After the large flocks break up in the winter, they form into three loose groups:
single males,
flocks of between two and fifteen females, and
a large flock of yearlings.

As winter approaches, males become more aggressive towards each other.
Then they start courting females by calling and raising the front of their body up while keeping their neck straight and ruffling their plumage.
They will raise their wings and may run some distance like this, sometime flapping their wings methodically.
After doing this and attracting females, they will continue calling at a specific female, and will start to either walk alongside her or in front of her while spreading his wings and lowering his head.
As the display continues, he will get more intense and animated and start waving his neck around and in figure eights.
Once he has attracted a first mate he will copulate with her and then lead her to his nest.

When it is time for the eggs to be laid, the male will typically be on the nest already and will act aggressive when approached by the female, covering his nest with his wings.
He will gradually relax and allow her to crouch and lay the egg at the edge of the nest.
The male will roll the egg into his nest.

Males are simultaneously polygynous, females are serially polyandrous.
In practice, this means that the females move around during breeding season, mating with a male and depositing their eggs with the male before leaving him and mating with another male.
Males on the other hand are sedentary, attending the nests and taking care of incubation and the hatchlings all on their own.
Recent evidence has shown that some males will utilize subordinate males to help incubate and protect the eggs.
If this happens, the dominant male will find a second harem and start the process over again.
The nests are thus collectively used by several females and can contain as many as 80 eggs laid by a dozen females; each individual female's clutch numbers some 5–10 eggs.
However, the average clutch size is 26 eggs laid by 7 different females.

Rhea eggs measure about 130 mm × 90 mm (5.1 in × 3.5 in) and weigh 600 g (21 oz) on average; they are thus less than half the size of an ostrich egg.
Their shell is greenish-yellow when fresh but soon fades to dull cream when exposed to light.
The nest is a simple shallow and wide scrape in a hidden location; males will drag sticks, grass, and leaves in the area surrounding the nest so it resembles a firebreak as wide as their neck can reach.
The incubation period is 29–43 days.
All the eggs hatch within 36 hours of each other even though the eggs in one nest were laid perhaps as much as two weeks apart.
As it seems, when the first young are ready to hatch they start a call resembling a pop-bottle rocket or even fireworks, even while still inside the egg, thus the hatching time is coordinated.
Greater rheas are half-grown about three months after hatching, and sexually mature by their 14th month.
After the large flocks break up in the winter, they form into three loose groups:
single males,
flocks of between two and fifteen females, and
a large flock of yearlings.
As winter approaches, males become more aggressive towards each other. Then they start courting females by calling and raising the front of their body up while keeping their neck straight and ruffling their plumage. They will raise their wings and may run some distance like this, sometime flapping their wings methodically. After doing this and attracting females, they will continue calling at a specific female, and will start to either walk alongside her or in front of her while spreading his wings and lowering his head. As the display continues, he will get more intense and animated and start waving his neck around and in figure eights. Once he has attracted a first mate he will copulate with her and then lead her to his nest.
When it is time for the eggs to be laid, the male will typically be on the nest already and will act aggressive when approached by the female, covering his nest with his wings. He will gradually relax and allow her to crouch and lay the egg at the edge of the nest. The male will roll the egg into his nest.
Males are simultaneously polygynous, females are serially polyandrous. In practice, this means that the females move around during breeding season, mating with a male and depositing their eggs with the male before leaving him and mating with another male. Males on the other hand are sedentary, attending the nests and taking care of incubation and the hatchlings all on their own. Recent evidence has shown that some males will utilize subordinate males to help incubate and protect the eggs. If this happens, the dominant male will find a second harem and start the process over again. The nests are thus collectively used by several females and can contain as many as 80 eggs laid by a dozen females; each individual female's clutch numbers some 5–10 eggs. However, the average clutch size is 26 eggs laid by 7 different females.
Rhea eggs measure about 130 mm × 90 mm (5.1 in × 3.5 in) and weigh 600 g (21 oz) on average; they are thus less than half the size of an ostrich egg. Their shell is greenish-yellow when fresh but soon fades to dull cream when exposed to light. The nest is a simple shallow and wide scrape in a hidden location; males will drag sticks, grass, and leaves in the area surrounding the nest so it resembles a firebreak as wide as their neck can reach. The incubation period is 29–43 days. All the eggs hatch within 36 hours of each other even though the eggs in one nest were laid perhaps as much as two weeks apart. As it seems, when the first young are ready to hatch they start a call resembling a pop-bottle rocket or even fireworks, even while still inside the egg, thus the hatching time is coordinated. Greater rheas are half-grown about three months after hatching, and sexually mature by their 14th month.
Greater rhea