It's not true that ostriches “bury their heads in sand” when danger approaches. No animal can afford to ignore danger. Ostriches are sharp-witted and aggressive creatures. They do, however, resort to a “disappearing” act when an enemy is chasing them and this strategy may have led people a long time ago to invent the “burying heads” theory.
Ostriches are excellent runners-they can go upto 80 kph for over half an hour at a stretch. They are usually 2.75 m tall and when running at full speed their strides can add another 1˝ m to their height. This can be a grave disadvantage. To overcome this they just simply “disappear”- they throw themselves full-length flat on the ground.