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Stress - The silent killer

Birds, like humans, do not like stress. It should be understood that stress in birds hampers the function of the immune system and may have a dramatic impact on bird performance. Reasons enough to prevent stress.

By Leo Antony, Staff trainer and poultry management consultant, Bangalore, India

So much has been written and discussed about stress and yet this familiar and little understood hazard remains one of the greatest challenges to modern day poultry management. Yes, stress has come to stay as a silent threat to the well-being and performance of chickens. I call it silent because when birds are under stress, they seldom display dramatic signs and therefore, both the problem as well as the losses that result from it go unrecognised and unaccounted for. I am afraid, it will continue to remain so, until we have understood the dynamics of this complex disorder or grow insensitive to it.

What is “stress”?
The Oxford dictionary defines stress as ‘Pressure, burden or compulsion when much energy is required’. All living beings have a limited amount of stored up resources at all times. These resources help them to adapt or adjust themselves to unstable conditions which at times may pose as a challenge or even as a threat. In chickens, for example, extreme weather conditions, vaccination, beak trimming, insufficient housing space and many such disturbing conditions are known to result in a greater demand for these resources.

As long as these challenges are minor and passing or are within tolerable limits, the animal or bird manages to make use of its reserves, adjust itself to the difficult situation and come out of it with little or no damage. It is only when these challenges come in more intense forms or in greater numbers at any given time, that serious chemical and physical changes take place within the bird with far reaching consequences. This could result in immune suppression, poor weight gain, high FCR and depression in egg production - in short, reduction in the general well-being as well as performance. This happens as a result of the bird’s response in trying to cope with the unfavourable factors which pose a threat to its very existence.

Triggering chemical reactions
The factors which interfere with the well- being of the bird are called stressors or stress factors, and the result of the response or the effort that costs the chicken while trying to cope with the stressor is what constitutes stress. Interestingly, during a training session, I asked the participants to express in their own words what the term stress meant to each one of them. Some of the notions that came to their minds were: tension, anxiety, worry, fear, uncertainty, problem, threat, discomfort, pressure, strain, torture, challenge, difficulty, distress, hardship, burden, effort and struggle.

Surely, these are concepts that are related to human experience, but not entirely. We are aware from experience as well as from medical science that certain emotions like fear, anxiety and frustration work not only on the mind, but can also produce physical ailments as well. To mention a few, today experts attribute medical problems like skin disorders, peptic ulcers, cardiac complaints and other health related issues in some patients to psychological disturbances which trigger off a chain of chemical and physical reactions within the body. If this is true in us, humans, it is also in some ways applicable to animals. This is more easily noticeable in larger animals like cattle and canines whose response to stress and the behaviour resulting from it is more pronounced than that of birds.

Chicken too are subject to negative stimuli like fright, shock, discomfort, deprivation, pain and so on. As a result, their response (a must for survival) to such stimuli is also negative. This is very evident from the number of occasions you would have experienced poor performance of your flocks in any number of ways for no obvious reasons.

Chicken are not mere egg machines
We often wrongly reduce chicken to creatures of the lower order thinking that they have a very low threshold for pain and feelings. It is not so, as chicken, like other animals have reasonably well developed physical senses. Unfortunately, chicken, unlike human beings do not have the faculty to ‘reason out’ and imagine a future where things could change for the better. They live only for the present and experience the immediate.

For this reason, they are unable to ‘reconcile to’ or ‘accept’ any negative experience, even if it happens to be a life-saving procedure like vaccination or beak trimming as something that will do them good. They remain victims of the situation and the only recourse they have in order to survive and cope with such traumatic situations is with the aid of those physical and chemical processes about which I mentioned earlier. In short, this is done with the release of corticosterone, a steroid from the adrenal gland whenever the chicken’s system, in an effort to survive, struggles to prepares itself to face any challenge.

Different types of stresses:
For practical purposes, we could make a distinction between what we know as unavoidable and avoidable stresses - those that form a necessary part of poultry operations such as vaccination, beak trimming, handling for the purpose of weighing or moving, high production or rapid rates of growth, etc. and those which we unnecessarily impose on the birds either through neglect or due to wrong management practices such as overcrowding the birds, making abrupt or sudden changes, providing faulty ventilation, exposing them to harsh environmental conditions, like extreme temperatures and so on. Good management sense should dictate that we altogether prevent any kind of avoidable stress and at the same time alleviate the unavoidable ones.

