FLIGHT AND WING STRUCTURE
By Dr Colin Walker BSc, BVSc, MRCVS, MACVSc (Avian health)
When assessing pigeons, fanciers almost invariably look at their wings. Just what characteristics of the wing should we be looking at and do they really have any effect on performance?
The pigeon uses its wings to stay in the air, move forward and manoeuvre. The ability to manoeuvre cannot be assessed through handling. However, as the structure of the wings does affect their ability to provide both lift and forward propulsion, their ability in these areas can to some extent be determined through examination.
When gliding, the bird’s wing behaves like an aeroplane wing and lift is generated by its forward movement through the air. In flapping flight, the wing is acting both as a lifting surface and as a means of propulsion.
When assessing the wing for lift, it is the curve of the wing, feather quality, the size of the secondary flights and the shoulder support of the wing that are important.
The curve of the wing
This refers to the curve, ‘hump’ or camber observed on the top of the outstretched wing when viewed from the front. To achieve lift, we need a lower air pressure on one side of the wing than the other . This is achieved by the wing having a curve. Air takes longer to pass over the top ‘humped’ or cambered surface of the outstretched wing, resulting in a lower air pressure above the wing than that created by air passing over the lower ‘flat’ wing surface. The difference in pressure results in a force acting upwards, which is called lift. This fascinating action is called the Bernoulli effect. Bernoulli postulated that air has a dynamic pressure (ie pressure generated by air) when it is moving such as is felt by someone walking into the wind) and a static pressure (which is simply the weight of the air). The total pressure of the air is the combination of these two and must always be the same. Therefore as air flows faster its weight must become less. This means that air rushing over the camber or curvature of the top surface of the wing generates a low pressure area, resulting in lift.
If the bird is to maintain its height with ease, the force of the lift must be sufficient to support the bird’s weight. Just as to what is the perfect curve is hard to qualify. Suffice to say that birds should have a noticeable camber to the wing and that birds with the flatter non-lift-generating wings will expend more energy staying aloft and therefore fatigue more readily. Such birds should be selected against.
To get maximum lift, it is vital that the air flows around the wing in unbroken ‘streamlines’. Air flow that does not flow in streamlines develops eddies, resulting in turbulence. Turbulence upsets a smooth air flow, causes a loss of lift and increases the drag of the wing (ie its resistance to passage through the air). Lift still occurs but there is less of it. Good feather quality is the result of good genes and good care and is readily assessed by handling.
Surface area of the wing
Lift depends on the surface area of the wing. More air is able to act on and develop more lift on a big wing rather than a small wing. The surface area depends on the length and width of the secondary and primary feathers. Having said that, it would seem that the bigger the wing, the better. However, as will be seen later in the section on propulsion, there are other limiting factors on the width and length of the primary flight feathers. The secondary flight feathers need to provide a full and complete base to the outstretched wing in order to ensure maximum lift but their size is limited by the fact that they must not interfere with the action of the primaries. Secondaries that are long compared to the primaries allow for fast but energy-draining flight of short duration, while secondaries that are short compared to the primaries allow for slower but more energy-efficient sustained flight. The most efficient design for endurance racing is for all secondaries to be slightly shorter than the first primary feather (ie the one closest to the body).
Many fanciers feel around the bird’s shoulder when assessing the wing, usually attempting to feel the length of the bone that runs from the shoulder to the elbow (the humerus). This bone differs in length from one bird to another and this different length affects the wing function. Muscles (such as the supracoracoideus and others) attach to this bone, which are responsible for pulling the wing up above the bird’s back after the downward stroke. Their importance for the competitive racing pigeon can be verified by the fact that when birds become fit and their pectoral muscles (responsible for the down stroke and also pulling on the humerus) become toned these muscles are also felt to become full and firm.
Just how long the humerus needs to be for optimal performance is impossible to say but rather than stressing a particular length it is more important that the bone is of a length that is proportional to the rest of the wing and that it not only provides a strong base for the wing but also supports the wing at the correct angle.
Additional lift can be created by angling the leading edge of the wing up into the air current. When the leading edge of the wing is lifted, air flow is directed downwards off the rear of the wing, resulting in a ‘down wash’. This downwash also contributes to lift. The more the wing is tilted against the air flow, then the greater the deflection of air downwards and up to a point the greater the amount of lift generated.
