Breeding

The nest is on the ground. The male selects a site, and tramples the surrounding vegetation with his feet. The male then sits on the site of the nest, rotating slowly in a circle while pushing backwards with the feet, raising a mound about 1.4 m across (range 1-2 m, n=87) with an interior depth of ca 30 cm. The nest scrape is lined with grass, which the male gathers with his bill (Bruning 1974).

Eggs are elliptical and shiny in texture. Fresh eggs are golden in color, but become white after being exposed for 5-6 days to the sun (Sick 1993). The mean clutch size of nests at one site in Argentina was 26.1 (Bruning 1974). Average mass of the egg is 605 g (Mato Grosso; Sick 1993).

The mean length of incubation at one site in Argentina was 36.6 days (Bruning 1974).

Breeding success in the Greater Rhea is remarkably low. Following the fate of 34 nests, Bruning (1974) found that only 7 (20%) hatched any chicks. From 119 eggs in the five nests of known clutch size, 74 chicks hatched (62%). Another study in Argentina followed 162 nests over the course of four years. Only 52 (31.7%) hatched any young (Fernández and Reboreda 1998). Nests established late (November and December) succeed more often than earlier nests (September and October). In most bird species, early nests succeed more often than late nests, and the cause of the rhea’s pattern of increasing success rates is likely related to the high rates of nest desertion. Males desert 65% of their clutches. This may be due to egg predation, egg breakage, disturbances by people, livestock, or predators, or hot or rainy weather, but males will sometimes desert for no apparent reason (Fernández and Reboreda 2002).

Fernández and Reboreda propose that the seasonal increase in nesting success may be due to differences in quality between early and late breeders, and/or seasonal environmental variation. Higher nesting success of later breeders does not necessarily imply higher reproductive success due to diminished certainty of paternity, possible lower rates of chick survival, and the possibility that early deserters may renest, and conclude that there is at present no clear explanation for seasonal increase in nesting success and its correlation with reproductive success.