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

FERNANDEZ, GJ; JUAN C. REBOREDA

AUK

Año: 1998 vol. 115 p. 340 - 348

Greater Rheas (Rhea americana), have an unusual breeding system which combines harem defense polygyny, communal egg laying and exclusive male parental care. We studied the survivorship of eggs and nests in relation to clutch size and time of the breeding season and we estimate the reproductive success of males and females. The study was conducted from 1992 to 1996 in a wild population of 500 individuals in the province of Buenos Aires, Argentina. 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 30-35 eggs had longer laying periods, lost more eggs during incubation and had a lower hatchability. As a result of this, the number of chicks hatched per nest increased with clutch size up to 30-35 eggs but decreased for larger clutches. More than 65% of the nests were deserted during incubation and only 5-6% of the male population bred successfully each year. Nesting 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 was on average 60%. We discuss possible energetic constraints that could explain the low proportion of breeding males observed.