For the most part, chickens aren’t especially long-lived. This is just the nature of the species. Those who are trying to make pets of a few of these birds should realize this. Keeping chickens as backyard pets is like trying to keep most short-lived animals (goldfish and gerbils come to mind); under ideal conditions some live longer than others but it is more exception than rule. If a hen lives to 5 years of age—at which point it will have probably stopped laying—it is a ancient bird. There are a lot of reasons, singularly or combined, for chicken losses.
Start with the genetics. Chickens have been domesticated for in excess of 8000 years. Over that time period a small number of ancestors have produced a large population by breeding within a line. This is especially critical when it comes to each separate breed. In order to fix the different breeds and improve on their ability to either lay more eggs or grow to butchering size, a lot of in-line breeding had to have been done. These back crosses are a recipe for genetic defects. While fairly successful, there are breeds and individuals that have recessive genes that, when passed on, can lead to birth defects. While some, like size, odd feather growth or small combs are desirable, others, like crossed beaks, deformed toes, weak limbs, and the like are not. In addition there may also be internal defects that, while not evident to an observer, can lead to egg laying problems as well as other organ defects that can cause sudden death. It is not surprising, then, that a normal, healthy looking chicken can suddenly die with no apparent sign of illness. Only an autopsy could provide a clue to the cause of this kind of death and, often as not, it will show some anatomical defect as the cause.
Next, consider the method of reproduction. The external egg is not the best form for generating a new generation. If it were, evolution would have stopped at reptiles and I would be typing this with talons. Since the egg requires external incubation it is subject to many occurrences that can alter what is taking place inside the shell. While reptiles simply deposit their eggs and hope for the best, birds incubate them by providing body heat and humidity. This means the brooding bird has to be present almost constantly once the incubation process begins and any time spent off the eggs must be short. Should the eggs become chilled, too dry/wet or should the brooding bird not rotate the eggs at the right time problems can arise with the developing chick. Not only that but the egg shell can be either too weak, which will allow microbes to enter or the shell to be crushed, or too hard, in which case the emerging chick may exhaust itself trying to get out. Given all the variables, even when the eggs are carefully artificially hatched, it is no wonder that many chicks emerge with defects or life-threatening abnormalities. While, especially in artificial conditions, some defective chicks survive, they will always carry these abnormalities which can, later, lead to unexplained death.
Then, for the females, there is the egg laying process itself. In creating an egg laying machine from the original wild bird, we have sped up a process that was not designed by Mother Nature to produce as many eggs as it does. A normal, wild chicken, would produce, at the most, a single clutch of eggs per year, set these eggs and care for the hatchlings. Humans, because they want eggs not more birds, have short circuited this process through selective breeding and by removing the eggs so the hen lays more. This is hard on the hen, puts a burden on her reproductive system, and certainly ages her. It is little wonder that chickens are subject to such maladies as internal laying, egg blockage, peritonitis, “blow out” and other problems that may also be compounded by genetic defects.
Then there are diseases—a lot of them. While most birds are healthy enough to overcome and build immunity to most of the common ones, there are sneaky microbes that can get into a flock from outside sources that will take out one or all of the birds. Unfortunately, many common diseases only show symptoms once the bird is too ill to recover or are not easily and cheaply cured so the infected bird succumbs. Also, given that the chickens are confined in small spaces these diseases can spread very rapidly.
The above doesn’t even get into the physical problems that can befall chickens like impacted crops, broken limbs, and other injuries brought on by bad food or their environment. These kinds of things can happen with little warning in spite of the owner’s vigilance. Often too, the bird dies before the problem is diagnosed.
Finally predators take more than their share. Chickens seem to be on the lower end of the food chain where anything from rats to dogs find them either eatable or fun to kill. Because they have been bred for size, ease of handling and are, generally confined to yards or pens, most chickens have neither the ability to get away from predators nor can they fight them off. A chicken’s defensive reaction to predation is flight (that’s where “to be a chicken” comes from) or sacrifice of members of the flock to save the others. Often this behavior results in death itself without any sign of wounds or struggle and a determined predator can easily decimate a flock in a short period of time.
With a large flock and a lot of room all of the above do not, normally, present much of a problem. The loss of a few chickens in a flock of 20 or more is, while not something that makes the owner happy, understandable. However, if one has only half a dozen birds to begin with a loss of one or two is a tragedy of great proportions, especially if the owner is unprepared for it. It is important then that if you are keeping a small flock that you understand that this kind of thing is going to happen. It will better prepare you for the eventuality.
While there are things you can do such as cull any obvious defective chicks so as not to allow them to breed, select birds from proven flocks, watch for signs of disease, isolate sick birds, practice good biosecurity, and do the best you can to prevent predators from getting to your birds, they will still die unexpectedly. Just remember this is part of keeping chickens as much as enjoying those cute little fuzzy butt chicks.