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Chickens and hens

About 50 billion chickens and hens are killed each year across the world. Their massive exploitation has resulted in their becoming the most numerous bird species in the world. If we only counted the farm animals raised on land, and not fishes and other aquatic animals raised on farms, chickens and hens would constitute the greatest number of animals killed for consumption.

The lives of chickens and hens are short and full of suffering. The vast majority raised for their flesh or eggs live crammed together in tiny spaces. They only see sunlight when they are taken to a slaughterhouse. They live under artificial light, which results in more economically profitable exploitation, but alters the hens and chickens’ biological cycles. Their living conditions generate tremendous stress, which leads to behaviors such as pulling one another’s feathers out and cannibalism.1 With the aim of minimizing the harm done by such behaviors, farmers normally partially amputate the chickens’ beaks, a painful operation usually carried out with a blade, or using other methods such as infrared beak trimming.2

As with other animals raised for animal products, the living conditions of chickens and hens contribute toward the development of a range of diseases, and many of these animals die on farms before they are even taken to the slaughterhouse.

The animals who do survive, after a life in harsh conditions, are taken to a slaughterhouse when they are still very young. They are crammed together in a truck, suffering great stress during their transport and at the slaughterhouse, and then they are painfully deprived of their lives.


Laying hens

Shortly after hatching, the chicks go through a selection process based on whether they are male or female. Females are raised to become laying hens. On some farms, the males are killed immediately, sometimes by being put through a grinder. On other farms, they are simply put in the trash while they are still alive, and die of suffocation or as a result of being crushed by the weight of other animals. Male chicks on egg farms are not raised for fattening because the variety of chickens genetically selected for fattening is not the same as that intended for egg laying. It is not worth it to egg farmers to raise them to be eaten, so it is more cost effective to simply kill them when they first hatch.

When the hens reach four months of age and can lay eggs, they are transferred to laying sheds. Many farms currently in operation consist of several of these buildings. Some reach the size of a football field.

In many cases, chickens live on these farms in what are known as “battery cages”. These are rows of wire cages, stacked vertically or on a series of steps on sloping ground so that the eggs roll along a conveyor belt.

As many hens as possible are packed into these cages, each occupying the space equivalent to about one sheet of paper. In these conditions, it is impossible for the hens to nest or clean themselves. They lose feathers when their bodies rub against the wire cages, as well as suffering from bruises and grazes.

The hens are forced to remain standing up, which often results in their legs becoming deformed. The wire can hurt their toes, sometimes causing cuts. When the weather is cold, their legs can freeze and even become stuck to the bars of the cage. Sometimes when they are taken out of the cages, their legs have become grafted to the bars and are torn off when the hens are pulled away.

Battery cages have been outlawed in the European Union, although several countries do not enforce it. So-called “enriched cages” continue to be legal. These can group several dozen chickens in an area that gives each hen slightly more room than a battery cage, and must contain space and material for nesting. They don’t allow enough space however for the hens to fly or flap their wings, and they do not protect the animals from the inevitable pecking among themselves due to stress. Hens in enriched cages are also susceptible to diseases from living too close to their own feces.

Several states in the United States have also banned battery cages. They are still heavily used in many parts of the world.

Factory farms contain many thousands of individual hens, and it is impossible to monitor the health of all of them. For this reason, if a hen has any health problems or is physically harmed by other animals, she will die without receiving any care. In cage farms, it is difficult to access and dispose of these hens’ dead bodies, so their bodies normally stay in the cages until all of the hens are taken to the slaughterhouse.

These days each hen lays an average of 260 eggs per year, although the figure was only half that in the early twentieth century. On some farms today, hens can even produce 300 eggs a year. This means fewer hens are exploited to meet the existing demand for eggs, but the hens who are exploited are subjected to greater harm. This is because hens who lay more eggs are at increased risk of diseases such as fatty liver disease and osteoporosis. Fatty liver disease, caused by excess eating and too little exercise, is when hens’ liver cells accumulate excess fat, making them prone to die of hemorrhaging in the liver. Osteoporosis is caused by the hens’ bodies using a lot of calcium to produce egg shells. A lack of physical exercise also contributes to it.

