1. M. H. Avé (), B. Voslamber and J. Stahl, Sovon Dutch Centre for Field Ornithology, PO Box 6521, NL-6500 GA Nijmegen, the Netherlands.
2. C. A. Hallmann, Inst. for Water and Wetland Research, Radboud University, PO Box 9010, NL-6500 GL Nijmegen, the Netherlands