Flight

There seems to be great extremes in the flight capability of birds, some can:

• Fly at high altitudes, such as migrating Whooper Swans at 8850 metres (29 000 feet).
• Stay aloft for months at a time, such as Swifts and Albatrosses.
• Travel long distances, such as Swifts, and Terns.
• Reach incredible speeds, such as a Peregrine exceeding 300 kph (about 190 miles per hour) while chasing prey.

Generally, however, most birds fly at 15-50 km/h (about 10-30 miles per hour) and at altitudes of less than 150 metres (about 500 feet), to go higher requires more energy and greater exposure to stronger, colder winds and birds of prey. This is a compromise that involves maximising distances and minimising metabolic rates, which in turn determines their energy requirements and how much food they must find and eat.

The shape and size of the wings is chiefly responsible for how a bird flies and not the size of the bird itself. For example:

• Passerines, such as House Sparrows and Blackbirds, have wings that taper to a point (reducing drag) and are short relative to their width which enables rapid take off but not sustained high speed, and agility to escape predators.
• Carrion Crows have "fingers" or slots at the end of the wings to reduce turbulence and so allow them to stay aloft at much lower speeds.
• Swallows and some falcons have long, flat wings that allow high speed flight, but hawks (like Sparrowhawk) and owls that hunt in woodland areas have shorter wings for more manoeuvrability at the expense of less speed.

The skeleton of most birds is greatly modified for flight:

• Some bones are fused together, for example the vertebrae and pelvic girdle.
• Others bones have been eliminated, for example, the fingers.
• Most bones contain hollows to reduce weight, and some are connected to the respiratory system so that they can be filled with air.

Interestingly, some scientists now believe that feathers evolved (see Evolution) primarily for insulation and evolved much later as flight-related features. However, while some feathers are undoubtedly the most efficient natural insulators - just think about eider down in duvets - others are ideally developed for flight.

So which came first, insulation or flight?