What does stress do to chickens?
Technically speaking, the hormone corticosterone, is released by the adrenal glands when the bird’s body prepares for the ‘flight or fight‘ syndrome. This actually helps the bird deal with stress, but at the same time takes a heavy toll. I would like to compare stress and it’s mechanism to our familiar ATM card which provides us with ready money but not without depleting our cash reserves! Whenever a bird is under stress there is a rapid release of glucose into the blood resulting in the depletion of glycogen which is a form of sugar that is stored up as a reserve in the liver and muscles. The respiratory rate gets altered. This hormone also causes chemical changes like alteration of the pH levels in the intestines which in turn upsets the balance of micro-flora in the gut.

Results are that these changes provide a suitable environment for certain types of bacteria and fungi. Gastrointestinal diseases can follow. Several studies have shown that this stress hormone can encourage the formation of, as well as the increase of, free radicals inside the body. Free radicals are substances which once produced in the system can be extremely destructive. They react with oxygen in the body and reduce its supply. In addition to this, they constantly turn hostile to several normal processes within the body. Stress causes damages even to the growing embryo as much as it can affect a grown up bird. In practical terms, this hormone can upset several vital systems in the chicken and gnaw into productivity. Chicken have limited resources drawn mainly from daily nutrition for purposes of maintenance, growth, response to environmental changes, support of the defence mechanism and reproduction. Whenever they undergo stress, there is a redistribution or diversion of these resources which include energy and protein, thus sacrificing health, growth, reproduction and other vital functions.

Growth pattern in birds as well as reproduction show declining trends. The percentage of dropouts and culls keeps rising. Depression of the immune system with lowered resistance to viral, bacterial, protozoan and fungal infections is another natural fall out of the stress syndrome. Metabolic malfunctions can also be experienced during periods of stress. We have more than enough evidence of all this happening in humans and it is equally true in animals, including chicken. What is even more relevant in chicken is the fact that more often than not, the number of stress factors and their combinations present at any given time can often be multiple and disastrous. It is also evident that the meat of birds and animals that have undergone stress before slaughter is low in quality in many ways. In short, stress could threaten the very reason for which we grow chicken.

Best way to handle stress?
1. The most important requisite for the farm manager when it comes to stress is to be aware whenever birds are in trouble or are likely to experience it. This includes planning for the occasions when birds have to be handled or subjected to difficult times, although they may happen to be necessary or natural. Let whatever has to be done, be done by always keeping the bird’s comfort and welfare in mind.

2. It is important to quickly recognise signs of stress, like abnormal feathering, constant preening of feathers even in the absence of external parasites, increased aggression like feather pecking or cannibalism and even aimless and restless pacing of birds that are housed on the floor. Delay in laying eggs is also an indication since birds under stress are believed to hold their eggs longer in the shell gland. It is also suggested that coloured egg laying birds under stress begin to lay eggs with pale coloured shells. The hatchability of these eggs is also lower when compared to normal coloured eggs.

3.Today, given the abundant availability of therapeutic formulations in the market, the manager should learn to judiciously pick just the right ones that will do his flock good. Many managers need to be educated in this area because the use of vitamin, mineral and herbal formulations on farms to fight stress is very often misunderstood or indiscriminate and in fact stressful for the birds and therefore counter-productive.

While the priority of poultry farming is the business consideration, we have the obligation of balancing it with humane methods of achieving it. Animal welfare in some countries is controlled by legislation which protects animals and birds from being subjected to unnecessary pain or distress. Even where we are not bound by law, without sounding sentimental, we owe our birds the quality of handling and care which they deserve for serving our business ends. Besides, everyone knows that the performance of chicken, like all living beings, is directly proportionate to a reasonable level of their well -being and comfort. As long as we stay ignorant or insensitive to any form of hardship that birds go through, stress will continue to remain a silent killer.

Leo Antony


  • Mr Luis Benavides Andrade - Stress this is a fight that all poultryman need to have every days, this is a cause of high mortality and the reason that this happens are multiples. To keep the risk low, I our expericen through our poultryman customers and is very helpful to have a information system that allow measure the key factors over all production process to detect abnormal behaviors.


    This is an excellent article so concise and understandable. The elusive stress word is banded around so much with poultry, but yet so few people really understand how it effects the health of a chicken. In my role as an educator I will be encouraging my clients to read this article .

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