A bird can fly with the wing straight and level but this is uneconomic. With the very edge slightly elevated, drag is low (because the wing offers only small resistance) but lift is small. As the leading edge is elevated, lift increases but past a certain point, the wing juts into the airflow, leading to so much increased resistance that forward movement becomes impossible. The best position for the outstretched wing to be held in for sustained flight is elevated 4° from the flat. Once the angle approaches 15°, lift disappears and the bird will stall. The anatomical support structure of the upper wing is genetically controlled. Within the range of function offered by this anatomy, birds then have control over the position in which the wing is held and therefore their speed through the air.
Lift is also affected by air density. More lift is generated when the air is thin and light as on a hot day than when the air is heavy and more dense. This is because there is less air by weight passing over the wings. This is why birds are keener to exercise and appear less tired when exercised on this type of day.
Air speed also affects lift. The faster the air flow, the less the pressure above the wing and the greater the lift. Interestingly, doubling the air speed over the wing quadruples the lift. We have all seen the way a bird with outstretched wings facing the wind on the loft roof can appear to suddenly rocket skywards and the difficulty that birds sometimes have in losing altitude as they come into a strong wind to land.
When gliding, a bird’s wing behaves like an aeroplane wing and lift is generated by its forward movement through the air. The magical difference about a bird’s wing is that not only the shape of the wing but also the shape of the flight feathers change during flight in the motion of the wing beat. Indeed, this must occur because the avian wing, unlike that of the plane, must not only provide lift but also forward propulsion.
When assessing the wing for propulsion. It is the length, shape and condition of the primary flights that are the most important.
The primary flight feathers
In each primary feather, the trailing edge of the vane (each feather is made up of the central quill with a web of filamentous material called the vane extending from two opposite sides) is broader and more flexible than the leading edge. As the wing beats down, the vane twists with the trailing edge going up and forcing air backwards to produce forward thrust. To fly faster, the bird flaps its wings faster to make the primaries twist more and increase thrust. On the up stroke, the wing is pulled closer to the body and the primaries are separated, allowing air to pass through them. This avoids the wing pushing the bird down. Interestingly, a small backward sweep of the wing as it comes up into the final upstroke position forces the top surface of the primaries down against the air to give the bird an extra forward push.
The wings of a racing pigeon must be of a size that can be comfortably moved and in proportion to the size of the bird’s body, otherwise flight will become inefficient; leading to premature fatigue. A good analogy here is a set of human rowers that are given a set of oars that are either too large or too small. Too small, and the oars can be moved very quickly but progress is slow; too big and enormous strokes can be made, covering a good distance, but fatigue quickly sets in. In the same way, the length of the primaries must match the size of the bird.
As a general rule, however, it does seem that distance birds have longer primaries (and also shorter secondary flights) than sprint birds. Short wings can be moved quickly up and down. Resulting in a rapid flight. By contrast, the longer wings of distance birds take longer to move up and down but each stroke propels the pigeon a greater distance (in the same way that the step of a human with long legs covers more ground). This means that the distance birds travel further for a given number of wing beats. This combined with other factors allows distance birds to fly further and longer without becoming tired. Because of the extra effort involved in flying on short wings, sprint birds can only maintain their speed for a short period of time. It is not unusual for successful distance birds to have the tenth flight the longest and some have the eight, ninth and tenth flights all the same length. In sprint birds, usually the ninth flight is the longest. One can appreciate the difficulty that birds forced to race when they are growing their outer flights must have in maintaining forward propulsion.
Distance birds also tend to have more spacing (or ventilation) between the last four primary feathers. This enables air to easily slip through during the upstroke, minimising the effort involved to bring the wing up. Sprint birds tend to have wider end flights with rounded fuller vanes at their tips. As it is the air trapped by the vane on the down stroke that generates forward propulsion, making the flights broader results in a faster flight. This does, however, make the wing harder to push down because of increased air resistance and also harder to pull up because of the lack of gaps for air to slip through. These factors contribute to a more rapid onset of fatigue.
It goes without saying that the primary flights need to be strong but supple to efficiently cope with the demands placed on them with each wing beat. As mentioned earlier, feather quality is determined by the bird’s genetics and level of care.
And so, what type of wing is a bird going to need so that it can win if it is good in other respects? In summary, the rules of aerodynamics tell us that the wing should be in proportion to the rest of the bird’s body, have an obvious camber, be well supported at the shoulder and covered in feathers of good quality. The secondaries should provide a full inner wing but be no longer than the shortest primary feather. In addition, for birds to be successful in long races, the primary flights, particularly the last four, should be longer but thinner at their ends while speed pigeons are more likely to have shorter wings with wider vanes. Like all rules, there will be exceptions but it is hoped that these notes help in enabling fanciers to make an informed assessment of their birds wings.
Please note my special thanks to Professor Gary Cross of Sydney University for his help in the preparation of this article.