1. School of Biological Sciences, Royal Holloway University of London, Egham, Surrey TW20 0EX, UK
2. Department of Zoology, University of Cambridge, Downing Street, Cambridge CB2 3EJ, UK
3. FitzPatrick Institute of African Ornithology, DST-NRF Centre of Excellence, University of Cape Town, Rondebosch 7701, Cape Town, South Africa