Breeding

Rather well studied in both captivity and, to a lesser extent, the wild; see summaries in, e.g., Davies (4 Davies, S. J. J. F. (2002). Ratites and Tinamous: Tinamidae, Rheidae, Dromaiidae, Casuariidae, Apterygidae, Struthionidae. Oxford University Press, Oxford and New York. Close ) and Sales (53 Sales, J. (2006). The rhea, a ratite native to South America. Avian and Poultry Biology Reviews 17(4):105–124. Close ).

The breeding season in South America varies regionally, but generally lasts from early spring to midsummer (July‒January), e.g., nests with eggs found in northern Argentina between late October and early January, and July‒September in Mato Grosso, Brazil (84 de la Peña, M. R. (1986). Guía de Aves Argentinas. Volume 1. Rheiformes, Tinamiformes, Sphenisciformes, Podicioediformes, Procellariiformes, Pelecaniformes, Ardeiformes, Phoenicopterifromes, Anseriformes. Literature of Latin America, Buenos Aires, Argentina. Close , 73 Fernández, G. J., and J. C. Reboreda (2002). Nest-site selection by male Greater Rheas. Journal of Field Ornithology 73:166–173. Close , 71 Fernández, G. J., and J. C. Reboreda (2003). Male parental care in Greater Rheas Rhea americana in Argentina. Auk 120:418–428. Close , 1 Sick, H. (1993). Birds in Brazil: A Natural History. Princeton University Press, Princeton, NJ, USA. Close , 53 Sales, J. (2006). The rhea, a ratite native to South America. Avian and Poultry Biology Reviews 17(4):105–124. Close , 85 Lèche, A., C. Hansen, J. L. Navarro, R. H. Marin, and M. B. Martella (2014). Influence of breeding season on fecal glucocorticoid levels in captive Greater Rhea (Rhea americana). Zoo Biology 34(1):71–75. Close ), whereas the non-native population in Germany breeds between April and August, or exceptionally from late March (86 Jokisch, N., F. Philipp, and A. Korthals (2011). Brutverhalten und Brutplatzwahl des Nandus (Rhea americana ssp.) in Mecklenburg-Vorpommern. Vogelwarte 49:261–262. Close ).

The nest is on the ground, in open, higher grassland (80 Belton, W. (1984). Birds of Rio Grande do Sul, Brazil. Part 1. Rheidae through Furnariidae. Bulletin of the American Museum of Natural History 178(4): 369–636. Close , 84 de la Peña, M. R. (1986). Guía de Aves Argentinas. Volume 1. Rheiformes, Tinamiformes, Sphenisciformes, Podicioediformes, Procellariiformes, Pelecaniformes, Ardeiformes, Phoenicopterifromes, Anseriformes. Literature of Latin America, Buenos Aires, Argentina. Close ). The male selects a site, and tramples the surrounding vegetation with his feet, but also cutting the grass using its bill, to produce a clean area 2‒3 m wide around the nest (1 Sick, H. (1993). Birds in Brazil: A Natural History. Princeton University Press, Princeton, NJ, USA. Close ), or 1.0 to 1.6 m in diameter (53 Sales, J. (2006). The rhea, a ratite native to South America. Avian and Poultry Biology Reviews 17(4):105–124. Close ). Thereafter, the male 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 (73 Fernández, G. J., and J. C. Reboreda (2002). Nest-site selection by male Greater Rheas. Journal of Field Ornithology 73:166–173. Close ).

During the egg laying and incubation periods, the male lines the nest scrape with grass, feathers, dirt, and sticks (73 Fernández, G. J., and J. C. Reboreda (2002). Nest-site selection by male Greater Rheas. Journal of Field Ornithology 73:166–173. Close ), gathering material with its bill (9 Bruning, D. F. (1974). Social structure and reproductive behavior in the greater rhea. Living Bird 1974:251–294. Close ). The nest is roughly circular, ca. 90 cm across, and about 30 cm deep (73 Fernández, G. J., and J. C. Reboreda (2002). Nest-site selection by male Greater Rheas. Journal of Field Ornithology 73:166–173. Close ).

