120 eggs of which only 15% hatched; of the chicks only a third (six) resulted in fledged young. This equates to 0.01 fledglings per pair, much lower than the estimated 0.33 fledglings per pair needed to maintain stable populations. We consider whether this was due to predation by Feral Cats, Kelp Gulls or Mole Snakes or nests being washed out by the large waves arising from the Sumatra tsunami. We conclude that predation by Feral Cats, whose population increased from about six in 1999 to 100 in 2004, is the most likely cause.