We talk about miracles, particularly during the holiday season, but one that we as bird owners often marvel at is the miracle of flight! The adaptations necessary for birds to fly are considered among the most intriguing puzzles of all in biology. Bats are the only mammal that have the ability to fly on a dynamic wing, while insects have true flight but usually on a fixed wing.
Humans have yearned to fly, but it took the last 100 years for us to perfect machines for flight — we just can’t do it even with wings strapped to our arms! And, in these 100 years, we have finally gotten closer to the abilities that birds have been endowed with — from “flying” deep into the water to flying far into the outer reaches of the atmosphere. How is it that birds can do this so effortlessly? What allows them to make this miracle happen?
A Warm-Blooded Heart
A number of factors are involved for this to happen. Having evolved from reptiles that had developed a four-chambered heart, birds also developed the ability to maintain their own internal body temperature, to be “homeothermic.” They can be, on average, 9 degrees warmer than mammals, which can account for a number of other issues that we can explore in the future. Warm bloodedness allows animals to maintain a constant body temperate, and that is important for providing the energy necessary for flight.
Flight takes an incredible amount of energy; up to 20 times the energy of just sitting. The ability to maintain an internal body temperature is also accompanied by the development of a four-chambered heart. That type of heart is important for providing the energy for pushing the blood to oxygenate the tissues, and it is critical in generating the power for flight.
A Balancing Act
So for flight, form does indeed follow function to be successful. There are strict limitations for the shape and size of a flying animal or machine. Aerodynamic efficiency needs to have sufficient power combined with structural and muscular strength. Part of this equation also requires that weight be kept to a minimum or even more power and strength is needed—stretch the limits too far and flight just does not occur! This has resulted in a certain uniformity of design in all birds that fly. So a parrot has a similar overall structure to that of an eagle, heron or a duck. The anatomical parts of birds are perfectly suited for life in the air. All are designed to allow for the Miracle — from weight reduction to power and all of the aspects needed to fly!
Efficient Respiratory System
In a previous column, I talked about the unique respiratory systems of birds. That uniqueness allows for one-way movement of air with the blood in a cross current multiplier to efficiently allow for the removal of carbon dioxide with the capture of oxygen in their very effective air capillaries. These air capillaries are smaller but have a thinner lining to whisk the oxygen into the red blood cells on the other side of the mucus membrane while dumping carbon dioxide to be exhaled.
Additionally, the ability to use the air sacs to move blood with a fixed lung allows for packing a huge number of these oxygen-capturing air capillaries into the lung surface, compared with the alveoli of mammalian lungs. So in effect, us humans just couldn’t get enough oxygen for lift off or to maintain flight even if we could get airborne! On the other hand, birds’ respiratory systems allow them to take in oxygen efficiently and to get rid of carbon dioxide, and to maintain flight.
Streamlined Shape For Reduced Drag
The body shape of birds allows for smooth, sweeping lines that reduce turbulence and minimize air resistance. Their shape is perfectly suited for flight – they have pointed front ends with smooth contours and a compact, but light, body. Even their legs disappear during flight to reduce drag – their landing gear comes up with takeoff and flight but, when landing, birds bring their legs down and spread their toes to increase drag needed for slowing the body for landing. You have probably seen ducks with their tails and legs down with toes out during landing!
So birds are like airplanes that put down their wheels for landing. Insects, by contrast, have a very different outward appearance. They are often blunted with a number of appendages that do not retract well so that they are sticking out in all directions. Insects are slow fliers at 10 to 15 miles per hour, while there are some birds that can fly upward of 100 to 200 miles per hour. Interestingly, there is 100 times greater drag when flying at 100 miles per hour than there is when flying at 10 miles per hour. A bird’s significantly greater efficiency of streamlining allows for those high airspeeds with a minimum of energy.
Weight Reduction with Strength and Flexible Rudder
Weight reduction is one of the important adaptations necessary for flight. The skeleton of birds has been fused in many locations to reduce weight while adding strength. In the skull, a large number of the bones have been fused and the infraorbital sinus has been added to fill it full of air to reduce weight and add buoyancy. In contrast, I am sure you have noticed how heavy your head becomes when you try to do a number of sit-ups properly!
Weight reduction with centralized strength is important for flight. The vertebral column in birds has been fused from the distal thoracic region with the lumbar vertebrae to just before the vertebrae of the tail, the coccygeal vertebrae. These fused vertebrae then make a rigid platform to work off of to thrust the body forward during the downstroke of the wing beat. This strong rod is fixed to the bird’s “landing gear” through a structure called the synsacrum.
The synsacrum represents the fused ilium, ischium and pubis, and it is then fused to the vertebrae, including the caudal thoracic, lumbar, sacral and proximal coccygeal vertebrae. The distal coccygeal vertebrae are moveable except at the tip, where the tail feathers insert. That allows the tail to move so that it can act as a rudder! Those of you whose birds have had broken tail feathers know that this can seriously compromise their ability to maneuver.
A Good Skeletal Architecture
As you would expect, the landing gear of the legs need to act to absorb shock. This is accomplished through the flexion of the joints, while the head of the femur is firmly placed in a confined hip joint that continues to transmit the landing forces through the fused synsacrum to the fused vertebral column. Pretty ingenious I would say!
The thoracic girdle also has fused elements where the wing is connected to the body. The thoracic girdle is composed of the clavicles that fuse to make the wishbone, the coracoids and the scapulae. The coracoid is the stout bone that connects the humerus of the wing to the sternum. The sternum, with its long keel, provides the broad surface for the insertion of the pectoral muscles for the down and upstroke of the wing. The ribs insert onto the sternum, and these ribs, in turn, attach to the vertebral column.
With each downstroke, the force moves from the wing across the sternum through the ribs to the vertebral column, which then, because of its fused parts, acts to propel the bird’s body forward. The clavicles act as a transverse spacer bar to not allow the chest cavity to collapse with the force exerted during the downstroke. The birds’ fused skeletal architecture is an elegant adaptation for flight, providing structure while managing the forces of flight effectively.
And More To Come …
In the next discussion, I will discuss how a bird reduces weight of some of the organs for flight, as well as some ingenious ways that anatomy has been modified to meet the physiologic demands during flight.