Written by a friend for Magnum magazine, originally. Copied here with his permission.

Diagrams in the article can be found at the original site, along with many other good reads of his, @:

http://www.nrvoutdoors.com/



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Anyone who has done much wingshooting knows that when you calculate kills made for shots fired, bringing a bird to bag is much less likely than it is for almost any mammal. People who’ve given some thought to this situation often attribute it to the difficulty of hitting a moving object, or to technical mistakes: excess range, too little shot, wrong degree of choke, unsuitable shot size, etc. While all these certainly are factors in determining whether a given shot will kill, cripple, or have no apparent effect, the truth is that by virtue of their anatomy and physiology, birds are much tougher targets than mammals of comparable size. It’s to be expected they’ll be harder to kill cleanly and consistently. Good wingshooting demands not only skill with the shotgun, but a basic understanding of avian structure and physiology; and how these confer on birds a good deal of protection from the effects of shot impact.

Consider just how a shotgun kills. A pellet is nothing but a very small bullet: shotguns are “area” weapons relying on large numbers of pellets to increase the statistical probability that one or more will impact some vital structure, effecting a kill. The next-best outcome is a disabled bird who can be dispatched with a second shot or by neck-wringing. While the fate of any individual pellet is unpredictable, adequate charges fired from properly-made guns produce consistent patterns, and one or both outcomes should happen if the shooter does his part. There is an oft-repeated mantra about needing to put “…a minimum of 3 pellets anywhere into the bird to assure a clean kill.” Like so many rules of thumb, this one has its root in truth, but it’s greatly oversimplified. The fact is that the absolute minimum is one pellet, properly placed. It’s that “properly placed” that’s the hard part. Let’s now consider just why.

A sphere is the least efficient of ballistic shapes. Pellets lose energy rapidly, limiting the shotgun’s range to that distance at which enough energy remains in individual pellets to penetrate a target to a vital zone. Every wingshooter has scratched his head over the question of whether pattern density is more important than individual pellet energy. Head-scratching will continue forever, because these two criteria are to some extent mutually exclusive. But most shooters have found loads that in their guns deliver a good, dense, and uniform pattern at typical ranges, with enough velocity and energy to drive the pellets deep enough to kill cleanly if something important is struck. Through the rest of this discussion I’ll assume we are talking about such a gun and load. Now, then: why is it so hard to kill birds cleanly?

A bird is essentially a ball of dense muscle covered with feathers. It’s possessed of a minimalist skeleton barely adequate to provide anchor points for the flesh. In most flying birds the feathers weigh more than the skeleton does! A few regions of the skeleton are pretty robust, all of them sites of massive muscle attachments. The principal such one is the “keelbone” or sternum, to which the big pectoral muscles are attached. Other comparatively “hard points” are the skull and the spinal column. The rest is spindly bits with little structural strength. The legs, for example, are designed principally for resisting vertical stresses of landing and weight-bearing on the ground; they snap under lateral pressure quite easily in most species. Like airplanes, birds put most of the weight of their airframe in the main axes of flight, the longitudinal spinal column and the horizontal wing spar that crosses it at a right angle. To this frame is attached the heavy musculature.

Birds are tremendously strong for their size, with most of their brawn in the pectoral muscles that work the wings. Muscle is dense, much more so than other tissue types. It’s heavily reinforced by connective fibers that surround and attach to every individual strand; these fibers transmit the force of muscle contraction to the limbs. The more work that has to be done, the heavier the muscle and the sturdier its reinforcing fibers will be. The fibers that invest the cells of the pectoral muscles are the strongest of all, and they merge into heavy tendons at the point of attachment to the wings. Other strong muscles work the tail for postural control in flight, arch the back, and move the neck and head. Birds that are strong and agile flyers, such as pigeons and doves, have disproportionately large breast muscles; and they also, not coincidentally, are almost universally regarded as hard to kill by those who hunt them.

While the muscles are an obstacle to pellet penetration, they aren’t the only one. Outside of the muscles is the skin, covered with feathers. In some birds the feathers are quite heavy, serving not only to provide lift in flight, but as insulation from heat loss or gain. Feathers are analogous to hairs on mammals. However, feathers are considerably heavier and stronger than hairs; plus, they’re usually layered and packed pretty tightly. Thus the outer integument can provide a considerable amount of “armor plating.”

