Reference

David J. C. Rodrigues, Antonio M. D. Fabiao, Maria E. M. A. Figueiredo, Paulo J. Q. Tenreiro (1999/2000): Migratory status and movements of the Portuguese Mallard (Anas platyrhynchos) – Vogelwarte - Zeitschrift für Vogelkunde – 40_1999: 292 - 297.