1. H H Wills Physics Laboratory, University of Bristol, Tyndall Avenue, Bristol BS8 1TL, UK.
2. Avian Demography Unit, Department of Statistical Sciences, University of Cape Town, Rondebosch 7701, South Africa.
3. Marine and Coastal Management, Department of Environmental Affairs and Tourism, Private Bag X2, Rogge Bay 8012, South Africa.
4. Robben Island Museum, Robben Island 7400, South Africa.