Effects of Clutch Size and Timing of Breeding on Reproductive Success of Greater Rheas

Greater Rheas (Rhea americana) have an unusual breeding system that com- bines harem-defense polygyny, communal egg laying, and exclusive male parental care. We studied survivorship of eggs and nests in relation to clutch size and timing of breeding and estimated the reproductive success of males and females. The number of eggs in a nest varied from 8 to 56, although in most cases it was between 20 and 30. Nests with more than 30 to 35 eggs had longer laying periods, lost more eggs during incubation, and had lower hatch- ability. As a result, the number of chicks hatched per nest increased with clutch size up to 30 to 35 eggs but decreased for larger clutches. More than 65% of the nests were deserted during incubation, and only 5 to 6% of the male population bred successfully each year. Nest- ing success was inversely correlated with rainfall during the breeding season and positively correlated with time of the breeding season. Chick survival during the first months after hatching averaged 60%. We suggest that the proportion of males that are able to attempt breeding is limited by energetic constraints. Received 16 December 1996, accepted 21 August