The eggs of laying hens, as well as those of broiler hens, are not normally incubated by the hens. In an exploitation-free situation, hens take care of their children. This is not the case on a farm. Incubators are currently used that maintain a temperature controlled environment and can hold hundreds or even thousands of eggs at once. When the chicks hatch from their eggs, the female chicks are separated from the males and the whole process starts again.

There are other types of laying hen farms where the hens are not kept in cages, but where similarly to broilers they remain confined and crowded together in situations that cause them considerable suffering, as explained below.

There are also hens reared in extensive farms who do not experience some of the conditions suffered by hens kept in cages. Yet as mentioned above, in order for laying hens to exist, male chicks are also born and subsequently killed. And as soon as the exploitation of individual hens ceases to be profitable, those hens are taken to the slaughterhouse.

On factory farms, hens are usually taken to a slaughterhouse after the first year of laying eggs. In extensive or organic farms, hens can live a few more years, but this may vary from one place to another and does not usually exceed six or seven years. Although the hens continue to lay eggs, when their productivity declines they are sent to their deaths. If they weren’t being exploited on farms, they could live up to 15 years.

When the hens are not killed after the first year, they are forced to molt in the dark. This can be done using pharmacological methods, by providing food containing substances that trigger molting, or by making them fast, which also triggers molting. Forced fasting involves depriving the hens of food for a period of time that may last from 10 days up to several weeks. About 10% of hens die of hunger or dehydration when molting in these conditions, and those that survive can lose up to 25% of their body weight. In any case, this process has a significant impact on their bodies.3 Molting brings about the beginning of the egg laying cycle, and a further extension of the economic productivity of the animals. Forced molting is illegal in a few places, but is a common practice.


Broiler chickens

In 2009 the Swedish government recommended the consumption of chicken to tackle climate change, given that the exploitation of these animals is less polluting than eating meat from cows and pigs.4 Since chickens are smaller animals than cows and pigs, following this recommendation will cause an increase in the number of animals killed. It is sometimes assumed that efforts to protect the environment will be good for nonhuman animals. This is not true. They can, as in this case, lead to an increase in animal deaths.5

Chickens are transported from farms to slaughterhouses shortly before they reach the age of two months. They are crammed into trucks where they are knocked about, deprived of food and water, stressed, and often suffer as a result of very high or low temperatures.

Chickens raised for fattening, like hens raised for their eggs, are social animals that organize themselves hierarchically, and this social order stands for long periods of time. In the poultry industry animals remain crammed into small spaces, resulting in frequent disputes6 either over available food or the introduction of new chickens.

Competition for food can be so strong that some animals may be forced not to eat in order to avoid being attacked, which will eventually result in the weakest individuals dying of starvation or dehydration.

Over the years, chickens raised for consumption have been genetically selected to ensure rapid growth for more profitable exploitation.7 They grow so fast that they are sent to the slaughterhouse after a few weeks of life.

At the beginning of the last century, a chicken could reach a weight of one kilo in 16 weeks, but today will reach a weight of over two kilos in just 6 or 7 weeks; this is more than twice the weight in less than half the time.8 Such extreme weight gain in such a short time can lead to many diseases and injuries. The chickens suffer injuries and deformities of their legs due to their inability to support their body weight, their tendons weaken and break, and they suffer from metabolic disorders.9 Their immune systems weaken.10 It is common for overweight animals to suffer from arrhythmia, which can lead to heart failure and sudden death syndrome.11 Between 1% and 4% of broiler chickens are at serious risk of sudden death syndrome.12

The chickens spend their short lives in crowded farms where they will barely have enough room to spread their wings. The ventilation is poor and as days go by, fecal matter will accumulate. This, combined with typically wet ground conditions, facilitates the proliferation of bacteria and consequent contraction of diseases.

