1. Dynamic soaring
Wind directly above the ocean surface is slowed by friction; as you rise higher above the water, air flows become less encumbered and so wind blows faster and faster. This is a speed gradient. Seabirds such as albatrosses and petrels have evolved a flight style called dynamic soaring, which involves flying upwards into these headwinds. They thereby obtain free energy to whisk them upwards and back with the wind at their tail, whereupon they turn back to face the headwind and are thrust up again. Seabirds can travel vast distances using hardly any energy. Unlike birds of prey or storks circling over land, no updrafts are involved.
2. Wing loading
This is the ratio of weight to wing area, calculated by dividing the former by the latter. Light birds with large wings have low wing loading; the reverse is also true. A high wing loading results in inefficient flight – the wings have a relatively small surface area to create lift. By contrast, the lower the wing loading, the more manoeuvrable a bird is, and it can take off at slower speeds.
3. Pectoral muscles
Bird flight, with the exception of soaring, dynamic soaring and gliding, generally requires enormous muscular power. This is supplied by the equally substantial pectoral, or breast, muscles. Much like the engine of a car, they represent a large proportion of a bird’s mass; for example, they make up 40 per cent of a city pigeon’s weight. The pectoral muscles are anchored to the huge, sail-like keel, an extension of the central breast bone, or sternum, one of the biggest bones in the otherwise lightweight avian skeleton.
In cross section, a bird’s wing is blunt at the leading edge, narrow at the trailing edge, and arced downwards (concave). This shape, known as an aerofoil, is perfect for flight and has long been copied by aircraft designers. It ensures that air passing over the top of the wing has to flow more quickly to meet the air taking the ‘short cut’ underneath the wing. Since fast-moving air exerts less atmospheric pressure than slower-moving air, the force is upwards from high to low pressure – in other words, lift is generated.