Kotrschal, Kurt, Josef Hemetsberger, and John Dittami. 1992. “Vigilance in a Flock of Semi-Tame Greylag Geese Answer Answer in Response to Approaching Eagles Haliaeetus Albicilla and Aquila Chrysaetos”. Wildfowl, January, 215-19. https://tidsskrift.dk/Wildfowl/article/view/155354.