Above & Beyond

Overachievers in the bird world

- Andrew Mckechnie

Birds are capable of feats of endurance that beggar the human imagination. They survive and reproduce almost everywhere on earth, from the hottest, most inhospitable deserts to the icy expanses of the Arctic and Antarctic. Some dive hundreds of metres below the ocean’s surface, while others migrate over the planet’s highest mountains. This article explores the adaptations of those species that push the avian limits of long-distance flight and altitude, heat or cold tolerance, and unpacks some of the physiology that makes these achievements possible.

FLYING HIGH

In late 1973, a Rüppell’s Griffon collided with an aircraft over Abidjan on the Ivory Coast. Bird strikes are not uncommon, but what made the abrupt demise of this particular vulture unusual was the altitude at which it happened: 11 kilometres above the ground. That any animal can survive and function at such an altitude is remarkable, even if in this instance the griffon probably reached this height by soaring on thermals rather than in flapping flight.

Birds are inherently better suited to operating at altitude than mammals are. One key factor is the structure of the avian respiratory system. Mammals have a respiratory cycle that involves the inhalation of oxygenated air into the lungs, followed by the exhalation of partially deoxygenated air before the next inhalation. Birds possess a network of air sacs surrounding the lungs, arranged in such a way that oxygenated air flows continuously through the lungs regardless of whether the bird is inhaling or exhaling, providing a far more efficient mechanism for getting oxygen from the atmosphere into the bloodstream.