The animals go through life permanently treading in their own excrement. This then decomposes and produces ammonia, a toxic and highly irritating gas that causes respiratory diseases and is linked to ocular, gastrointestinal, and tracheal irritation in animals.13 Prolonged contact with excrement and the resulting ammonia causes contact dermatitis.14 It has been confirmed that lameness can affect up to 20% of birds.15

A study conducted in the United States concluded that chickens were suffering a mortality rate of 1.1% due to leg problems (total mortality was 3.8%),16 and that 2.1% had to remain on the ground due to leg deformities.17 Also, between 1% and 5% of chickens are affected by problems such as this when they arrive at the slaughterhouse.18

The chickens’ deaths when they have reached the set weight are very early given their life expectancy. They commonly die when they are only 6 or 7 weeks old. The chickens in extensive or ecological farms are killed a little, but not much later, when they are about three or three and a half months old. As mentioned above, however, chickens can potentially live for up to 15 years.

Further readings

Appleby, M. C. & Hughes, B. O. (1991) “Welfare of laying hens in cages and alternative systems: Environmental, physical and behavioral aspects”, World’s Poultry Science Journal, 47, pp. 109-128.

Appleby, M. C.; Mench, J. A & Hughes, B. O. (2004) Poultry behaviour and welfare, Wallingford: CABI Publishing.

Bestman, M. W. P. & Wagenaar, J. P. (2003) “Farm level factors associated with feather pecking in organic laying hens”, Livestock Production Science, 80, pp. 133-140

Burt, D. W. (2002) “Applications of biotechnology in the poultry industry”, World’s Poultry Science Journal, 58, pp. 5-13.

Cooper, J. J. & Albentosa, M. J. (2003) “Behavioural priorities of laying hens”, Avian and Poultry Biology Reviews, 14, pp. 127-149.

Craig, J. V. & Swanson, J. C. (1994) “Review: Welfare perspectives on hens kept for egg production”, Poultry Science, 73, pp. 921-938.

Dawkins, M. S. & Hardie, S. (1989) “Space needs of laying hens”, British Poultry Science, 30, pp. 413-416.

Edgar, J. L.; Paul, E. S. & Nicol, C. J. (2013) “Protective mother hens: Cognitive influences on the avian maternal response”, Animal Behaviour, 86, pp. 223-229.

European Food Safety Authority (2005) “Welfare aspects of various keeping systems for laying hens”, The EFSA Journal, 197, pp. 1-23 [accessed on 18 November 2012].

Evans, T. (2004) “Significant growth in duck and goose production over the last decade”, Poultry International, 43 (11), pp. 38-40.

Farrell, D. J. & Stapleton, P. (eds.) Duck production and world practice, Amidale: University of New England, pp. 229-237.

Gentle M. J.; Hunter, L. N. & Corr, S. A. (1997) “Effects of caudolateral neostriatal ablations on pain-related behaviour in the chicken”, Physiology & Behavior, 61, pp. 493-498.

Gregory, N. G. & Wilkins, L. J. (1992) “Skeletal damage and bone defects during catching and processing”, in Whitehead, C. C. (ed.) Bone biology and skeletal disorders in poultry, Abingdon: Carfax, pp. 313-328.

Grigor, P. N.; Hughes, B. O. & Appleby, M. C. (1995) “Effects of regular handling and exposure to an outside area on subsequent fearfulness and dispersal in domestic hens”, Applied Animal Behaviour Science, 44, pp. 47-55

Guesdon, V.; Ahmed, A. M. H.; Mallet, S.; Faure, J. M. & Nys, Y. (2006) “Effects of beak trimming and cage design on laying hen performance and egg quality”, British Poultry Science, 47, pp. 12-23.

Harrison, R. (1964) Animal machines: The new factory farming industry, London: Vincent Stuart.

Huber-Eicher, B. & Wechsler, B. (1997) “Feather pecking in domestic chicks: Its relation to dustbathing and foraging”, Animal Behaviour, 54, pp. 757-768.

Janczak, A. M.; Torjesen, P.; Palme, R. & Bakken, M. (2007) “Effects of stress in hens on the behaviour of their offspring”, Applied Animal Behaviour Science, 107, pp. 66-77

Keeling, L. & Svedberg, J. (1999) Legislation banning conventional battery cages in Sweden and subsequent phase-out programme, Skara: Swedish University of Agricultural Sciences.

Kjaer, J. & Sørensen, P. (1997) “Feather pecking behaviour in White Leghorns, a genetic study”, British Poultry Science, 38, pp. 333-341

Koelkebeck, K. W.; Amoss, M. S. & Cain, J. R. (1987) “Production, physiological and behavioral responses of laying hens in different management environments”, Poultry Science, 66, pp. 397-407.

