Streamlining in Egyptian Swifts (ES)

by Adel Salem(Clinical Laboratory Scientist)

In order for a long bird of an average length of 11 to13 inches long (29 to 33 cm as measured from the tip of the chest to the tail end) to be able to fly smoothly in the air, with speeds of 30 to 60 miles per hour (50-95 km/hr) it must be aerodynamically built. The distribution of the muscle mass and the feather arrangement on the body play a major role of thrusting and keeping the ES buoyant in the air for at least 2 hours during leisure flying and over 6 hours while racing.

General Appearance Of The Flying ES:
Short beak curved downwards, fused directly in a small compacted head and a conspicuously short neck. The broad erected chest blends in directly with a long concave back that ends with a long tail. Wings are shorter than the tail, pressed tightly against a heart-shaped chest and resting usually above the back. Short powerful legs containing 6% of the ES body weight helps in jetting the bird rapidly in the air.

Wings And Feather Arrangement:
Because of a massive length of the wing span reaching over 30 inches (75 cm.), they have to be designed so articulately other wise creating excessive weight, drag and air turbulence reducing the bird's ability to fly. The outer vane, which springs from the side of the Rachis(the shaft or the middle portion of the feather that supports the vane on each side) should be overlapping the next outer vane with the exception of the first two primaries (the first nine to ten feathers in the pigeon's wing) to allow for some air flow to the upper wing. In non-flying ES the overlapping is minimal. The primaries act as a propeller by providing forward motion. While the secondary (the 9 feathers that follow the primaries) give the proper support of lift in the air.

 

Slots: 

The first two primaries don't show the typical vane overlapping or webbing as the rest, creating gaps or slots. Slots prevent stalling and increase lift by making airflow fast and evenly over the upper surface, thus reducing turbulence. Slots also noted between the tips of the outer most primaries and another between the alula (three stiffened feathers springing from the outer most edge of the wing) and wing margin. Broad-winged soaring birds such as eagles and vultures show slots that are very much larger than any bird. The ES take advantage of this modification by flying so slowly and effortlessly at times, as they become tired.

Feather Characteristics Of A Weak Flying ES:
Stretched wing displays long and narrow primaries bent inwards. Rachis of weak flyers is brittle, snip easily, and grow slowly and weakly. Weak Vanes are thin with weak Barbs (plates of hair extending obliquely from the shaft) showing evidence of stress marks across the wings and the tail. The general appearance of that bird is of long wings that are of the same length or even longer than the tail. Wings in weak flyers are permanently dragging, presenting an overall look of loose feathers. Not only are these birds’s weak flyers, but also have a lower fertility rate. The extra length of the primaries reflects entirely on the wing's lining as Coverts (small feathers covering the upper wing) and Axilars (under wing feathers lying close to the body) showing also an excessive length. Loose feathers represent the general appearance of these birds. Good ES should be compacted with short feathers and streamlined in order to sail fast and smooth in the air or even look sharp in a show room. ES that occasionally exhibit wings slightly below the tail are not considered at fault as long as wings are shorter than the tail with all the wing feathers tucked together without evidence of fins or sails and most importantly they have to be pressed tightly against a full chest. Many of these birds are excellent flyers although hard to pinpoint accurately even for an experienced eye. The method of choice then is to test them in the air. I would like to speculate that the droopiness in the flying birds could be due to the weight exerted on the Triosseum tendon by a long heavy wing.

Triosseum Tendon:
It is a tendon of 1.2 centimeters (0.48 of an inch) that fastens the lesser pectoral muscle to the humorous (shoulder bone), thus pulling the wing upward while the bird in flight. It also helps in keeping the wing tight to the chest and above the back while the bird is not flying.
As the lesser pectoral muscle contracts, The attached tendon pulls the humorous lifting the shoulder and the wing up in the air. The greater pectoral muscle which is attached to the lower humorous then contracts generating the needed power needed, for the primaries that act as a propeller, pulling the bird's wing down and moving it forward.

 

The Tail:

The long tail is a modification to help balancing their large surface area. It contains occasionally more than 12 feathers. The ES make tight circles before landing as compared to a racing homer that has to make a much wider circle. Ducks and many other short-tailed birds although fast, their circles are very wide and almost close to a straight line. This adaptation makes them highly maneuverable in the air, especially as air predators at high speed chase them.

The Muscle Mass: 

The chest or the breast is composed of two muscles: the great pectoral and the lesser pectoral muscle. They rest on an oblong sternum one of the most highly specialized bones of the avian skeleton system. A good flying Egyptian Swift with a weight of 300 to 320 grams (an average of 10.5 ounces or 0.7 of a pound) should have up to 32% of its body weight concentrated between the wings and the chest area. The breast muscle by itself compromises a 20% of the total body weight or 1:5 ratio of wing muscle to body weight. If the ratio is increased as the case of utility pigeons or meat pigeons, the bird becomes too heavy to fly. If decreased the bird becomes so light that it would not have enough power to push forward in the air. The result will become evident as the bird Hoover's high in the sky like a kite in the same spot for hours as noted in many high flying breeds. In the case of the non-flying ES their air movement will resemble a glider with very little forward movement.

Flying pigeons that have strong wing-breast muscles is able to develop a force equal to 10 times the weight of the pigeon or a little more than 100 ounces. It is for that reason a racing pigeon could be trained to carry a message or a container of few ounces when properly attached. This scheme has been used for useful purposes and also abused in cases of drug smuggling.

The Sternum Bone Or The Breast-Bone:
Two kinds of sterna exist in the birds: Ratite and Carnite. The sternum of a ratite bird has the ventral surface flattened, like the bottom of a raft. Flightless birds like the Ostrich and utility pigeons posses this kind. It allows for a large build up of chest muscle; however lacks wing-muscle attachment; witch is essential to a flying bird. The Carinite sternum has the ventral surfaced keeled, like the bottom of a sailing boat. Flying pigeons have this kind, because it permits more surfaces for the attachment of the more vital wing muscles. The prominent heart-shaped chest in a racing homer is a good example. Some experience is required to evaluate the muscle attachment o f the ES because of the massive amount of feathers covering their body.

Streamlining:
Since air has weight and exerts pressure streamlining is an essential for flight. It allows air to flow smoothly over the wing surface and at the same time prevents excessive pressure from building up in the front, reducing pressure on the upper and lower surfaces, and lessens vacuum and turbulence behind. During flapping flight more pressure of the oncoming air stream is thus met by the under surface and deflected downward than is allowed to flow over the upper surface. This gives the required lifting force because the pressure under the wing is greater, while the pressure on the upper surface is less. As the wings tilt while in flying the lifting force increase until a point where air eddies build on the upper wing interfering with the bird's ability to continue on flying. The effect of the air turbulence is partially offset by the wing's aspect ratio, which is the proportion of the length to breadth. In other words, a wing is long and broad enough to allow for disturbances and yet has sufficient surface for lifting purposes. Birds with long wings as the ES, don't need to flap as many times (3.0 times per second) as compared to a racing Pigeon with shorter wings (more than 3.0 times Per second) or a Herring Gull of very long wings (2.3 times per second), Pelican (1.2 times per second) and the Humming bird with wing beat of 60 times a second.

Summary: 
Excessive length and poor wing design is a hindrance to the ES. On the other hand long properly constructed wings aided by longer tail could enhance the ES both in the sky and the show room.

References:
Ornithology by Olin Sewall, - Beebee; The bird and its function - Roberts; on the normal flight of birds - The pigeon by Levi on chest muscle