How Do Birds Fly?
Freedom Of The Air
The Mastery Of Flight by Sir David Attenborough
How Flight Works
If numbers of species are a worthwhile guide, birds-with over 8000 species that fly compared with 4000 non-flying mammal species are arguably the most successful class of vertebrates today. Their success is due in large part to the powers of movement, and thus the wide choice of habitat, that flight allows.
The mechanical problem concerned with flight is how to lift a weight clear of the ground, keep it airborne for a practical time and propel it forwards at the same time. For this, an upward force (lift) and a forward force (propulsion) are required.
A glance at the wing cross-section of a model aircraft shows the solution to this problem. Rounded and thicker at the front than at the slender ‘trailing edge,’ shallowly convex above and concave below, this is called an aerofoil section. Move an aerofoil section forwards through the air and it causes air to flow in such a way across its surfaces as to produce lift. The discovery of the aerofoil section enabled man to fly: birds of course have used it for eons. The inner portion of a bird’s wing, which is anatomically equivalent to the human forearm, has a classic aerofoil section.
The next requirement is propulsion. For this, birds rely on the beating of their wings, powered by the breast muscles (the pectorals) which are highly developed. Propulsion is provided by the outer part of the wing, from the angle (where the front edge bends) to the tip. This part is anatomically equivalent to the human hand, from wrist to fingertips. The ten or twelve long, strong feathers at the end of the wing (the flight feathers) are vitally important and merit their technical name ‘primaries.’
How An Aerofoil Works
Nature's Fighter Jet- The Peregrine Falcon
The vast majority of aircraft have wings and tail securely fixed in position, and only the provision of flaps and ailerons allows their shape to be altered so that the aircraft is manoeuvrable. Some of the most sophisticated modern fighter planes are famed for their ‘variable geometry’ but this merely allows the wings to swing in and out from the fuselage. It’s easy to see, therefore, how much more manoeuvrable birds are than aircraft, with an almost infinite variety of positions, not just of the wings and tail, but of the parts of the wings and often of the individual feathers.
A Running Start
Taking Off And Landing
To complete their manoeuvrability, birds have to be able to take off and land at will. Many survive precisely because they can do this at a split second’s notice, escaping the ever present danger of attacking predators. Taking off requires the maximum output of propelling force (thrust) that the bird is capable of producing.
The smaller species are capable of exerting sufficient effort to lift themselves off the ground with no preliminary ‘run.’ Heavier birds, on the other hand, have to survive without the ability to respond to danger by an instant take off. Large water birds, such as swans, attain safety by spending much of their time on the open water, far away from the predators on dry land. Their take off is so difficult that they often have to ‘run’ some distance across the water surface, and can only occupy stretches of water large enough to allow space for this long take off.
To land, birds need to ‘spill’ the lift that their wings and tails have been providing during flight. One way to lose lift is to raise the tail, thus steering into a descent towards the landing place. To halt their flight, birds rear up in mid-air to an upright posture as they approach the selected spot. This tilts the wings until they are nearly in a vertical plane, so that they can act as ‘brakes,’ or even beat against the direction of flight. The tail is lowered, also acting as a brake against forward movement.
The hard impact of landing is absorbed by flexible legs and strong muscles, which are attached to the most rigid part of the bird’s skeleton- the pelvis, which is fused on to the lower spine for added strength.
Types Of Bird Flight
- High manoeuvrability: The wren can fly neatly among dense undergrowth. While the partridge is an expert at the sudden escape flight.
- General purpose: Robins, finches and geese are among the many 'all-rounders.'
- High performance: Long, narrow primaries give the swift and the hobby (a species of falcon) maximum power with the least air resistance.
- Gliding:The gannet and the Manx shearwater have narrow wings with a long lift section for sea gliding.
- Soaring: The wings of the raven and the buzzard are broad with a long lift section, useful for catching upward air currents.
In Perfect Formation
Shapes Of Wings
In the course of evolution, families of birds have developed the capability of flight in a number of different ways to suit the various modes of life they have adopted.
First, there are naturally many birds without a specialised form of flight and these have a ‘general purpose’ wing layout. In this layout, the lift and power sections of the wing are roughly of equal length. The wing is twice as long as it is wide. Garden birds such as the greenfinch and the robin fit into this category. So, too, do larger birds like geese.
If the ‘general purpose’ category is placed in the middle of a range of flight styles, one of the extremes on the scale would be ‘high manoeuvrability.’ Birds in this category have wings with lift and power sections of equal length, but the wing is much rounder in appearance, rather than long, for it is almost as broad at its base as it is long from base to tip. Examples include game birds, which need fast and almost vertical take-off to escape predators, and birds of thick cover such as the tiny wren. The wing pattern provides strong propulsion, but the high manoeuvrability results from the broad wing area.
The other extreme consists of the birds that specialise in extra-efficient flight. Among these species, flight is developed to the highest standards of performance. For maximum speed, their wings do not have the same proportions as those of the other categories of birds. The power section of the wing (the primary feathers) is much longer than the lift section- in some cases by as much as a factor of four- and the wing is long and thin. The swift is a prime example of this.
A Perfect Glider
Fly Like An Eagle
Ways Of Gliding
Gliding (in the case of birds) is flying without flapping wings. Two groups of birds are adapted for this type of flight: those that use wind power to travel over the sea (gliding in the common sense of the word) and those that not only glide but also exploit rising air currents to gain height (soaring).
The ‘gliding’ birds have extremely long and narrow wings, with a lift section either as long as the power section or longer. Most of them are oceanic birds, best exemplified by the albatrosses of the southern hemisphere but represented in British waters by the fulmar, the gannet, and the Manx shearwater.
The ‘soaring’ birds use up-currents at a ridge of hills or, in warm conditions, columns of rising air called thermals. In warmer countries these currents are the province of the vultures, but in Britain and Ireland the best examples of soaring flight are given by the raven, the buzzard and the golden eagle.
These birds are quite different in silhouette from the seagoing gliders: they too have a lift section as long as the power section or else even longer, but their wings are much broader. Further, the wingtips, rather than being pointed, have a broadly splayed ‘fingered’ appearance which helps to reduce turbulence. The birds flap slowly and laboriously into the air in wide circles, but once they catch the rising air they soar effortlessly.
© 2013 James Kenny