Lay, Jr., D. C.; Fulton, R. M.; Hester, P. Y.; Karcher, D. M.; Kjaer, J. B.; Mench, J. A.; Mullens, B. A.; Newberry, R. C.; Nicol, C. J. & Mench, J. A. (2002) “Broiler breeders: Feed restriction and welfare”, World’s Poultry Science Journal, 58, pp. 23-29.

Mitchell, M. A. & Kettlewell, P. J. (1998) “Physiological stress and welfare of broiler chickens in transit: Solutions not problems!”, Poultry Science, 77, pp. 1803-1814.

de Mol, R. M.; Schouten, W. G. P.; Evers, E.; Drost, H.; Houwers, H. W. J. & Smits, A. C. (2006) “A computer model for welfare assessment of poultry production systems for laying hens”, NJAS – Wageningen Journal of Life Sciences, 54, pp. 157-168.

Newman, S. & Leeson, S. (1998) “Effect of housing birds in cages or an aviary system on bone characteristics”, Poultry Science, 77, pp. 1492-1496.

Nicol, C. J. (1987) “Behavioural responses of laying hens following a period of spatial restriction”, Animal Behaviour, 35, pp. 1709-1719.

O’Sullivan, N. P. & Porter, R. E. (2011) “Hen welfare in different housing systems”, Poultry Science, 90, pp. 278-294

Onbasilar, E. E. & Aksoy, F. T. (2005) “Stress parameters and immune response of layers under different cage floor and density conditions”, Livestock Production Science, 95, pp. 255-263.

Patterson, P. H. & Siegel, H. S. (1998) “Impact of cage density on pullet performance and blood parameters of stress”, Poultry Science, 77, pp. 32-40.

Savory, C. J. (2004) “Laying hen welfare standards: A classic case of ‘power to the people’”, Animal Welfare, 13, pp. S153-S158.

Savory, C. J.; Jack, M. C. & Sandilands, V. (2006) “Behavioural responses to different floor space allowances in small groups of laying hens”, British Poultry Science, 47, pp. 120-124.

Sherow, D. (1975) Successful duck and goose raising, Pine River: Stromberg.

Sherwin, C. M., Richards, G. J. & Nicol, C. J. (2010) “Comparison of the welfare of layer hens in 4 housing systems in the UK”, British Poultry Science, 51, pp. 488-499.

Silverin, B. (1998) “Stress response in birds”, Poultry and Avian Biology Reviews, 9, pp. 153-168.

Smith, S. F.; Appleby, M. C. & Hughes, B. O. (1993) “Nesting and dustbathing by hens in cages: Matching and mis-matching between behaviour and environment”, British Poultry Science, 34, pp. 21-33.

Tactacan, G. B.; Guenter, W.; Lewis, N. J.; Rodriguez-Lecompte, J. C. & House, J. D. (2009) “Performance and welfare of laying hens in conventional and enriched cages”, Poultry Science, 88, pp. 698-707.

Tauson, R. (1985) “Mortality in laying hens caused by differences in cage design”, Acta Agriculturae Scandinavica, 34, pp. 193-209.

Tauson, R.; Wahlstrom, A. & Abrahamsson, P. (1999) “Effect of two floor housing systems and cages on health, production, and fear response in layers”, The Journal of Applied Poultry Research, 8, pp. 152-159.

United Egg Producers (2016 [2003]) Animal husbandry guidelines for U.S. egg-laying flocks, 2016 ed., Alpharetta: United Egg Producers [accessed on 7 February 2016].

Webster, A. B. (2004) “Welfare implications of avian osteoporosis”, Poultry Science, 83, pp. 184-192.

Wilkins, L. J.; McKinstry, J. L.; Avery, N. C.; Knowles, T. K.; Brown, S. N.; Tarlton, J. & Nicol, C. J. (2011) “Influence of housing system and design on bone strength and keel-bone fractures in laying hens”, Veterinary Record, 169, pp. 414-420

Zimmerman, P. H. & van Hoof, J. A (2000) “Thwarting of behaviour in different contexts and the gakel-call in the laying hen”, Applied Animal Behaviour Science, 69, 255-264.

