An ostrich may reach speeds of up to 45 miles per hour and sustain that speed for extended periods of time. This is the fastest land speed of any bird on the planet.
The birds are able to run so fast over long distances because they use half as much energy as humans do at top speed. According to Professor Jonas Rubenson of the University of Western Australia, this is because they store twice as much energy in their leg tendons as humans and other animals. This explains why species such as lions and tigers outrun the ostrich in short sprints, but the flightless birds are faster over long distances.