Common swifts (Apus apus) are migratory birds, which spend almost their entire life in the air and land only by accident or in order to breed. They feed on airborne insects and sleep either in colonies flying in circles or thanks to air currents at an altitude of 5000–6600 ft. A common swift can live as long as 21 years. During its life, it may have flown 4 million miles, which is equivalent to eight round trips between the Earth and the Moon!