The southern cassowary is a solitary bird, which pairs only in breeding season, in late winter or spring. The male builds a nest on the ground, a mattress of herbaceous plant material 5 to 10 centimetres (2–4 in) thick and up to 100 centimetres (39 in) wide. This is thick enough to let moisture drain away from the eggs. The male also incubates the eggs and raises the chicks alone. A clutch of three or four eggs are laid measuring 138 by 95 millimetres (5.4 in × 3.7 in). They have a granulated surface and are initially bright pea-green in colour although they fade with age. Southern cassowaries make a thunderous call during mating season, and hissing and rumblings otherwise. Chicks will make frequent high-pitched contact whistles and chirps to call the male.
Southern cassowaries have a reputation for being dangerous to humans and animals, and are often regarded as aggressive. The birds can jump quite high and kick powerfully with their blade-like claws. However, deadly encounters with southern cassowaries are rare. Only two human deaths have been reported since 1900. A 2003 historical study of 221 southern cassowary attacks showed that 150 had been against humans: 75% of these had been from southern cassowaries that had been fed by people, 71% of the time the bird had chased or charged the victim, 15% of the time they kicked. Of the attacks, 73% involved the birds expecting or snatching food, 5% involved defending their natural food sources, 15% involved defending themselves from attack, and 7% involved defending their chicks or eggs. Only one human death was reported among those 150 attacks.
The first documented human death caused by a southern cassowary was on 6 April 1926. In Australia, 16-year-old Phillip McClean and his brother, age 13, came across a southern cassowary on their property and decided to try and kill it by striking it with clubs. The bird kicked the younger boy, who fell and ran away as his older brother struck the bird. The older McClean then tripped and fell to the ground. While he was on the ground, the cassowary kicked him in the neck, opening a 1.25 cm (0.5 in) wound that may have severed his jugular vein. The boy died of his injuries shortly thereafter.
Another human death due to a southern cassowary was recorded in Florida on 12 April 2019. The bird's owner, a 75-year-old man who had raised the animal, was apparently clawed to death after he fell to the ground.
Being fed by people tempts southern cassowaries into closer associations with human-inhabited areas, increasing the already high risk of vehicle strikes – a major cause of southern cassowary mortality – and increasing the likelihood of encounters with humans. Many "aggressive" birds are simply responding to having been fed by humans in the past. Unfortunately the poor reputation of this species leads to confusion and misinformation among the public, which hampers conservation efforts of this shy bird.
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