The following is from Williams (1995), which draws in part from Boersma (1974). Calls include:
1) "donkey-bray", which consists of "several low introductory huffs followed by three, long, higher-pitched brays" (Williams 1995: 261). This call primarily is uttered during the breeding season, especially early in the breeding season when the call is given by males attempting to attract females to a nest site. This call also is given in agonistic interactions between males; females give this call only rarely. Donkey-bray calls are individually distinctive.
2) "courtship" or mutual bray. This call is given by both sexes, "during nest reliefs or at other times when birds are reunited at the nest; males may follow the courtship bray with the donkey bray" (Williams 1995: 261).
3) "haw" call, which functions as a contact call. This call is uttered both on land and in the water.