Chickens are raised for either meat (referred to as ‘broilers’) or eggs (see our page about eggs here). As a species, chickens endure far more days of suffering than most other farmed animals, living in some of the worst conditions on farms in massive numbers. Over 900 million chickens are slaughtered in the UK annually, or 2.5 million every day.
This figure does not include the hundreds of millions of male chicks who are gassed or crushed to death simply because they can’t lay eggs for the industry to sell and are the wrong breed to be raised for profitable meat.
Contrary to public perception, chickens are remarkably intelligent and sociable animals who learn quickly. Multiple studies have demonstrated that not only do chickens feel joy, boredom, frustration and fear, they have a sense of self-awareness. Even newly-hatched chicks can perform simple arithmetic! They are also socially complex animals who are capable of intentional deception one as well as displaying empathy for one another. Research has also shown that the birds possess self-control and an understanding of the future: when presented with a smaller amount of food with a shorter waiting time versus a larger amount of food with a longer waiting time, chickens choose the second, more valuable option.
'As a trick at conferences I sometimes list [their] attributes without mentioning chickens...and people think I'm talking about monkeys.'
Dr. Chris Evans Professor of Psychology, Macquarie University
Broiler chickens have been manipulatively bred to grow much more quickly than they would do naturally. To increase the industry’s profitability, chickens are now reared to reach slaughter weight as quickly as possible; in fact, and they are sent to slaughter at just six weeks old. (In nature, these young birds would still be with their mothers and indeed, they never grow old enough to cluck like an adult; they are still peeping like chicks when killed.) If humans grew at the same rate as broiler chickens do today, we’d weigh 25 stone at age two! As Dr Toby Knowles from the UK’s Bristol University Division of Food Animal Science states:
In the past 50 years, broiler growth rates have increased by over 300 per cent from 25g per day to 100g per day. 
The birds are four times heavier than they were in the 1960s and their breasts are 80% larger , and this unnatural rate of growth puts increasing pressure on the chickens’ legs. Many of them are unable to support their own body weight and eventually collapse.
As birds struggle to stand, they will often squat to the ground where high concentrations of ammonia (from their faeces) burn their legs and breasts. These painful lesions are called 'hock burns' and researchers have found that these burns, which take the form of small areas of dark discolouration around the knee joints, are visible on 82% of British supermarket chicken. Many other birds starve or die of dehydration when their bones break and they are simply unable to get to food and water. In 2000, the European Commission’s Scientific Committee on Animal Health and Animal Welfare stated that “Leg disorders are a major cause of poor welfare in broilers”.
Heart attacks and lung issues are also common.
That millions of these birds will die before they are old enough to be taken to slaughter is an expected and widely acknowleged cost for the farming industry, which openly admits that 'is it more profitable to grow the biggest bird and have increased mortality...[S]imple calculations suggest that it is better to get the weight and ignore the mortality.” (Tabler GT, Mendenhall AM, “Broiler Nutrition, Feed Intake and Grower Economics,” Avian Advice 5(4) (Winter 2003): 8–10)
An undercover investigation carried out at four Red Tractor-certified Moy Park farms, at least 500 birds died in one day in one shed; workers admitted on camera that this death rate was 'normal'.
Broiler chickens come from birds known as 'broiler breeders', and they face their own forms of cruelty.
The majority of broiler chickens in the UK live in large sheds or barns, the largest of which hold more than a million birds at a time. 
As the birds grow, the space available for each chicken diminishes. Soon the chickens become cramped and frustrated – as shown in this footage taken from inside a UK broiler barn:
WHAT ABOUT FREE-RANGE?
Free-range systems are often advertised as being a humane alternative. In the UK, only about 5% the approximately 980 million chickens slaughtered for meat every year are free-range, and unfortunately, they don't fare much better than their conventionally-raised counterparts. The 'free-range' label only requires that the birds have access to the outdoors--and this can come in the form of a single pop-hole at the end of a barn filled with tens of thousands of birds. Thus even free-range chickens are still predominately confined in barns with a stocking density of up to twelve or thirteen hens per square metre. Due to inadequate pop-holes for outside access and the protection of these exits by dominant hens, less than 10 per cent (on average) of the chickens are outside at any given time, and many never go outside at all. As with conventionally-raised birds, this overcrowding leads to similar welfare problems of aggression and feather-pecking. Free-range birds are killed at the same age as well--approximately six weeks old.
(We have a separate page for free-range egg production; check it out here.)
Whether raised for meat or eggs, chickens are caught by their legs and carried (usually several at a time) upside down before they are loaded into small crates for the journey to the slaughterhouse. This rough handling, combined with the speed that chickens are loaded into transport cages (more than a thousand per hour), causes great stress to the birds and often results in painful leg dislocations and broken bones in their wings and legs.