Birds are shot in flight as they pass overhead; as they cross in front of the shooter; or from behind as they rise to take flight. The vital organs that have to be hit for a clean kill are thus behind some pretty formidable protection. In shooting at a bird passing overhead, the pectoral muscles and the keelbone form an effective shield to deflect shot and absorb its energy before it can reach something important. A big bird can have three to five centimeters of dense down, overlaid by hard-surfaced flight feathers, as the first obstacle the shot has to deal with. Then, after losing considerable momentum and energy, they have to get through the underlying muscle and the hard shell of bone surrounding the heart. On overhead shots it’s common for a pellet never to make it into the body cavity at all. In a crossing shot, the wings and the lateral portions of the breast muscles are interposed between the vital regions and the pellets: not so good a protection as the underside, but not too bad. The rising-and-climbing presentation is the one which exposes the bird to the most danger. It provides only a minimal layer of feathering and the relatively skimpy back muscles as protection.

There are only two ways to kill a bird quickly and cleanly. The first and fastest way is put a pellet into the central nervous system (CNS, the brain and the spinal cord). This immediately “cuts the wires” between the command-and-control centers and the rest of the body, and down the bird comes, stone dead. A good CNS hit is more likely than not to cause an instantaneous kill, with the bird dead before it hits the ground.

But even on a large bird the CNS is a very small target, representing only a small part of the overall area as seen from most presentations, as Diagram One shows. From above or below, it resembles a lollipop; from head-on or behind, it’s an insignificant portion of the total area where a shot might strike. Hence the likelihood of a perfect hit is small. Worse, as we’ve seen, it’s heavily protected. Increasing the number of shot in the charge is one way to compensate, but if that means reducing shot size to stay within a certain charge weight, even a pellet striking smack on the right spot may not have enough energy to penetrate the “armor” and the bone around the CNS. Using a larger shot size to increase penetration reduces pattern density and the chances of a perfect hit.




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Figure 1: A bird as seen from below, and head-on presentations, to illustrate the very small vulnerable area of the central nervous system. The CNS structures are protected from below by a substantial "armor" of feathers, muscle, and bone.
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The other way to kill a bird is by making a hole or holes in it, large enough for it to bleed to death. This is not so easy to do as you might think. The reliable and deadly heart/lung shot that anchors nearly any mammal right away isn’t appropriate for birds, nor nearly so effective.

Mammalian lungs are isolated in a sealed chamber whose internal air pressure is lower than that of the atmosphere by a considerable margin. The pressure differential between the inside of the lung (open to the atmosphere) and the space between it and the chest wall is what keeps the lung inflated. This condition is maintained by sealing the chest cavity at the top where the windpipe enters, and at the bottom by the muscular diaphragm. A mammal inhales by expanding its chest and lowering the diaphragm. By these actions the pressure difference is increased, atmospheric pressure inflates the lungs still further, and the blood flowing through the lung capillaries is bathed in a fresh batch of oxygen-rich air. When the chest muscles contract and the diaphragm rises, the chest volume is reduced, and air is forced out again. This in-and-out cycle is “tidal breathing.”

Perforating a mammal’s chest causes its lungs to collapse. Even if the lungs themselves are untouched, tidal breathing has to stop when the pressure is equalized. When that happens no oxygen can be passed to the blood. A chest-shot mammal is a dead mammal, whether or not you find him: his odds of survival are almost zero. If he can't make his lungs work, even if he doesn't bleed to death (as he most likely will, since the lung’s colossal and very fragile capillary plexus is almost always damaged by the bullet) he'll die of oxygen deprivation within a few minutes*.

The chest shot so deadly to mammals doesn’t work well for birds because of fundamentally different arrangements of the respiratory system. You can’t rely on chest hits to kill birds cleanly. Birds don't ventilate their lungs the way mammals do: there’s no chest cavity with a pressure differential to inflate the lungs, and birds aren’t tidal breathers. Anatomically they don't have a chest cavity at all, since they have no diaphragm to separate the thorax from the abdomen.