Eggs are elliptical and shiny in texture (84 de la Peña, M. R. (1986). Guía de Aves Argentinas. Volume 1. Rheiformes, Tinamiformes, Sphenisciformes, Podicioediformes, Procellariiformes, Pelecaniformes, Ardeiformes, Phoenicopterifromes, Anseriformes. Literature of Latin America, Buenos Aires, Argentina. Close ). Fresh eggs are golden-yellow in color, but become white after being exposed to the sun for 5‒6 days (1 Sick, H. (1993). Birds in Brazil: A Natural History. Princeton University Press, Princeton, NJ, USA. Close , 81 Navarro, J. L., M. B. Martella, and M. B. Cabrera (1998). Fertility of Greater Rhea orphan eggs: conservation and management implications. Journal of Field Ornithology 69:117–120. Close ). Mean size is 118‒139 × 87‒97 mm (87 Prinzinger, R., V. Dietz, and B. Nagel (1997). Respiratory quotient and embryological development of metabolic heat production in the Rhea (Rhea americana). Journal of Thermal Biology 22:223–226. Close ) or 120‒141 × 81‒103 mm (mean 130.8 × 92.3 mm, n = 18) (84 de la Peña, M. R. (1986). Guía de Aves Argentinas. Volume 1. Rheiformes, Tinamiformes, Sphenisciformes, Podicioediformes, Procellariiformes, Pelecaniformes, Ardeiformes, Phoenicopterifromes, Anseriformes. Literature of Latin America, Buenos Aires, Argentina. Close ), with individual size varying dramatically in the same nest (1 Sick, H. (1993). Birds in Brazil: A Natural History. Princeton University Press, Princeton, NJ, USA. Close ). Mean egg mass in the wild is 605 g (Mato Grosso, Brazil; 1 Sick, H. (1993). Birds in Brazil: A Natural History. Princeton University Press, Princeton, NJ, USA. Close ), or 626 g in captivity (n = 12; 88 Navarro, J. L., M. L. López, D. M. Maestri, and D. O. Labuckas (2001). Physical characteristics and chemical composition of Greater Rhea (Rhea americana) eggs from wild and captive populations. British Poultry Science 42:658–662. Close ).

Mean clutch size in nests at one site in Argentina was 26.1 (9 Bruning, D. F. (1974). Social structure and reproductive behavior in the greater rhea. Living Bird 1974:251–294. Close ), and nests in the wild may contain from 8 to 56 eggs, although the average seems to be between 20 and 30 (53 Sales, J. (2006). The rhea, a ratite native to South America. Avian and Poultry Biology Reviews 17(4):105–124. Close ). Sick (1 Sick, H. (1993). Birds in Brazil: A Natural History. Princeton University Press, Princeton, NJ, USA. Close ) reported that in the past nests could contain as many as 60 or more eggs, making it impossible to incubate them all. A single female can lay as many as 10‒18 eggs in a single season (the product of up to 12 different males), at two-day intervals, although up to the final 30% of the total can be infertile (74 Bruning, D. F. (1973). The greater rhea chick and egg delivery route. Natural History 82:68–75. Close , 9 Bruning, D. F. (1974). Social structure and reproductive behavior in the greater rhea. Living Bird 1974:251–294. Close , 1 Sick, H. (1993). Birds in Brazil: A Natural History. Princeton University Press, Princeton, NJ, USA. Close , 72 Fernández, G. J., and J. C. Reboreda (1998). Effects of clutch size and timing of breeding on reproductive success of Greater Rheas. Auk 115:340–348. Close ). In captivity, clutches of 12‒18 eggs produce the highest ratio of hatchlings (89 Paschoal, F. R., and F. S. Magnani (1990). Obtenção de maior indice de nascimentos com um controle do número de ovos por incubação em ema. Resumos de XVII Congreso Brasileiro de Zoologia, Londrina: 173. Close ).

In captivity, most egg laying occurred during the time period 14.00 h to 1600 h (60 Sales, J., D. C. Deeming, P. J. U. van Deventer, M. B. Martella, and J. L. Navarro (2000). Diurnal time-activity budget of adult Greater Rheas (Rhea americana) in a farming environment. Archives für Geflugelkunde 64(5):207–210. Close ).

By male alone, reported to last as few as 27 and as many as 43 days (4 Davies, S. J. J. F. (2002). Ratites and Tinamous: Tinamidae, Rheidae, Dromaiidae, Casuariidae, Apterygidae, Struthionidae. Oxford University Press, Oxford and New York. Close ); the mean length of incubation at one site in Argentina was 36.6 days (9 Bruning, D. F. (1974). Social structure and reproductive behavior in the greater rhea. Living Bird 1974:251–294. Close ), commencing 5‒8 days (1 Sick, H. (1993). Birds in Brazil: A Natural History. Princeton University Press, Princeton, NJ, USA. Close ) or 5‒7 days after the females start laying (53 Sales, J. (2006). The rhea, a ratite native to South America. Avian and Poultry Biology Reviews 17(4):105–124. Close ). Males spend around 64% of their time on the nest during egg laying (days 1‒10), increasing to 97.5% during the mid and late incubation period (days 20‒40). They leave the nest during the warmest hours of the day, and the length of non-attendance is positively correlated with environmental temperature (71 Fernández, G. J., and J. C. Reboreda (2003). Male parental care in Greater Rheas Rhea americana in Argentina. Auk 120:418–428. Close ).

