1. a Department of Ecology and Evolutionary Biology , The University of Tennessee , 569 Dabney Hall, Knoxville, TN, 37996, USA E-mail:
2. b Department of Wildlife Ecology and Conservation , University of Florida , Gainseville, FL, 32611-0430, USA E-mail:
3. c Percy FitzPatrick Institute of African Ornithology, University of Cape Town , Rondebosch, 7700, South Africa