Powerful legs help the cassowary run up to 31 miles per hour (50 kph) through the dense forest underbrush”
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I wouldn’t be so quick to assume the guy simply ‘fell’ and was then attacked.
I read the Wikipedia article on cassowaries. It sounds like the big danger from them is being kicked while one is down. The center claw is sharp and something like 5 inches long. A nasty puncture.