The male frequently shifts position, making a complete 360° rotation during the course of 24 hours, moving some eggs with each change in position in order to ensure that they are all regularly incubated (9 Bruning, D. F. (1974). Social structure and reproductive behavior in the greater rhea. Living Bird 1974:251–294. Close ). A threat display, during which the bird crouches low on the nest, wings spread laterally, and giving a head-forward posture, is given by an incubating male if approached by another rhea (90 Raikow, R. J. (1969). Sexual and agonistic behaviour of the Common Rhea. Wilson Bulletin 81:196–202. Close ). Some eggs often break during incubation; males will eat them, as well as any flies attracted by the liquid (1 Sick, H. (1993). Birds in Brazil: A Natural History. Princeton University Press, Princeton, NJ, USA. Close ).

Temperatures around 36.5°C, with a relative humidity of 60‒80%, resulted in a high hatching success during artificial incubation (87 Prinzinger, R., V. Dietz, and B. Nagel (1997). Respiratory quotient and embryological development of metabolic heat production in the Rhea (Rhea americana). Journal of Thermal Biology 22:223–226. Close ). Slightly higher temperatures have been reported in zoos (91 Brown, D. A., and R. H. Kimball (1972). Maintaining rheas Rhea americana at Fort Worth Zoo. International Zoo Yearbook 12:187–189. Close ).

Cases of Joint Nesting

Males begin incubating 2‒3 days after the females start to lay, becoming increasingly aggressive towards any other conspecifics, and eventually driving the females away. Males almost always breed alone and aggressively expel other males from their nest sites. However, two studies have reported intriguing cases of peaceful male partnerships, including apparent cases of cooperative breeding. Fernández and Reboreda (69 Fernández, G. J., and J. C. Reboreda (1995). Adjacent nesting and egg stealing between males of the Greater Rhea Rhea americana. Journal of Avian Biology 26:321–324. Close ) described four cases in which two wild males nested <1 m apart and periodically stole eggs from each other throughout the incubation period. In each case, a second male sat beside the nest of a male already incubating and began stealing eggs, instigating a peaceful clandestine exchange. All cases occurred at the end of the breeding season, when access to sexually active females had decreased. After hatching, both males shared the care of the chicks (69 Fernández, G. J., and J. C. Reboreda (1995). Adjacent nesting and egg stealing between males of the Greater Rhea Rhea americana. Journal of Avian Biology 26:321–324. Close ).

The young all hatch within 6‒8 hours of each other on the same day (9 Bruning, D. F. (1974). Social structure and reproductive behavior in the greater rhea. Living Bird 1974:251–294. Close ), despite that the last eggs to be laid may have been incubated ca. 12 days less than the first; it has been suggested that the loud calls uttered by the chicks (see Sounds and Vocal Behavior) from inside the egg might stimulate near-synchronized hatching (1 Sick, H. (1993). Birds in Brazil: A Natural History. Princeton University Press, Princeton, NJ, USA. Close ). The hatching eggs emit a strong smell, which attracts many flies; within 5‒6 hours of emerging the chicks are already capable of feeding on these (1 Sick, H. (1993). Birds in Brazil: A Natural History. Princeton University Press, Princeton, NJ, USA. Close ). Belton (80 Belton, W. (1984). Birds of Rio Grande do Sul, Brazil. Part 1. Rheidae through Furnariidae. Bulletin of the American Museum of Natural History 178(4): 369–636. Close ) reported that a rancher informed him of a male with newly hatched young breaking unhatched eggs in a nest.

Weight of chicks at hatching is 354‒425 g (92 Bruning, D. F. (1973). Breeding and rearing rheas in captivity. International Zoo Yearbook 13:163–172. Close , 93 Navarro, J. L., P. E. Vignolo, M. R. Demaría, N. O. Maceira, and M. B. Martella (2005) Growth curves of farmed Greater Rheas (Rhea americana albescens) from central Argentina. Archives für Geflugelkunde 69:90–93. Close ), but the young amay lose up to 15% of their initial mass during the first 3–4 days after hatching, whereafter weight increases (93 Navarro, J. L., P. E. Vignolo, M. R. Demaría, N. O. Maceira, and M. B. Martella (2005) Growth curves of farmed Greater Rheas (Rhea americana albescens) from central Argentina. Archives für Geflugelkunde 69:90–93. Close ).