Unlike other farmed animals, there is no maximum journey time for transporting chickens. Some birds will have to face long distance journeys in transport lorries; legal obligations only require that the birds be provided with food and water after 24 hours for newly hatched birds or 12 hours for adults. It is perhaps not surprising, given these stressors, that approximately more than a million annually are already dead by the time they arrive at UK slaughterhouses. An anonymous whistleblower who works at poultry slaughterhouses as a government vet spoke on the issue in early 2021, claiming that birds were being denied water as well as being held in lorries without any temperature control: 'In the summer months they can be sat in the cages with temperatures getting to 40C in some parts of the truck. They would be panting and die from heat stress in a pretty rough way. In the winter, they are being taken out of [...] sheds where they have been kept at 21C and can quickly get cold stress from the sudden temperature drop...' The chicken industry responded by stating that they were 'totally opposed' to any proposals that would require them to use vehicles fitted with proper ventilation and heating.
UK legislation states that while waiting for slaughter, chickens can be shackled by their legs and hung upside down for up to three minutes, a procedure which has been shown to cause significant pain. Before they are slaughtered, their heads are meant to be immersed into an electrical stun bath so as to render them unconscious. This isn't always unsuccessful, however--the struggling birds frequently avoid the water by lifting their heads up and away from it, a movement the industry calls 'swan-necking', or their wings dip into the water instead, causing painful electrical shocks but not unconsciousness. A back-up employee is meant to catch these birds and decapitate them, but with a line speed of up to 9,000 an hour (or 150 a minute) precision is impossible and chickens are missed. This, of course, means that they are carried along the conveyer to have their throats cut whilst fully conscious.
Even when the stun bath is successful, it is not always enough: birds require longer stunning than mammals to fully lose consciousness, and they recover more quickly. Additionally, the industry tends to use a lower current than is necessary, as higher currents damage the carcass. Studies have shown that broiler chickens will recover after 52 seconds; egg-laying hens, just 22 seconds. Unless the stun bath has actually caused cardiac arrest, the birds routinely wake up after their necks have been cut. Depending on whether one or both carotoid artery was cut, they will can take anywhere from three to five minutes to die. This means that sometimes, they are still awake when they reach the next step of the slaughter process--the scalding tank, which is meant to loosen their feathers--and they are boiled alive.
Another method of slaughter is Controlled Atmosphere Stunning (CAS). This has become much more common in the UK over the past decade; sometimes for the purpose of stunning the birds prior to slaughter and sometimes for killing them outright. CAS includes any system which uses gases which work by limiting the oxygen available to the birds, who are conveyed via their transport crates into a tunnel or a pit, or put into a closed cabinet. While this has the advantage of less handling, stunning is not instant, and if the mixture of gases is not correct, the birds can suffer considerably before finally losing consciousness.
Rugani, Rosa, Laura Fontanari, Eleonora Simoni, et al, Arithmetic in newborn chicks, Proceedings of the Royal Society B, 1-4-2009
Barras, Colin, Despite what you might think, chickens are not stupid, 2017-1-11, BBC Earth
Abeyesinghe, S.M., Can domestic fowl, Gallus gallus domesticus, show self-control?, Animal Behaviour, Volume 70, Issue 1, July 2005
Welfare concerns for broiler chickens, 2008-2-6, Department of Clinical and Veterinary Science, University of Bristol
Tahirali, Jessie, Chickens four times bigger than they used to be: study, 2014-10-7, CTV News
Broom, Donald M. and Nadine Reefman, Chicken welfare as indicated by lesions on carcases in supermarkets, 2005-9, British Poultry Science
Stevenson, Peter, Leg and Heart Problems in Broiler Chickens, 2003, Compassion in World Farming
Dalton, Jane, More than 500 chicks ‘too small to be profitable’ die in 24 hours on farm supplying Tesco, 2020-8-14,The Independent
Wasley, Andrew, Fiona Harvey, Madlen Davies and David Child, UK has nearly 800 livestock mega farms, investigation reveals, 2017-7-17, The Guardian
De Castella, Tom, Do people know where their chicken comes from?, 2014-10-23, BBC News
Milliken, Alison, Andrew Wasley and Cahal Milmo, Dead on arrival: More than a million chickens die before reaching the slaughterhouse, 2018-1-22, iNews
Levitt, Tom, ‘Dying like flies’: A million chickens die on way from farm to abattoir each year, The Guardian, 2020-2-26
Research - Humane Slaughter: Broiler Chickens, Compassion in World Farming
Readhead, Harry, Pigs and chickens were boiled alive at slaughterhouses, animal welfare report reveals, 2016-8-30, Metro News