Emus form breeding pairs during the summer months of December and January, and may remain together for about five months. During this time they wander around in an area a few miles in diameter. It is believed they guard or find territory during this time. Both males and females increase in weight during this time and the female is slightly heavier at between 45 and 58 kg. This weight is lost during the incubation period, the males losing around 9 kg.Mating occurs in the cooler months of May and June, and the exact timing is determined by the climate, as the birds nest during the coldest part of the year. During the breeding season, males experience hormonal changes, including an increase in luteinizing hormone and testosterone levels, and their testicles double in size.
It is the females that court the males, and during the mating season, they become physically more attractive. The female's plumage darkens slightly and the small patches of bare, hairless skin just below the eyes and near the beaks turn turqoise-blue, although this is a subtle change. The female strides around confidently, often circling the male, and pulls its neck back while puffing out her feathers and crying out a low, monosyllabic sound that has been compared to human drums. This calling can occur when the males are not in view and more than 50 metres (160 ft) away and when the male's attention has been gained, the female can circle in a radius of 10–40 m. As the female circles its prospective mate, it continues to look towards him by turning its neck, while keeping its rump facing him. During this time, the female's cervical air sac may remain inflated as it calls outs. The passive male retains the same colour hair, although the bare patches of skin also turn a light blue.The female has more black hairs on its head but gender differentiation can be difficult for humans. If the male shows interest in the parading female, he will move closer; the female continues to tantalise its target by shuffling further away and continuing to circle him as before.
Females are more aggressive than males during the courting period, often fighting one another for access to mates. Fights among females accounted for more than half of the violent incidents in one mating season study. If a female tried to woo a male that already had a partner, the incumbent female will try and repel the competitor by walking towards her challenger and staring in a stern way. If the male showed interest in the second female by erecting his feathers and swaying from side to side, the incumbent female will attack the challenger, usually resulting in a backdown by the new female.
Some female-female competitions can last up to five hours, especially when the target male is single and neither female has the advantage of incumbency. In these cases, the animals typically intensify their mating calls and displays, which increase in extravagance. This is often accompanied by chasing and kicking by the competing females.
Males lose their appetite and construct a rough nest in a semi-sheltered hollow on the ground from bark, grass, sticks, and leaves. The nest almost a flat surface rather than a segment of a sphere, and although in cold conditions the nest is taller, up to 7 cm tall, and more spherical to provide more insulation. When other material is lacking, it can also use spinifex grass bushes more than a metre across, despite the prickly nature. The nest can be placed in open ground or near scrubs and rocks, although thick grass is usually present if the emu takes the former option. The nests are usually placed in an area where the emu has a clear view of the surrounds and can detect predators.
If a male is interested, he will stretch his neck and erect his feathers and bend over and peck at the ground. He will then sidle up to the female, swaying his body and neck from side to side, and rubbing his breast against his partner's rump, usually without calling out. The female would accept by sitting down and raising her rump.
The pair mates every day or two, and every second or third day the female lays one of an average of 11 (and as many as 20) very large, thick-shelled, dark-green eggs. The shell is around 1 mm thick also indigenous Australians say that northern eggs are thinner. The number of eggs varies with rainfall. The eggs are on average 134 by 89 millimetres (5.3 × 3.5 in) and weigh between 700 and 900 grams (1.5 and 2.0 lb), which is roughly equivalent to 10–12 chicken eggs in volume and weight. The first verified occurrence of genetically identical avian twins was demonstrated in the Emu. The egg surface is granulated and pale green. During the incubation period, the egg turns dark green, although if the egg never hatches, it will turn white from the bleaching effect of the sun.