Hatchability (% of total number of eggs) among eggs collected daily from a captive population and stored for four days or more before incubation (28%) was ca. 30% lower than that of non-stored eggs or eggs stored for three days or less (94 Lábaque, M. C., J. L. Navarro, and M. B. Martella (2004). Effects of storage time on hatchability of artificially incubated Greater Rhea (Rhea americana) eggs. British Poultry Science 45:638–642. Close ). Total hatchability is higher in captivity (60%) than in semi-captivity (45%), or in the wild (30%) (53 Sales, J. (2006). The rhea, a ratite native to South America. Avian and Poultry Biology Reviews 17(4):105–124. Close ).

By male alone, taking care of the chicks for 4‒6 months, during which they allocate less time to feeding and more time to vigilance than males in groups of adults or solitary males, but vigilance decreases as chicks get older (71 Fernández, G. J., and J. C. Reboreda (2003). Male parental care in Greater Rheas Rhea americana in Argentina. Auk 120:418–428. Close ). The young remain together in a well-united flock.

Belton was informed by a rancher that he had watched a male rhea save its nest from an approaching fire, by constantly running around the nest, wings outstretched and dragging on the ground, with the combined action of the bird’s wings and feet ridding the area of dry grass, and thereby creating a firebreak (80 Belton, W. (1984). Birds of Rio Grande do Sul, Brazil. Part 1. Rheidae through Furnariidae. Bulletin of the American Museum of Natural History 178(4): 369–636. Close ).

Cooperative breeding between pairs of males has been reported in Brazil (5 Codenotti, T. L., and F. Alvarez (1997). Cooperative breeding between males in the Greater Rhea Rhea americana. Ibis 139(3):568–571. Close ). Of 35 observed harems, eight contained a dominant male as well as a younger subordinate from the same original flock. Dominant adults in these two-male groups defended the harem, performed territorial calls and aggressive displays, and expelled other males from the area more often than males without helpers. However, they invariably tolerated the presence of their subordinate helpers. Females approached the dominant males 656 times and the subordinate males 48 times, and while the dominant males appeared significantly more successful, it is possible that beta males garnered occasional reproductive opportunities.

In such cases, a dominant male builds a nest in which the females lay, leaves the subordinate to incubate it, and establishes another nest in which the same females lay a second clutch that he incubates himself. In recorded cases, both clutches had relatively equal hatching success, and the combined clutch sizes laid by harems with two males were roughly equivalent to those laid by single-male harems. More chicks survived to age five months in broods guarded by dominant males (75%) than those led by subordinate males (17%), but overall survival rates were higher than broods belonging to solitary males (22%) (5 Codenotti, T. L., and F. Alvarez (1997). Cooperative breeding between males in the Greater Rhea Rhea americana. Ibis 139(3):568–571. Close ).

The duties of incubation and chick rearing incur costs, reducing feeding periods, and increasing the risk of predation (72 Fernández, G. J., and J. C. Reboreda (1998). Effects of clutch size and timing of breeding on reproductive success of Greater Rheas. Auk 115:340–348. Close ). The dominant males in such partnerships may benefit from increased chick survivorship, but benefits to the subordinate helpers are unclear. This is one of the principal questions in cooperative breeding literature, and four hypotheses are typically considered: (1) direct reproductive success by fathering some young, (2) indirect reproductive success via kin selection, (3) gaining reproductive experience increasing future success, and (4) higher reproductive status if females recognize and value helping behavior. The second hypothesis proposes helpers may gain inclusive fitness benefits by caring for the young of close relatives, and has been studied in several other species of cooperatively breeding birds. Survivorship data coupled with genetic analysis can address the first two hypotheses by determining if the subordinate fathers a proportion of the combined clutch, what proportion survives, and whether the two males are related and therefore subject to inclusive fitness benefits.

Apparently unknown.

At age six months young birds have achieved almost the same size as adult females; they reach sexual maturity at 2‒3 years old (1 Sick, H. (1993). Birds in Brazil: A Natural History. Princeton University Press, Princeton, NJ, USA. Close , 53 Sales, J. (2006). The rhea, a ratite native to South America. Avian and Poultry Biology Reviews 17(4):105–124. Close ). In captivity, two-year-old males construct nests, gather females, and start to incubate eggs (but desert after no more than three weeks), while of three males in their third year, just one hatched chicks, which it then attacked; the same male successfully raised some young the following year (9 Bruning, D. F. (1974). Social structure and reproductive behavior in the greater rhea. Living Bird 1974:251–294. Close ).