1 Savory, C. J. (1995) Feather pecking and cannibalism”, World’s Poultry Science Journal, 51, pp. 215-219

2 Breward, J. & Gentle, M. J. (1985) “Neuroma formation and abnormal afferent nerve discharges after partial beak amputation (beak trimming) in poultry”, Experientia, 41, pp. 1132-1134. Duncan, I. J.; Slee, G. S.; Seawright, E. & Breward, J. (1989) “Behavioural consequences of partial beak amputation (beak trimming) in poultry”, British Poultry Science, 30, pp. 479-489. Gentle, M. J.; Waddington, D.; Hunter, L. N. & Jones, R. B. (1990) “Behavioural evidence for persistent pain following partial beak amputation in the chicken”, Applied Animal Behaviour Science, 27, pp. 149-157. Hester, P. Y. & Shea-Moore, M. (2003) “Beak trimming egg-laying strains of chickens”, World’s Poultry Science Journal, 59, pp. 458-474. Glatz, P. C. (2005) Beak trimming, Nottingham: Nottingham University Press.

3 Brake, J. & Thaxton, P. (1979) “Physiological changes in caged layers during a forced molt. 2. Gross changes in organs”, Poultry Science, 58, pp. 707-716.

4 Rosenthal, E. (2009) “To cut global warming, Swedes study their plates”, The New York Times, October 23 [accesed on 9 August 2011].

5 Ball, M. (2010) “Animals as the bottom line: Global warming, human psychology, and net impact for animals”, [accessed on 15 January 2014]; (2013) “Equitable ethics vs easy environmentalism: The essence of Earth Day”, The Vegan Outreach Blog, April 15 [accessed on 15 November 2013]; (2014) “Lesson learned: Advocacy can hurt animals”, A Meaningful Life, A Better World, November 3 [accessed on 3 December 2014]

6 Banks, E. M. & Allee, W. C. (1957) “Some relations between flock size and agonistic behavior in domestic hens”, Physiological Zoology, 30, pp. 255-268; Al-Rawi, B. & Craig, J. V. (1975) “Agonistic behavior of caged chickens related to group size and area per bird”, Applied Animal Ethology, 2, pp. 69-80.

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9 Julian, R. J. (2004) “Evaluating the impact of metabolic disorders on the welfare of broilers”, in: Weeks, C. & Butterworth, A. (eds.) Measuring and auditing broiler welfare. Wallingford: CABI Publishing. Bessei W. (2006) “Welfare of broilers: A review”, World’s Poultry Science Journal, 62, pp. 455-466.

10 Rauw, W. M.; Kanis, E.; Noordhuizen-Stassen, E. N. & Grommers, F. J. (1998) “Undesirable side effects of selection for high production efficiency in farm animals: A review”, Livestock Production Science, 56, pp. 15-33.

11 Riddell, C. & Springer, R. (1985) “An epizootiological study of acute death syndrome and leg weakness in broiler chickens in Western Canada”, Avian Diseases, 29, pp. 90-102. Gardiner, E. E.; Hunt, J. R.; Newberry, R. C. & Hall, J. W. (1988) “Relationships between age, body weight, and season of the year and the incidence of sudden death syndrome in male broiler chickens”, Poultry Science, 67, pp. 1243-1249.

12 Olkowski, A. A. & Classen, H. L. (1997) “Malignant ventricular dysrhythmia in broiler chickens dying of sudden death syndrome”, Veterinary Record, 140, pp. 177-179.

13 Kristensen, H. H. & Wathes, C. M. (2000) “Ammonia and poultry welfare: A review”, World Poultry Science Journal, 56, pp. 235-245.

14 Whates, C. M. (1998) “Aerial emissions from poultry production”, World Poultry Science Journal, 54, pp. 241-251.

15 Gregory, N. G. (1998) Animal welfare and meat science, Oxon: CABI Publishing, p. 184.

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17 Morris, M. P. (1993) “National survey of leg problems”, Broiler Industry, 56 (5), pp. 20-24.

18 Gregory, N. G. (1998) Animal welfare and meat science, op. cit., p. 183.

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