Ventilation isn’t accomplished by expanding and contracting the chest, and birds aren’t tidal breathers. Instead they have a “flow-through” pattern. Air movement is caused by expansion and contraction of large air sacs located at the ends of the complicated system of respiratory tubes. As the bird moves its muscles and wings these air sacs expand, drawing air through the lung, not into it; and then expelling it out again as they contract, like working paired bellows at the end of a set of pipes. Ventilation is mechanically independent of blood oxygenation. Unlike the mammalian pattern, a perforated chest can’t cause the lung to collapse, because it isn’t inflated at all, and the movement of air isn’t dependent on a pressure difference between the lungs and the chest cavity.


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Figure 2: Approximate location of the lungs and air sacs in the avian body cavity.
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The bellows-like avian ventilation mechanism not only pulls in air, it controls its internal flow, routing fresh air and “used” air to different sections of the system. At any given point in the avian breathing cycle, about a third of the total volume of air in the avian respiratory tract is in the air sacs, another third in the lungs proper, and the final third “in transit,” moving from one region to another, or being exhaled. This system is exceptionally efficient at extracting oxygen from air, much more so than mammalian lungs. It has to be: flight is metabolically expensive and demands an uninterrupted supply of oxygen to the muscles involved. The efficiency is enhanced by the arrangements of the air-exchange sites in lungs themselves.

The lungs of birds are small, stiff, and non-expanding. Tucked up in the top of the body cavity, at the microscopic level these spongy masses structurally resemble thousands of small tubes laid side-by-side. Half of them carry blood, and half carry air. Picture a box of drinking straws, half red and the other half white: the red straws are blood capillaries and the white ones air capillaries. As blood flows through the system, oxygen from the air flowing through diffuses across the walls of the air capillaries and into the blood passing it in the blood capillaries. This system is what an engineer would call “redundant.” Because flight is so energetically expensive, and requires a constant supply of oxygen, the side-by-side arrangement of capillaries has quite a bit more exchange area than a mammal of similar size would show. There’s an enormous reserve capacity for oxygenating the blood; truly massive damage to the capillary beds is required to shut oxygenation down completely. It’s unlikely that enough pellets will reach the lung itself to cause enough physical injury to shut the system down: not only is it afforded the same protection as the rest of the internal organs by the feathers and muscles, the redundancy built into it means that blood and air can simply be re-routed through the undamaged parts of the lung.


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Figure 3: The Lung (L) and Air Sacs (AS). Note how the recurrent loops of the lung are arranged so as to permit a "flow-through system." The Inset shows the side=by-side arrangements of the air and blood capillaries and the countercurrent flow mechanism in them. The major airway (AW) is connected to the oral cavit and allows for entry and exit of air.
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Pellets in the lungs will cause hemorrhage, of course, and a bird may eventually bleed to death from such an injury. But this isn’t a “given” as it is with mammals, as we have seen. It’s almost impossible to inflict enough damage to cause the immediate and catastrophic interruption of his oxygen supply that a mammal would suffer. Thanks to that reserve capacity, the chances are very good he’ll survive long enough to get away, if he can still fly. If his blood clots fast enough to control the hemorrhage, he will live, something nearly unheard of in mammals. In practical terms the typical lung-shot bird will have enough functional lung tissue left to fly a long way before he has to stop, and his odds of becoming a lost cripple are great.

The wingshooter has a tough job. He has to contend with the vagaries and unpredictability of shotgun patterns; with the randomness of pellet flight, and inevitable errors in estimating range, target speed, and lead. On top of that he is trying to bring down a weaving, jinking, fast-moving quarry whose evolutionary development has conferred on it not only the power of flight, but substantial physical and physiological protection against his best efforts. But with a proper understanding of what needs to be done and how to do it; and with a resolve to suit his tools and his skill level to the job, he can substantially increase his odds of success in the fascinating game of wingshooting.



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* When President Reagan was shot, his actual gunshot injury was minor: a small hole in the chest caused by a non-expanding .22 caliber bullet. But that little hole was enough to cause collapse of one entire lung and part of the other. He was saved because that one lung remained functional enough, long enough, to get him to a hospital 2 km away. The surgery sealed the hole and once the pressure differential in his chest was re-established he recovered. A perforation of this type, causing partial or complete lung collapse is a “traumatic pneumothorax.” Had the bullet opened a hole big enough to collapse both lungs he’d have died before he could have been treated.